Although noise can be generated by turbulence in highspeed flows, most noise is generated by mechanical motion caused by forces acting on structures. The motion can be very complex—for example, in the case of a panel on a machine. Consequently, the coupling between the moving structure and air, required to generate noise, depends on details of the motion as well as frequency. Generally, low-frequency vibration is less efficient in generating sound than high-frequency vibration. Sound reaches the ear by propagation from the source to the receiver and can be complicated by reflections from nearby surfaces as well as atmospheric conditions outdoors. Some motion is unavoidable; for example, fan blades must move and vehicle tires must rotate. In many cases the function of the machine is unrelated to the noise generated. An example is the mechanical suspensions that attach airplane engines to an airplane but also allow the transmission of vibrations to the fuselage. This transmission into the fuselage and subsequent radiation of sound can be (and is) minimized by good design—which can also save money by reducing wear and fatigue.
This chapter is concerned with new technologies in materials and systems to reduce noise, the modeling and analytical tools used to design products for reduced noise, and experimental methods of gathering and interpreting data to test and determine how much noise is generated by different product designs. It will be immediately obvious that there are enormous disparities among programs, facilities, and resources for addressing noises of different types. For example, although engineering tools may be available for reducing aircraft noise and highway noise, the former has been deemed a national priority, while the latter has received less attention. Resources allocated for noise reduction are not always commensurate with noise exposures and impacts.
Many tools for designing and developing quieter products have become available in the past few decades, driven largely by increases in computational power and reductions in computational costs. Even so, access to new tools is as uneven as the allocation of resources; corporate budgets for capital equipment are generally tight and there is competition between departments for available funds. Furthermore, organizations that are doing only routine testing of products according to national and international standards find expensive new tools hard to justify. Thus, even though noise mechanisms in aircraft, automobiles, rapid transit and trains, consumer products, and industrial machinery are fundamentally similar, the availability and application of tools for addressing them are not. The question is whether ways can be found to give industry and academia access to these tools for the benefit of manufacturers, workers, and the public.
AEROSPACE AND AEROACOUSTICS
SOURCES OF AIRCRAFT NOISE
Noise from aircraft includes both noise from airplanes and noise from helicopters. At commercial airports, airplanes are the major noise source and will be emphasized here because of the widespread annoyance issues that have affected the quality of life for many persons. Noise from helicopters is also an important issue and affects people living near heliports and in densely populated areas where helicopter flights are not uncommon. The Federal Aviation Administration was asked to prepare a report to Congress on nonmilitary helicopter noise (FAA, 2004). One important issue relates to noise metrics; the impulsive character of the noise requires that metrics in addition to the widely used day-night average sound level (DNL) be used to assess its effects on people.
The noise heard when an airplane flies overhead comes from many sources, but the main contributors are engine noise and airframe noise. Engine noise comes from the fan/propeller, compressor, turbine, combustor, and jet exhaust. Airframe noise is produced mostly by airflows around lifting and control surfaces, such as flaps and slats, and around landing gears.
The relative contribution of these sources depends on the engine and airframe designs and the operating conditions. For example, during takeoff, when the engines are at full thrust, jet noise is the largest contributor to the noise signature of an aircraft. At approach, when the engine is throttled back, noise comes more from the airframe. Other sources, such as the fan, are significant contributors during both takeoff and landing.
Typical noise sources for a fixed-wing aircraft are shown in Figure 5-1. The noise received by an observer depends on the sources and propagation effects. The noise sources for a propeller-driven aircraft are shown in Figure 5-2.
RESEARCH IN AEROACOUSTICS
The aeroacoustics community has made significant progress over the years in understanding and reducing aircraft noise. Figure 5-3 shows comparative contributions from different noise sources for 1960s and 1990s engines. The figure, which originally appeared in Rolls-Royce (2005a), shows that the development of the turbofan engine and reduction in noise from individual engine components resulted in smaller, more evenly matched noise contributions from engine sources (SBAC, 2009).
Over a period of 30 years, these improvements, coupled with advances in aircraft aerodynamics and weight technologies, have reduced aircraft noise by about 20 dB, which corresponds to a reduction in noise annoyance of about 75 percent (EU, 2007). The new Airbus A380, the largest commercial aircraft ever produced (average of 525 passengers), has takeoff and approach noise levels comparable to those of heavy road traffic, a lower noise level than in an underground train. The noise footprint of the A380 is about half that of older, large commercial aircraft (Rolls-Royce, 2005b).
Despite these impressive results, airport community noise continues to be a significant environmental problem, and research and development (R&D) continue in the United States and Europe to meet increasingly stringent noise requirements set by regulatory bodies, such as the Federal Aviation Administration (FAA), the International Civil Aviation Organization (ICAO), and individual airports (Rolls-Royce, 2005b). Over the years, the FAA and ICAO have required comparable reductions.
INFRASTRUCTURE AND PROGRAMS THAT SUPPORT RESEARCH AND APPLICATIONS
A Note on Test Facilities
Both the United States and Europe have first-class aero-acoustics test facilities. Anechoic flight simulation facilities, the most useful for testing both jet noise and airframe noise, are available on both sides of the Atlantic on a rental basis. In the United States, high-quality anechoic chambers for model-scale testing are available at the National Aeronautics and Space Administration (NASA) Langley and Glenn Research Centers, as well as at Boeing, General Electric, United Technologies, and some U.S. universities, such as Georgia Institute of Technology, which inherited Lockheed Georgia’s aeroacoustics facilities. Rolls Royce in England has used the NASA Glenn jet noise acoustic chambers, and Boeing researchers have used facilities in England. NASA
Langley researchers have also used the Dutch anechoic wind tunnel to make helicopter noise measurements. Both the United States and Europe also have access to state-of-the-art flow measurement equipment (including particle imaging velocimetry) and modern phased microphone array systems. Most of these facilities have been described in great detail by Ahuja (1995).
U.S. NOISE REDUCTION PROGRAMS
The FAA and NASA have primary responsibility in the United States for R&D on aviation noise. The FAA focuses on the impacts of noise on communities, while NASA focuses on noise at its source—namely, aircraft engines and airframes. A recent congressionally requested report on aviation noise addresses (1) how well the FAA and NASA’s R&D plans are aligned and (2) the likelihood that noise reduction goals will be met (FAA, 2008).
The FAA and NASA’s R&D plans, aligned through partnerships and planning and coordinating mechanisms, include a wide range of projects for addressing aviation noise. The FAA sponsors aviation noise R&D in noise measurement, noise effects, interrelationships between noise and pollutant emissions, and flight procedures and technologies to mitigate
the impact of noise on communities. Much of this R&D is funded through partnerships with universities, other federal agencies (including NASA), and industry. NASA’s R&D can eventually lead to new technologies for substantially quieter aircraft. However, industry will have to integrate the research results into production-ready aircraft designs.
The FAA and NASA have worked with interagency planning and coordinating groups to establish objectives for the nation’s aeronautical R&D and for specific research on the environmental impacts of next-generation aviation technologies. The strategic plans for the National Airspace System indicate how each agency’s R&D will contribute to meeting noise reduction goals, which are designed to reduce public exposure to aviation noise primarily by reducing noise at its source (GAO, 2008).
In 1994, NASA initiated a seven-year program, the Advanced Subsonic Transport (AST) Noise Reduction Program, to develop technology to reduce jet transport noise by 10 dB relative to 1992 levels. Most of the goals of AST were met by 2001. However, because of an anticipated annual increase of 3 to 8 percent in passenger and cargo operations well into the twenty-first century and the slow introduction of new noise reduction technology into the fleet, the global impact of world aircraft noise is expected to remain essentially constant until 2020 (or perhaps 2030) and thereafter begin to increase. Therefore, NASA has begun planning with FAA, industry, universities, and environmental interest groups in the United States for a new noise reduction initiative.
One of the most important noise reduction technology programs in the United States is the so-called Quiet Technology Demonstrator (QTD1) Program, a partnership among Boeing, Rolls Royce, and American Airlines initiated in 2000 (Bartlett et al., 2004). A second phase, QTD2, a partnership among NASA, General Electric, Goodrich, and ANA, was begun in 2005. These programs have validated new, advanced noise reduction technologies, including nacelle inlet acoustical treatments and chevrons on engine exhaust ducts.
After rigorous testing, including measurements taken on the ground, in the passenger cabin, and on the airframe (Herkes, 2006), many noise reduction technologies, including nozzle chevrons, spliceless inlet linings, extended lining locations, and redesigned wing anti-icing systems (see Figure 5-4), as well as smooth fairings to reduce landing gear noise (see Figure 5-5), have been incorporated into existing airplanes and designs for future Boeing airplanes. Thus, Boeing’s newer airplanes are significantly quieter for both passengers and airport communities (Herkes et al., 2006). A third phase, QTD3, is in the planning stages at Boeing.
Over the years the FAA has defined requirements for the reduction of aircraft noise emissions in terms of stages (1–4). The metric for describing the noise emissions is the effective perceived noise level in decibels (EPNdB), and well-defined microphone positions are used for the measurement. Note that this is quite different from the immission metric (DNL) used to describe the effects of aircraft noise on communities.
The goal of NASA’s current Subsonic Fixed Wing Project is to reduce aircraft noise by 42 EPNdB cumulative below Stage 3 for conventional, small, tube-with-wing twin-jet aircraft, what NASA calls “N + 1 generation” aircraft, by 2012 to 2015 (Collier and Huff, 2007). An even more ambitious goal, set for the 2018 to 2020 period, is to reduce aircraft noise by 52 EPNdB cumulative below Stage 3 for N + 2 generation aircraft, which NASA envisions as an unconventional hybrid wing-body aircraft (see Figure 5-6). In addition to reducing noise, NASA expects dramatic improvements in the emission and performance of these aircraft.
EUROPEAN NOISE REDUCTION PROGRAMS
Driven by increasingly stringent noise requirements and strong competition from the United States, Europe has set
very ambitious goals for reducing aircraft noise by 2020 (Collier and Huff, 2007). For example, as shown in Figure 5-7, the Advisory Council for Aeronautical Research in Europe (ACARE) has set a goal of a 50 percent reduction in noise annoyance (relative to their 2000 counterparts) for aircraft entering into service in 2020. This is equivalent to a 10-EPNdB reduction in the day-evening-night averaged sound level from fixed-wing airplanes. Along with the noise reduction, there must be a 50 percent reduction in specific fuel consumption (again relative to engines introduced into service in 2000).
Over the years a number of ambitious programs for reducing aircraft noise and, more recently, reducing aircraft emissions have been launched in Europe. Significant investments have been made under the so-called Framework Programs in which European Union (EU) industry and researchers in many countries work together to perform well-funded, coordinated R&D. A total of 20 aircraft noise R&D initiatives were launched in Europe between 1998 and 2006 with considerable participation by industry, small and medium enterprises (SMEs), research establishments, and government agencies (see Figure 5-8).
Four of the most noteworthy programs are (1) the Silence(R) Program, (2) the Silent Aircraft Initiative (SAX-40), (3) the EnVIronmenTALly (VITAL) Friendly Aero Engine, and (4) the EU Clean Sky Initiative. Each program is described in detail in a Society of British Aerospace Companies (SBAC) Aviation and Environment Briefing Paper (SBAC, 2009).1 The summaries that follow are based on the SBAC briefing paper, a discussion at a Council of Academies of Engineering and Technical Sciences workshop in June 2008 (CAETS, 2008), and a presentation at the Workshop on Technologies for a Quieter America (Ahuja, 2008).2
The SILENCE(R) Program
SILENCE(R), the largest European aircraft noise research project ever undertaken, was a six-year program that began in 2001. Coordinated by Snecma, a French company, the €112 million program was a collaboration of 51 partners, including all major European airframe and engine manufacturers, major research institutes, and universities. The program addressed both engine noise (including jet noise, fan noise, compressor noise) (see Figure 5-9) and landing gear noise and airframe noise (see Figure 5-10).
Technologies for reducing jet noise included the ultra high bypass ratio fan; low-noise core and fan nozzles designed to improve the mixing of exhaust and bypass flows; internal and external exhaust plugs; and technologies to attenuate fan noise, including a zero-splice passive liner, active noise control technologies, and a negatively scarfed intake design to reflect fan noise away from the ground (see Figure 5-11). In flight tests the negatively scarfed fan was shown to reduce perceived noise by about 2.5 dB for an observer at a 60 degree angle to the engine (Rolls-Royce, 2005a).
Acoustical liners have traditionally been constructed from two or three pieces to facilitate manufacturing and maintenance. However, a continuous, zero-splice design greatly improved the absorption of fan noise, and the new technology is now being used in Rolls-Royce’s Trent 900 engine on the Airbus A380. The change has resulted in a 4- to 7-dB reduction in fan-tone noise on takeoff and a 2-dB reduction in fan noise on approach (Coppinger, 2007). (A similar device was demonstrated by QTD2 in the United States.)
An active noise control system was also successfully demonstrated (SBAC, 2009). The system consisted of microphones mounted in the fan duct and actuators mounted on the stator vanes. The microphones measured fan noise and sent signals to the actuators, which generated “antinoise” (sound waves that were out of phase with the sound waves generated by the fan), canceling out the fan noise.
To reduce landing gear noise, some new low-noise designs for the nose and main landing gears were investigated. Ultimately, the noise was reduced by shielding the gears from each other and aligning them in the direction of the flow. Two aligned nose landing gears were demonstrated to be as much as 3 dB quieter than two independent gears (Coppinger, 2007).
Some of the noise technologies validated in SILENCE(R) are now in production engines. Others are either undergoing further work in R&D programs by individual manufacturers or have been carried over to other projects (e.g., VITAL, described below).
Silent Aircraft Initiative (SAX-40)
The Silent Aircraft Initiative (2006) (SAX-40) was a £2.3 million three-year research project run by Cambridge University and the Massachusetts Institute of Technology—with input from industry and government. SAX-40 culminated in a revolutionary concept design for a very quiet aircraft (see Figures 5-12 and 5-13). The concept design includes an airframe and engines designed specifically for a steep, low-speed climb and a low-noise approach that reduces both the amount of noise generated and the ground area of noise exposure. Some of the noise reduction technologies are listed below:
a novel three-fan design that allows UHBR and hence lower jet noise
low fan speeds that emit less noise
extensive use of acoustic liners to absorb fan noise
engines embedded in the fuselage, with intakes above the wings, to shield much of the engine noise from the ground
variable area nozzles that allow engines to operate with low-speed, low-noise exhaust jets at takeoff and on ascent and then can be optimized for minimum fuel burn and carbon dioxide emissions at cruise
elimination of flaps and slats
low-noise fairing on the undercarriage
SBAC is the Society of British Aerospace Companies. After a merger of three companies, it is now ADS, which is Aerospace|Defence|Security.
Ahuja, K.K. 2008. Summary of the Aircraft Noise Day of the CAETS Workshop on Transportation Noise Sources in Europe, June 2–4, 2008, Southampton, United Kingdom. Presentation at the Workshop on New Technologies for a Quieter America, National Academy of Engineering, Washington, DC, June 11–12, 2008. Unpublished. A summary of the CAETS workshop is available online at http://www.noisenewsinternational.net/docs/caets-2008.pdf.
SAX-40 is predicted to achieve a reduction in noise of 25 dB based on current standards and also a reduction in fuel consumption of about 25 percent for a typical flight. Although these results are impressive, the SAX-40 is a concept design only. Further work must be done to confirm the feasibility and develop and validate the novel technologies.
EnVIronmenTALly (VITAL) Friendly Aero Engine
The VITAL program is a four-year, €90 million, EU-wide R&D program that began in January 2005 and has 53 partners. The partners, major stakeholders in the European aviation industry, include all major engine manufacturers, Airbus, and equipment makers, as well as innovative small businesses, universities, and research centers.
The goal of this Snecma-led program is to integrate the results and benefits in noise reductions of the SILENCE(R) program with the emission reductions achieved in the Affordable Near Term Low Emissions and Component vaLidator for ENvironmentally friendly Aero Engine programs. By the end of VITAL, there should be a noise reduction of 8 dB per aircraft operation and an 18 percent reduction in carbon dioxide emissions, compared to engines in service prior to 2000.
To reduce engine noise, very high bypass ratio engines with novel low-noise, low-speed fan designs are being studied. One of these designs, the contrarotating turbo fan, is shown in Figure 5-14.
VITAL also plans to demonstrate a low-pressure compressor and turbine technologies designed for low noise and weight and compatible with the novel fan designs. An overview of the VITAL project was given by Bone (2009) at a European Engine Technology Workshop in Warsaw, Poland.
EU Clean Sky Initiative
The goal of the Clean Sky Initiative is to create a radically innovative air transport system with a reduced environmental impact based on less noise and gaseous emissions and better fuel economy. The specific objective is to reduce carbon dioxide emissions by about 40 percent, nitrogen oxide emissions by 60 percent, and noise by 50 percent in time for a major fleet renewal in 2015. The approach is to conduct an overall assessment of individual technologies at
the fleet level to ensure the earliest possible deployment of research results.
The budget for Clean Sky is up to €800 million from the 7th Research Framework Program, which will be matched by funds from industry. The total budget could be as high as €1.6 billion. The research partners include all major aeronautical players in Europe, almost 100 organizations that are active in aeronautical R&D and many SMEs, research centers, and universities. The technical and geographical scope of a typical team is shown in Table 5-1.
The program is organized around six technical areas, called integrated technology demonstrators (ITDs), that will (1) perform preliminary studies, (2) select research areas, and (3) lead large-scale demonstrations either on the ground or in flight. The ITDs are “smart” fixed-wing aircraft, “green” regional aircraft, “green” rotorcraft, sustainable and “green” engine systems for “green” operations, and eco-design.
OVERALL OBJECTIVES OF ALL AERONAUTICS RESEARCH PROGRAMS
The goal of all of the programs described above is to have a “silent” aircraft in the future, that is, for the average sound pressure levels from all aircraft noise sources not to exceed sound pressure levels from other sources beyond airport boundaries during departure and arrival operations. In the next 20 years, newly designed aircraft are likely to be introduced at a rapid rate. These aircraft will likely be based on current aircraft but designed to achieve significant reductions in noise and carbon emissions. In the longer term (beyond 2025), further reductions in noise and carbon emissions are likely to require the development of entirely new aircraft and engine configurations.
TABLE 5-1 Team Members Available to Work on European Noise Reduction Programs
X-3 Team Partners
Ain Shams University
Budapest University of Technology and Economics (U.T.E.)
Czech Technical University (T.U.)
EPLF (Ecole Polytechnique Fédérale de Lausanne)
Federal University of Santa Catarina
FFT (Free Field Technologies)
Gediminas T.U. (Technical University)
Republic of Lithuania
Institute of Aviation
Instituto Superior Tecnico
National Aviation University
The enabling technologies for both phases of development are becoming apparent. It appears that one version of the futurist aircraft, based on lessons learned from SILENCE(R) and SAX-40, will mimic a hybrid wing/body (HWB) configuration. As NASA continues to work toward the introduction of a new generation of highly fuel efficient large aircraft as early as 2020, it is already planning wind tunnel tests of low-noise HWB aircraft (Figure 5-15 shows a typical HWB). Convinced that the HWB is the only way it can meet its goals, NASA is providing funding for Boeing to study improvements to the configuration to further reduce noise and improve fuel burn.
NASA’s subsonic N+2 research is now focused on a cargo version of the HWB, and if all goes well, an HWB freighter could be available by 2020, with a passenger version to follow within 10 years. According to a report in Aviation Weekly (2009), Boeing, with funding from NASA and the U.S. Air Force, will test two low-noise HWB configurations—N2A and N2B—in a wind tunnel in 2011. N2A has padded engines mounted above the aft fuselage. N2B has embedded engines and S-duct inlets for lower drag. Both designs incorporate hybrid laminar flow control to further reduce drag.
For NASA to achieve its goals of aircraft noise of 42 EPNdB cumulative below Stage 3 for the N+1 generation aircraft, considerable research will be needed in the following areas:
target next-generation single aisle
noise reduction technologies for fans, landing gears, and propulsion airframe aeroacoustics
lightweight acoustic treatment in multifunctional structures
To meet the goal of aircraft noise of 52 EPNdB cumulative below Stage 3 for the N+2 generation aircraft, considerable research will be needed in the following areas:
noise reduction from wing shielding of engines
drooped leading edge
continuous-mold line flaps
landing gear fairings
long-duct, low-drag acoustic liners
distortion-tolerant fans with active noise control
Objectives for the Air Transport System
Meeting NASA’s Goals
If NASA can meet its targets for the next three generations of aircraft, successively quieter aircraft would enter into service by 2015, 2020–2025, and 2030–2035, respectively (GAO, 2002). The likelihood of meeting these targets depends on a number of factors. First, federal funding will have to be available not only for NASA’s research but also for later-stage R&D, which NASA expects will be conducted by others.
Second, even if funding is available, the development of noise reduction technologies may be limited by concerns about global warming, because advances in noise reduction technologies could make it more difficult to achieve reductions in aircraft emissions of greenhouse gases. Third, manufacturers must be willing to integrate newly developed technologies into aircraft and engine designs. Finally, airlines must purchase new aircraft or retrofit existing aircraft with the new technologies in sufficient numbers to achieve targeted reductions in exposure to aviation noise.
If the FAA and NASA’s noise reduction goals are not met, this could impede efforts to reduce congestion by expanding the capacity of the National Airspace System (FAA, 2007).
U.S. and European Visions of the Future
In 2002 the Federal Transportation Advisory Group published Aeronautics Research and Technology for 2050: Assessing Visions and Goals, which compares civil aeronautics in Europe and the United States. Although the United States recognizes that its national well-being depends on a national transportation system with a strong aviation element, there is no explicit goal to ensure the primacy of the U.S. aeronautics industry. On the contrary, competitiveness is central to the European vision, so much so that it appears in the title of the document that defines this vision: European Aeronautics: A Vision for 2020—Meeting Society’s Needs and Winning Global Leadership (DG Energie et Transport and DG Recherche, 2001).
NASA’s Blueprint (2002) and the European Aeronautics vision both specify that the ultimate goal in terms of operational impact is that aircraft noise be reduced to the point at which it is no longer a nuisance beyond airport boundaries and that airports be free of operational restrictions related to noise. The European Aeronautics vision highlights two areas not emphasized in any U.S. visions: (1) the quality and affordability of air transportation and (2) the global primacy of the aeronautics industry (FTAG, 2002a; NRC, 2002).
According to the GAO report, by including quality and affordability issues, the European vision acknowledges the importance of structuring R&D programs to focus on providing air transportation services that users want to buy and can afford. NASA’s original goals issued in 1997 included reducing the cost of air travel by 50 percent in 20 years. However, this goal fell out of favor with Congress, which argued that meeting customer demands is an industry responsibility and not an appropriate goal for NASA’s research. Congress then reduced NASA’s aeronautics budget to eliminate research related to this goal (GAO, 2002).
The European Aeronautics document foresees the future in the following way:
In 2020, European Aeronautics is the world’s number one. Its companies are winning more than 50% shares of world markets for aircraft, engines, and equipment. The public sector plays an invaluable role in this success story. Crucially, they are coordinating a highly effective European framework for research cooperation, while funding programs that put the industry on more equal terms with its main rivals.
Future Operational Procedures
Limiting—on a yearly basis—the cumulative noise footprint in areas surrounding airports will effectively limit the capacity of the national aerospace system. Present departure and arrival procedures, which were developed when a limited range of navigational aids was available, are far from optimal from an environmental point of view. Therefore, in combination with new “silent” aircraft, the introduction of new approach, navigation, and flight management systems will make environmentally friendly procedures feasible.
FINDINGS AND RECOMMENDATIONS
A generation ago, “Higher, Farther, Faster” was the imperative for the future of air transport. Today, it is “More Affordable, Safer, Cleaner, and Quieter.” This change reflects the new emphasis on combining cost effectiveness with safety and environmental objectives. Significant investment is being made on both sides of the Atlantic to meet the demands of the market as well as the needs of the community. In the United States much of this effort has been led by NASA; in Europe significant investments have been made under the Framework Programs, in which EU industry and researchers in many countries work together in well-funded, coordinated R&D programs.
The major challenge in the development of noise reduction technology for the future is that the design requirements
for an aircraft with low emissions of chemical pollutants differ and sometimes conflict. Some design considerations are common to achieving both low noise and low pollution; for example, improved engine/aircraft aerodynamics result in lower noise as well as reduced fuel burn and thus reduced carbon dioxide emissions. Similarly, operating practices such as continuous descent approaches can reduce both pollution and noise (see SBAC Aviation and Environment Briefing Paper titled “Aircraft Traffic Management and Operations” at www.sbac.co.uk for more details). However, other design requirements are in direct conflict with each other, forcing engine and aircraft designers to make difficult compromises. One example of this is the requirement to increase the bypass ratio of the engine beyond the optimum to reduce fuel burn and reducing the fan speed to achieve a reduction in noise (particularly at takeoff), but at the cost of increasing fuel burn and chemical emissions at cruise speed. Future increases in air traffic, combined with the industry’s desire to reduce contributions to global warming, will certainly necessitate even more such difficult decisions.
On the positive side, the “silent” aircraft concept, as envisioned under the SILENCE(R) program and the Silent Aircraft Initiative (SAX-40), promises reductions in both noise and pollution. The realization of a silent aircraft may be possible with known concepts, but bringing the enabling technologies to a suitable level of readiness constitutes a significant barrier and will require a significant investment in R&D.
Even if government investment in R&D in the United States and Europe remains strong, new quiet aircraft technologies may still not be available until 2020. Even then it will take many years for current airplanes to be phased out and for the full benefits of quiet aircraft to be realized.
The impact of increased number of airports and aircraft in service is likely to exceed the mitigating effects of near-term technological advances and operational improvements, and the number of people exposed to aircraft noise is likely to increase. In addition, the sensitivity of people to noise, or at least vocal objections to it, which often depends on attitudes and socioeconomic conditions, may also increase as people become more affluent. The implication for the aviation noise research community and government agencies that must support this community is that they cannot afford to be complacent.
Recommendation 5-1: The National Aeronautics and Space Administration (NASA) should continue to fund collaborative projects by engine, airframe, and aircraft systems manufacturers. Drawing on expert knowledge in research organizations and academic institutions, research should focus on the complex interrelationships between engine and airframe and the importance of reducing each constituent noise source to reduce the overall noise signature of aircraft. These projects should develop improved prediction tools, for example, for advanced propulsion designs; acoustic scattering and propagation models, including adequate weather and terrain models; models of the effects of interactions between engine installation and airframe configuration; and benchmark measurements necessary for the development and validation of these advanced tools.
Recommendation 5-2: The Federal Aviation Administration should continue to fund the development of novel operational and air traffic management procedures to minimize noise and should work with NASA and industry to make intelligent trade-offs between competing noise mitigation and chemical pollution goals.
NEW TECHNOLOGIES FOR REDUCING NOISE FROM ROAD TRAFFIC
DEFINING THE PROBLEM
Noise from motor vehicles is undoubtedly the most pervasive noise in our society (Bowlby, 1998; Sandberg, 2001). Individually, passenger cars, trucks, buses, and motorcycles emit relatively low levels of noise compared with aircraft and rail transit at equivalent distances. However, the sheer number of these vehicles in close proximity to sensitive receptors more than offsets their lower noise levels (Donavan and Schumacher, 2007).
Based on figures from the Environmental Protection Agency (EPA), approximately four times as many people are exposed to highway noise with DNL values of more than 65 dB than are exposed to aircraft noise and almost eight times as many are exposed to highway noise than are exposed to rail noise (Waitz et al., 2007). The high numbers of impacted people are, in part, institutionalized by federal policy, which uses as a threshold a worst-hour equivalent noise level (Leq) “approaching” 67 dB for when noise abatement should be considered near new or expanding highways (23 CFR 772). Given a typical day/night urban freeway traffic distribution (Greene, 2002), the hourly level leads to DNL values of about 69 to 70 dB.
This DNL for road traffic noise is expected to “highly annoy” almost 30 percent of people exposed to it (Waitz et al., 2007) and is 15 dB higher than the 55-dB DNL criterion established by EPA as necessary to protect public health and welfare with an adequate margin of safety (EPA, 1974). A DNL value of 65 dB has also been identified by the EU as the threshold for negative health effects caused by noise.
Road traffic noise is the result of a combination of noise from several different vehicle types, each of which has its own characteristics. These include light vehicles (passenger cars, pickup trucks, sport utility vehicles, and passenger-size vans), medium trucks, heavy trucks, buses, and motorcycles. Of these, light vehicles and trucks tend to dominate traffic noise.
To support the modeling of traffic noise, an extensive database of vehicle pass-by noise emissions was collected in the mid-1990s by the Federal Highway Administration (FHWA) characterizing each vehicle type as a function of speed (see Figure 5-16). These data reveal a number of attributes. At speeds of more than 20 to 30 kilometers per hour, noise emissions from trucks and light vehicles increase rapidly with speed. At highway speeds, 80 kilometers per hour and above, noise from heavy trucks is about 10 dB higher than from light vehicles; medium trucks fall somewhere in between. For this reason, the level of traffic noise is strongly influenced by the mix of cars and trucks.
Contributions to overall noise from each vehicle type are typically considered in three categories: power train; tire/pavement interaction; and aerodynamic noise. Power train noise includes all sources associated with vehicle propulsion and strongly depends on engine speed. This source tends to dominate the overall noise emission at lower speeds at which speed has little effect on noise levels. At very high speeds, beyond legal speed limits in the United States, aerodynamic noise caused by flow over and under the vehicle becomes important. Noise emissions from this source are typically proportional to 60 times the logarithm of vehicle speed. Between these extremes, noise emissions from all three vehicle types are dominated by noise from tire/pavement interaction (as shown in Figure 5-17 for light vehicles).
Noise levels can also be visualized using acoustic beam-forming technology, as shown for a light vehicle and heavy truck cruising at about 88 kilometers per hour in Figure 5-18. In this speed range, at which tire/pavement noise is the dominant source, vehicle noise emission levels increase at about 30 to 35 dB times the logarithm of speed (Sandberg and Ejsmont, 2002).
Power train noise, the dominant noise source at low vehicle speeds, will be greatly reduced as new hybrid and plug-in hybrid vehicles are introduced into the fleet. This is an example of noise being reduced not by noise control engineers but by the introduction of a new technology by manufacturers. Urban dwellers will benefit from reduced noise levels as these vehicles are introduced, but new problems are created. The sound of a vehicle in some cases serves as a warning signal, especially to children and sight-impaired persons, and consideration is being given to adding sound when vehicles are very quiet. This, however, creates opportunities for engineers interested in the product sound quality issues discussed later in this chapter.
To address noise from motor vehicles, EPA has set a noise emission limit of 80 dB for new heavy and medium trucks, buses, and motorcycles. Although there is no federal limit for light vehicles, a sufficient number of state and local jurisdictions require that emissions be limited to a level of 80 dB, effectively making this a de facto national limit. The test procedures used to obtain these levels, which are conducted at low speed and under full-throttle acceleration, essentially deal with power train noise. Under these conditions, typically 40 percent or less of total noise emissions for light vehicles is due to tire noise (Donavan et al., 1998). As a result, these limits do little to address traffic noise under highway conditions (Sandberg, 2001). In Europe a limit on tire noise has been established for moderate speeds; however, there is no equivalent regulation in the United States.
Because there are no pertinent source emission regulations, road traffic noise is abated in the United States al-
most exclusively by erecting noise barriers or sound walls alongside freeways. All but six states have used this method of noise abatement to some degree (Polcak, 2003). The extensive use of sound walls is driven largely by FHWA policies (23 CFR 772). For new highway projects or when the capacity of an existing highway is planned, and if federal funds are being used, federal policy allows only five types of noise abatement to be considered: traffic management, such as speed limits or vehicle restrictions; alterations of the highway alignment away from sensitive receptors; barriers in the form of sound walls or earthen berms; the creation of buffer zones along the highway; and sound insulation for some public buildings. For practical reasons, barriers are almost always selected.
Once an abatement method has been selected, it must pass tests in terms of “feasibility” and “reasonableness.” Feasible in this context means: Will the barrier provide at least a 5-dB reduction in the predicted noise level for impacted residences? Reasonable has several dimensions, one of which is the cost of the barrier compared to the benefit received by the impacted residences. To implement the federal policy, each state develops its own policies and guidelines to define other parameters, such as the level at which noise abatement will be considered and the value of each impacted residence. If the state determines that a barrier or other form of noise mitigation is not feasible or reasonable, no abatement measures are required. Federal policy explicitly forbids the selection of pavement type for noise abatement, largely because of concerns that it will not maintain a given level of noise reduction performance over the life of the highway project.
Notwithstanding federal policy, because of the initial cost of highway barriers (see Chapter 7) and because they are not always feasible, both state and federal governments have an interest in investigating other possibilities for reducing road traffic noise. This interest has focused largely on source control at the tire/pavement interface. Although the two components of tire/pavement noise are inherently inseparable, when either the pavement or the tire remains constant, it appears that the greatest potential for noise reduction is in the pavement (if all pavement types are considered).
For most pavement types, the noise level from different
tires has a range of about 3 dB (see Figure 5-19). If all-terrain tires, such as those that might be used on four-wheel-drive vehicles, are included, the noise range increases by about 1 dB. Even comparing typical treaded tires to blank-tread tires produces a range of 3 dB or less, depending on the pavement. However, pass-by noise levels at 97 kilometers per hour can easily demonstrate a 10-dB range (see Figure 5-20) on different pavements. Surveys of many highway pavements using measurements made on board the vehicle near the tire/pavement interface demonstrate that the range may be even greater than 13 dB (Donavan, 2006).
According to the FHWA Tire Pavement Noise home page, eight states are investigating and testing quieter pavements (FHWA, 2009), and others are considering such programs. In the longest-running program, by Caltrans in California, 9 kilometers of older dense-grade asphalt concrete (DGAC) was overlaid with 25 millimeters of new open-grade asphalt concrete (OGAC) on a six-lane portion of I-80 near Davis, California. Initially, this produced a reduction of about 6.5 dB in traffic noise levels measured alongside the freeway, and a level 6 dB lower than was predicted by the FHWA Traffic Noise Model (TNM). After 10 years, the performance
of this pavement has deteriorated by a little more than 1.5 dB (Lodico and Reyff, 2009). In other projects, Caltrans has documented reductions of 3 to 6.5 dB by overlaying existing Portland cement concrete (PCC) with a rubberized open-graded asphalt concrete.
The largest application of quieter pavement in the United States was in the greater Phoenix area by the Arizona Department of the Transportation (ADOT). This Quiet Pavement Pilot Program (QPPP), conducted jointly with FHWA, allowed ADOT to take a 4-dB credit or reduction in the TNM predicted level attributable to a 25-millimeter overlay of rubberized asphalt friction course applied over new and existing PCC (Donavan, 2005; Reyff and Donovan, 2005). As measured at five wayside noise measurement research sites, this overlay reduced traffic noise levels by 6 to 12 dB measured 15 meters from the freeway. Using TNM predictions, this reduction was equivalent to a barrier height of 4 meters placed alongside the roadway. The QPPP also committed ADOT to continue to monitor the performance of the overlay for 10 years. After four years, the average tire/pavement noise reduction slipped from 8 to 6 dB, with most of the degradation in the first two years of the project.
The selection and modifications of PCC surface textures have also resulted in reduced tire/pavement and traffic noise levels (Donavan, 2005). As part of the Quiet Pavement Research Program completed by ADOT leading up to the QPPP, the agency investigated PCC texturing. Two types of rakelike tining textures applied perpendicular (transversely) to the direction of travel were compared to tining applied longitudinally to (with) the direction of travel. In the most extreme case, the longitudinal tining was found to reduce tire/pavement noise and the noise from light vehicle pass-bys by 7 dB compared to random transverse-tined PCC. The longitudinal tining also produced a level 5 dB lower than uniformly spaced transverse tining, which was the standard texture used by ADOT up to that time. Following the lead of California, Arizona—and several other states—have now adopted longitudinal tining as their design standard.
Modifying PCC surface texture by grinding has also been found to reduce tire/pavement and traffic noise. In an extreme example, a reduction of 10 dB was documented when Caltrans ground away an aggressively transverse bridge deck over the Sacramento River (Donavan, 2005). In less extreme cases and depending on the texture of the existing PCC, reductions of 2 to 9 dB have been reported (Donavan, 2005; Herman and Withers, 2005).
The Technology of Quieter Pavements
The examples of quieter pavements described above are primarily quieter versions of existing pavement designs that reduce road traffic noise. From research in Europe and more recently in the United States (Rasmussen et al., 2007), the two primary attributes of conventional pavement that dictate noise performance are surface texture and porosity. For asphalt concrete (AC), texture is largely determined by aggregate size; smaller sizes produce lower noise levels. Another factor is whether the texture is negative (embedded in the surface) or positive (protruding from the surface); negative texture is quieter.
For PCC, texture is more consciously designed on the surface by tining or dragging a material over the surface before it cures. As described above, the direction of tining is important; surfaces tined longitudinally are usually quieter than those tined transversely. Even in the longitudinal direction, however, tining can introduce some unwanted texture as the material is dug out of the surface. Surfaces that are ground or textured by dragging burlap over the uncured surface typically produce less textured surfaces and less resultant noise.
Porosity, typically associated with AC pavements, is dictated by the percentage of void area in the pavement. To be most effective in reducing noise, voids must be interconnected so that the structure provides some degree of acoustic absorption. Porous pavements reduce tire/pavement noise both by absorbing sound propagated over the surface and by reducing the air-pumping mechanism created by air being trapped and ejected out of tire tread void areas.
OGAC mix designs generally achieve higher pavement void ratios. However, OGAC constructions do not always ensure a porous pavement. Other AC pavement designs, such as DGAC and stone matrix asphalt, typically have very low void ratios and hence are only quieter if smaller aggregate sizes are used (Donavan, 2006).
There are also porous PCC pavements, although these are very rarely used for highways. Porous PCC can produce tire/pavement sound levels that rival quieter AC pavements, provided that the texture is managed through grinding or other techniques.
As can be inferred from the discussion above, the quietest AC pavements measured to date fall into two categories: (1) highly porous, small aggregate OGAC pavements and (2) very fine textured, small aggregate DGAC pavements. The latter is rarely used in highway construction, and the former is not regularly achieved in practice. In fact, high porosity is often achieved with larger aggregate OGAC designs that tend to actually increase texture-generated noise.
In Europe there are examples of double-layer porous pavements (Figure 5-21), with a large aggregate lower layer and a small aggregate upper layer (2 millimeters layer over a 6 millimeters layer). These pavements have produced the lowest noise levels measured in either the United States or Europe to date. As a next step, research is being conducted in Sweden and Japan with poroelastic surfaces that have air void ratios of 20 to 40 percent and are constructed of resilient materials, such as recycled rubber from used tires.
Many studies have been done in Europe on the effect of tire tread patterns on the generation of noise in tire/road interaction. Regulations have been promulgated to control the tire contribution, but these have been largely ineffective. The most important factors from a tire manufacturer’s point of view are safety and performance. Next the manufacturer considers rolling resistance and durability. Noise is also “on the list” of considerations but is not likely to be a major factor in tire design, although there is a weak correlation between noise radiation and rolling resistance (Sandberg, 2001).
The demand for quieter tires comes primarily from vehicle manufacturers. For passenger vehicles in particular, interior noise from tires is a development issue as important as other noises, such as power train and wind noise. Vehicle manufacturers work closely with tire manufacturers to tune and develop original equipment tires to meet specific targets. However, this does not necessarily result in lower levels of exterior noise, because interior noise can often be controlled by structure-borne noise paths into the vehicle.
On occasion, exterior tire noise becomes an issue for meeting the regulated pass-by noise limits determined under the ISO 362 test procedure. Although this procedure is initiated at relatively low speed (50 kilometers per hour) and performed at wide-open throttle conditions, tire noise can contribute as much as 41 percent to the overall A-weighted pass-by noise level (Donavan, 2005). For vehicles that are over the limit in the vehicle development stage, selecting or developing quieter tires becomes a consideration.
Costs and Benefits of Reducing Noise from Tire/Road Interaction
The costs and benefits of reducing traffic noise have not been extensively studied in the United States, but there have been studies in Europe. Chapter 7 in this report includes a review of EPA and FAA cost-benefit activities and a summary of European activities. Because of the extensive use of noise barriers in the United States, the costs of barriers, although highly dependent on a number of factors, are reasonably well known. Several examples of the cost of low-noise road surfaces are also given. As the reader can tell, benefits have been described in terms of decibel reductions but not yet in terms usually used by economists (i.e., hedonic pricing, willingness to pay).
FINDINGS AND RECOMMENDATION
More people in the United States are impacted by road traffic noise than by aircraft noise. At highway speeds the main source of noise emission is interaction between vehicle tires and road surfaces. Considerable progress has been made in understanding this noise source, and development work in both the United States and Europe has shown that considerable reductions in noise emissions can be achieved by changing the design of the road surface. This technology requires further development both to increase noise reductions and to study other factors, such as durability of the road surface over time.
The primary defense against road traffic noise in the United States has been noise barriers. However, barriers are expensive and provide relief to a relatively small number of residents—they are most effective within about 200 feet of the highway. To allocate costs effectively, a cost-benefit analysis of the two alternatives (quiet pavements and noise barriers) should be undertaken.
Recommendation 5-3: Current activities of the Federal Highway Administration and several states to investigate noise reduction through new pavement design should be continued and expanded to speed up development and application of new technologies. Studies on the durability of pavement surfaces are essential, because durability has a direct effect on the life-cycle costs of applying quiet pavement technology, which has the potential to reduce noise where barriers are not feasible—for example, where homes are located on a hillside overlooking a busy highway.
TYPES OF RAIL TRANSPORTATION
Rail transit systems are ideal for densely populated urban and suburban areas. Typically, these systems include three types of transit: light rail transit (LRT), rail rapid transit (RRT), and commuter railroads (CRRs).
LRT systems operate in two forms: (1) exclusive rights-of-way in tunnels, at grade level, or on aerial structures and
(2) nonexclusive rights-of-way on streets or medians. LRT consists3 tend to be one or two cars operated manually in urban areas with a maximum speed of 50 mph. Vehicles are generally powered by electricity supplied by overhead wires, and tracks are located close to buildings where people live, work, and shop. Stations are located every few blocks in downtown areas and less than a half-mile apart in outlying areas (AREMA, 2009).
RRT systems operate in fenced, exclusive rights-of-way in tunnels, at grade, or on aerial structures; no street crossings are permitted. RRT consists are typically made up of four or more cars operated either manually or automatically with a maximum speed of 80 mph. Vehicles are powered by electricity supplied by a third rail at trackside, and tracks are located close to buildings where people live, work, and shop. Stations are approximately one-half mile apart in downtown areas and 1 to 2 miles apart in outlying areas (AREMA, 2009).
CRRs, which carry passengers on intra- and intercity routes, transport people from the outer fringes and suburbs to center cities on a regular schedule and are concentrated heavily on morning and evening rush hours. Intercity railroads also carry people over longer distances with schedules distributed throughout the day and night. Passenger railroads in the United States are “conventional,” that is, they have steel wheels and steel rails and generally operate at lower speeds than trains in other parts of the world. Only one Amtrak intercity operation—some sections of the Northeast Corridor—can be called “high-speed.”
Conventional passenger railroads operate on exclusive rights-of-way, primarily at grade, but road crossings are common. Railroad consists are typically made up of locomotive-hauled trains of six or more cars operated manually at speeds up to 80 mph. Locomotives are generally powered by diesel engines, with some exceptions in electrified areas around major cities. Tracks are located on shared rights-of-way with freight railroads and are not very close to buildings where people live, work, and shop. Stations separated by many miles are located in city or town centers.
There are no high-speed passenger railroads (HSRs) in the United States—with the exception of Amtrak’s Northeast Corridor, where speeds of 150 mph are possible on limited sections of track in Massachusetts, Rhode Island, and New Jersey. According to EU Directives, the general definition of “high speed” requires operation at sustained speeds of 200 kilometers per hour (125 mph) or more. HSRs operate on exclusive, fenced, and grade-separated rights-of-way with as few road crossings as possible. Instead of traditional locomotives, HSR consists include one or more power cars driven by electrical motors that take the current from overhead wires. Because HSRs require considerable distances for acceleration and deceleration, stations are separated by many miles.
Two types of HSR are currently in operation: conventional trains with steel wheels on steel rails and magnetically levitated trains (maglevs) on special guideways. The speed record for a steel-wheel train, 357 mph, was set by a French TGV. The record for a maglev is 361 mph.
Freight railroads in the United States set the standard for efficiency and service and are the envy of the railroad world. Private companies operate the freight sector, with individual companies owning the rights-of-way over which their trains travel. Road crossings are common. Consists are made up of diesel locomotives pulling trains of freight cars, ranging from short lines and local freight trains that have one locomotive and 10 cars or less to typical coal trains in the Midwest that have six locomotives and more than 100 cars. Origins and destinations for freight trains are railroad yards or specialized facilities where cars are either unloaded for local distribution or switched to new trains for continuing transportation.
NOISE IMPACTS FOR EACH MODE OF RAIL TRANSPORT
Estimates have been made comparing rail transportation noise impacts with noise from other transportation modes. A study sponsored by EPA concluded that 4 million people in the United States were impacted by rail noise, 2 million by urban mass transit system noise and 2 million by railroad operations and yards (EPA, 1981; FRA, 2002). The current numbers are probably higher because of the expansion of public transit systems, especially LRT and CRRs, and the increase in freight operations in the past 30 years. Even more people are impacted by train horns at rail/highway grade crossings. In 2005 the Federal Railroad Administration (FRA) estimated that 6 million people were impacted by train horns (FRA, 2006). The EPA (1981) estimated that 2.5 million persons were impacted by rail noise. A reasonable estimate would be that at least 10 million people are impacted by rail transportation noise in the United States today.
With few exceptions, rail technology is associated with steel wheels rolling on steel rails.4 Noise generated by wheels and rails can be categorized into three types:
Rolling noise—roughness on the running surfaces of wheels and rails generates the ubiquitous rolling noise associated with moving trains. The condition of the running surfaces of wheels and rails ranges from smooth to rough, depending on how well they are maintained. Microscale roughness is associated with what appears to be smooth surfaces; macroscale defects, such as corrugations in the rail or skid flats on the wheel, are clearly visible. Noise is radiated by both wheel and rail vibrations, each with its own sound characteristics. Because rolling noise is a major source for rail systems, controlling it has been a focus of research, and reliable models have been developed to explain it.
Impact noise—gaps or discontinuities in the rail running surface generate a distinctive impact noise, sometimes called the “clickety-clack of the railroad track.” This noise occurs when steel wheels encounter joints (e.g., special track work associated with switches or crossings). Both wheels and rails radiate sound from the sudden application of force during the encounter.
Squeal noise—sharp curves in the track cause steel wheels to radiate a piercing squeal. The noise is generated by a stick-slip mechanism as the wheel’s running surface skids over the top of the rail on a curve. Because wheels are connected by solid axles and because the inside wheel has a shorter distance to travel than the outside wheel, one wheel has to slip in the curve. The slippage of wheel running surface and flange along the top and sides of the rail causes the wheel to resonate at its natural frequencies, some of which are in the most sensitive range of human hearing and are veryannoying.
Propulsion and Equipment
Mechanical equipment associated with propulsion, braking, and air conditioning is a major source of noise:
Traction motor noise—electric motors are used for propulsion in both diesel and electric locomotives, as well as electric transit systems. Although normally considered a minor noise source when compared with the powerful diesel engines in freight locomotives, electric motors generate considerable noise at certain rotational frequencies.
Fan noise—cooling fans, which are required for all propulsion systems, can be a major noise source, especially at low speeds.
Diesel engine noise—noise from the diesel engine emanates from both the exhaust and the engine casing. Exhaust noise generally dominates at all speeds.
Compressor noise—air compressors are key components of all trains because braking systems rely on air pressure for their operation. Compressors are electrically driven pumps that generate noise as they work to fill the reservoirs.
Air brake discharge noise—brakes on rail vehicles are released from wheels (tread brakes) or disks (disk brakes) by air pressure from the compressed air reservoirs. When air pressure is released, the brakes are engaged. Noise is generated as the air escapes the brake units. When the train comes to a complete stop, all of the air is “dumped” from the reservoir in the locomotive, causing a very loud sound.
HSTs generate noise from the sources listed above and one additional source—aerodynamic sound. The air surrounding the body of a moving train is pushed out of the way as the train moves through it. At low speeds the air moves away and closes back in without much disturbance. As the train speed increases, however, forces on the air also increase, causing turbulence at the boundary layer surface, vortex shedding at edges and appendages, and interactions with stationary objects beside the tracks. These aerodynamic phenomena generate noise levels that increase with speed faster than any of the noise from mechanical sources. At speeds of more than 150 mph, aerodynamic noise becomes the major noise source.
Interactions between rail and highway systems invite disaster unless they are controlled by warning systems. Motorists and pedestrians must be warned of approaching trains, typically by the train horn, where roads and footpaths intersect railroad tracks (DOT, 2002). Unfortunately, these intersections often occur in residential areas where the warning sounds are a continuing source of annoyance.
Horns—railroads that operate on the interstate railroad network are required to sound horns at all road crossings at grade level unless special conditions are met, in which case a waiver may be granted. Regulations specify a minimum sound level to ensure that sufficient warning is given, as defined by the Federal Railroad Administration (FRA). In a recently adopted rule, both minimum and maximum sound levels are specified (FRA, 2006). Very high sound levels are emitted from horns on top of the locomotive for one-quarter mile leading up to a grade crossing. In a residential area, one-quarter mile can cover many homes and generate a great deal of annoyance. Urban transit systems also use horns for emergencies to alert motorists and pedestrians at street crossings. Transit horns are unregulated, and a wide variety of sounds are used for warning purposes, depending on the policies and practices of the transit agency.
Bells—stationary bells are used at busy street crossings of tracks, often in connection with gates to block traffic. Bells give pedestrians and motorists advance warning that a train is approaching and that the gate arms are about to descend. Sound is emitted in all directions from a crossing before, during, and after the passage of a train, disturbing neighbors with an annoying ringing sound for significant periods of time in the course of a day.
Heavy trains rolling on tracks on bridges and elevated structures cause the structures to vibrate, resulting in rumbling sounds like thunder emanating from steel and concrete beams. In urban areas this rumbling sound can be extremely annoying, especially when it occurs many times an hour throughout the day.
Noise from rail transportation is a major concern of nearby residents. Both federal agencies that oversee rail transportation, the Federal Transit Administration and FRA, have developed noise guidelines for new projects based on the “source-path-receiver” noise model (see Figure 4-1), with abatement approaches identified for each element in the model (FRA, 2005; FTA, 2006).
Treatment of Noise Sources
Control at the source is generally the most cost-effective way to approach a noise problem. In the case of rail systems, the owners of rail systems are usually in control of the trains operating on their systems and therefore can be deemed responsible for their noise. Diagnostic techniques and applications of new technologies are focused on rail vehicles themselves and on their interaction with tracks.
Treatment of the Sound Path
Interrupting the path between the source and the receiver is a traditional way of handling a noise problem, especially in locations where the source is not under the control of the mitigating agency. Typically, noise barriers are used to screen the noise source. Although these can be effective, they are permanent structures—blank walls that are present whether or not the noise source is present. The cost of erecting a noise barrier must always be balanced against its effectiveness in reducing noise as well as the number of persons who benefit from the reduced noise level.
Treatment of the Receiver
Treatments for individual residences that receive unwanted sounds from rail systems are the least cost-effective approach. However, if sources cannot be controlled (e.g., horns at grade crossings) and path treatments are not feasible (e.g., roadway site lines prohibit noise walls), the only noise abatement approach may be to treat the windows, doors, and walls of homes exposed to the noise.
The history of efforts to control rail noise in the United States dates back to the early 1960s. The San Francisco Bay Area Rapid Transit District was the first new transit system to identify noise abatement as an important part of the design of rail systems. In the 50 years since then, transit systems, railroads, and public agencies have all addressed the problem of noise as a public concern. Methods of analyzing noise and noise control treatments have proliferated. Some new technologies for reducing rail system noise are described in this section.
The track wheel interaction noise system model, based on seminal research at Bolt Beranek and Newman, was developed in the mid-1970s and is used throughout the world to estimate the mechanisms in noise generation. In 1983, Remington provided the basis for development in Europe of a noise model to define the causes of wheel/rail noise.
Modern, high-speed computers have enabled the development of microphone arrays that can pinpoint the sources of noise on trains as they pass by on a track. This technology is especially valuable for locating sources of aerodynamic noise on HSTs.
Research has shown that roughness on the running surfaces of wheels and rails, even on the microscale, is the root cause of rolling noise. Improved wheel truing and rail grinding practices are used to reduce this source of noise. The radiation of noise from steel wheels, especially on tight curves where squeal is a problem, can be reduced by a variety of damping devices attached to the webs of the wheels. Radiated noise from rails is a major problem on tangent (straight) track. Damping devices attached to the rails have been demonstrated to reduce this noise (Remington, 1983).
Train wheels traversing tight curves emit a high-pitched squeal sound via a stick-slip phenomenon at the wheel/rail interface that vibrates the wheels at their natural frequencies, similar to the way a bow generates vibrations in a violin string. Changing the friction coefficient between the wheel and rail has been found to eliminate this phenomenon and eliminate squeal. Various fluids, including plain water, have been used to modify friction by lubrication.
Because sounds from locomotive horns are generally omnidirectional, the sounding of horns creates noise along the sides of the track ahead of a train as it approaches a highway grade crossing. Directional horns focus the sound forward where the warning is needed and eliminate much of the wayside noise. An even more effective technique is to install horns at the grade crossing pointing toward oncoming traffic. This technology eliminates the use of locomotive horns entirely, thereby eliminating horn noise in residential areas on both sides of the tracks.
Research supported by FRA provided proof-of-principle that loudspeakers placed around the exhaust stack of a diesel locomotive can be tuned to cancel out the sound of idling locomotives (Remington et al., 2005). Current research (Johnson et al., 2009) related to the design of next-generation locomotive cabs, also supported by FRA, has shown that low-frequency noise from a diesel engine can be drastically reduced by active noise cancellation techniques. Low-frequency noise is the main cause of crew fatigue on long-distance runs.
Applying damping materials to steel beams and isolating tracks are two proven ways of reducing the low-frequency rumbling noise that emanates from bridges and elevated structures as trains roll over them.
Maglev trains are supported above their guideways by magnetic forces, thereby reducing noise from rolling contact with tracks. However, because these systems are capable of traveling at speeds of more than 300 mph, they must be designed to reduce aerodynamic noise. At high speeds the noise is generated by aerodynamic forces interacting with the vehicle body. Consequently, maglevs are designed to be extremely smooth in all configurations. Designs are carefully shaped in wind tunnels to minimize air resistance and aerodynamic noise-generating mechanisms. Full-scale testing of the designs is conducted on dedicated test tracks where large microphone arrays are used to diagnose noise sources.
Special conventional HSTs can be built that are capable of speeds of more than 300 mph. These trains must also be designed to reduce aerodynamic noise.
FINDINGS AND RECOMMENDATION
It has been estimated that 10 million people are impacted by rail transportation noise. As in the case of road traffic noise, interaction between two surfaces—in this case a steel wheel and, typically, a steel rail—is a major source of rail noise. To ensure that technology to control this noise and other noise sources is applied to the design of new systems, there must be careful planning, and manufacturers must understand what noise emission levels must be to control noise immission in a particular environment. Available planning tools can be used on the local level for developing and planning new projects.
In Europe there are major problems with noise from freight wagons that the United States does not have. However, considerable progress has been made in controlling rail noise that could be applied to problems in the United States. Both Europe and Japan have considerable experience with controlling noise from high-speed trains that would be useful if new high-speed lines are developed in the United States. Noise from warning horns must also continue to be addressed. Horn blowing at highway grade crossings is the dominant noise source for many railroad corridors, not wheel/rail interaction or diesel noise. Horns are essentially omnidirectional, even though they appear to be pointing in the forward direction.
Recommendation 5-4: Planning tools available from modal agencies of the U.S. Department of Transportation, such as the Federal Railroad Administration and the Federal Transit Administration, should be used in planning new rail transportation systems, and supplemental metrics should be developed and used to estimate the effects of noise on people. The public would benefit if warning horns were made more directional; research and development related to warning horn directivity should be undertaken to better understand the effects on safety and benefits to the public.
NOISE CONTROL IN BUILDINGS
Noise in and around buildings affects 100 percent of the population all or most of the time. Although acoustics are a factor in many aspects of building design and construction, the best technologies are not always used in buildings for a variety of reasons. This section provides an overview of significant demands for the building design community, particularly for acoustics in health care facilities, multifamily dwellings, classrooms, and entertainment structures.
Because of the sheer size of the building industry, widespread educational efforts are needed and design standards must be readily understood if satisfactory acoustical conditions are to be achieved in buildings. There is a need for better information on the acoustical characteristics of building materials and for the will to implement noise control measures—sometimes at added cost. At one time the National Bureau of Standards (now the National Institute of Standards and Technology) had an active program in building acoustics, but that program no longer exists. One result of the program was publication of Quieting: A Practical Guide
to Noise Control by Berendt et al. (1976). As mentioned later in this chapter, there is an active program on building acoustics at the National Research Council of Canada, and design guidelines are being produced. Examples of currently available information are Cavanaugh and Wilkes (1998) and Harris (1994). Two documents related to noise in buildings published by the U.S. Department of Housing and Urban Development (HUD, undated; HUD, 1967) badly need updating. HUD noise policy can be found in 24 CFR 51.
To make building acoustics more accessible to the design community, two fundamental principles must be understood: (1) the source-path-receiver model (see Figure 4-1) and (2) the classification of building acoustics into six interrelated areas: exterior noise, interior room finishes, interior room noise levels, sound isolation between rooms, alarms and other electroacoustical systems, and building vibration (Cavanaugh et al., 2010). Although understanding these principles does not eliminate the need for special expertise, it does encourage the inclusion of someone with acoustical expertise on the design team.
HEALTH CARE FACILITIES
Unnecessary noise, or noise that creates an expectation in the mind, is that which hurts a patient. It is rarely the loudness of the noise, the effect upon the organ of the ear itself, which appears to affect the sick. Of one thing you can be certain, that anything which wakes a patient suddenly out of his sleep will invariably put him into a state of greater excitement, do him more serious, aye, and lasting mischief, than any continuous noise, however loud.
I have often been surprised at the thoughtlessness, (resulting in cruelty, quite unintentional), of friend or of doctor who will hold a long conversation just in the room or passage adjoining to the room of the patient, who is either every moment expecting them to come in, or who has just seen them, and knows they are talking about him…. If it is a whispered conversation in the same room, then it is absolutely cruel; for it is impossible that the patient’s attention should not be involuntarily strained to hear.
Notes on Nursing: What It Is, and What It Is Not (1860)
Florence Nightingale’s words are echoed by many, perhaps most, current hospital patients. Noise is cited as the first or second source of complaints by patients and staff in hospitals today (Anjali and Ulrich, 2007). This level of concern has led to growing interest in the acoustics of health care facilities.
In “Are Acoustical Materials a Menace in Hospitals?” a presentation by Charles Neergaard at the third meeting of the Acoustical Society of America in 1930, he referred, at least obliquely, to noise in hospitals as a design issue that requires control of reverberations (Neergaard, 1930). Neergaard’s studies of noise in hospitals were later described in another presentation, “What Can the Hospital Do About Noise?” at the twenty-sixth meeting of the Acoustical Society of America in 1941 (Neergaard, 1941). In a study of nine hospitals in Berlin, de Camp (1979) found noise levels that could interfere with patient well-being.
The use of modern instrumentation to study noise in hospitals was described as early as 1958 (Taylor, 1958). More recently, MacLeod et al. (2007) demonstrated a firm connection at Johns Hopkins hospital between treating surfaces to control sound reverberation and patient and staff satisfaction with improvements in the acoustical environment. In a recent paper, Pelton et al. (2009) demonstrate the need for noise isolation between rooms, especially when painful procedures are being performed. Unfortunately, the Institute of Medicine did not include a discussion of the adverse effects of noise in hospitals in its influential report, To Err Is Human (IOM, 2000). This may be a good subject for a future study. Almost every aspect of hospital design and operation is in dire need of noise control.
The Facility Guidelines Institute (FGI) Health Guidelines Revision Committee, in conjunction with the American Society for Healthcare Engineering (ASHE), has adopted recommendations from the Interim Sound and Vibration Design Guidelines for Hospital and Healthcare Facilities (FGI, 2009). The Interim Guidelines provide comprehensive recommendations for incorporating sound and vibration controls into the 2010 edition of the FGI/ASHE Guidelines for the Design and Construction of Health Care Facilities, which is issued every four to five years (ASHE, 2010) and is the primary reference for all aspects of hospital design in the United States.
Among the several needs is the development of a broader understanding of the implication of the use of sound-absorptive materials in nosocomial infection in health care settings. This includes not only whether sound absorptive materials pose a health risk, but also how that risk is measured and what the terms “clean-ability” and “microbial resistant” mean and how they can be measured.
Other materials that should be in design guidelines include controlling noise from air distribution systems, especially in operating rooms that require air curtains to control infection, and the use of pagers in hospitals and alarm systems. Many hospitals are considering eliminating areawide pagers in favor of silent pagers. In most hospitals today a gaggle of instrumentation is used to monitor patient health, each with its own alarm and of varying importance to patients’ health, comfort, and safety; these instruments can produce a bewildering cacophony that places the patient at risk. Inaudible warning signals and signals with easily identifiable tonal qualities are being studied, and some are already available (McNeer et al., 2007).
Finally, some less obvious methods of reducing sound in hospitals have been found to have some success. For example, dark colors in corridors seem to encourage staff and
visitors to lower their voices. Music can also be beneficial, as Florence Nightingale (1860) noted:
The effect of music upon the sick has been scarcely at all noticed. In fact, its expensiveness, as it is now, makes any general application of it quite out of the question. I will only remark here, that wind instruments, including the human voice, and stringed instruments, capable of continuous sound, have generally a beneficent effect.
As a result of the promulgation of privacy requirements applicable to all health care facilities under the Health Insurance Portability and Accountability Act (HIPAA) of 1996, hospitals are now under added scrutiny by regulators (Sykes and Tocci, 2008). The American National Standards Institute Committee S12, Noise, Working Group 44 Speech Privacy, is currently developing a draft standard on speech privacy in health care facilities in response to the HIPAA legislation as a service to both regulators and health care facilities. The need is urgent as hospitals have been cited by the Joint Commission on Accreditation of Healthcare Organizations for permitting conditions of insufficient speech privacy, even though there are no speech privacy guidelines for health care environments.
The desire for access to public transportation, close proximity to workplaces, and lifestyle changes have led to an increase in the construction of condominium units in many urban areas. However, perceived, or actual, inadequate sound isolation between units is a chief complaint of many residents. Putting aside complaints that arise from adjacent residents with incompatible lifestyles (e.g., a young couple with a new baby next to a young man who parties with his friends late into the night), there may be legitimate problems in the design and construction of these buildings.
Residential condominiums are predominantly of two types: (1) wood frame and (2) concrete deck/steel post and beam construction. Both are lightweight, and both have some advantages. Wood-frame buildings tend to be found in suburban areas where they blend in with existing wood-frame houses and have height limitations of three to four stories. Concrete and steel buildings can be taller and often fit better in urban environments. There are no hard-and-fast rules about which type of building is constructed, but concrete and steel buildings generally have a clear advantage in isolating sound.
The three most common sound isolation problems in multifamily buildings are floor/ceiling sound isolation, common-wall sound isolation, and plumbing sound isolation. Separate stud frames, doubling layers of gypsum wallboard, and glass fiber in common-wall cavities nearly always provide satisfactory sound isolation. However, in wood buildings, common walls are often used as shear walls for seismic restraint, which sometimes precludes double- or staggered-stud frames, and a single-stud frame must be constructed with plywood attached to both sides. In these cases, resilient channels do not work, so a separate sound isolation wall must be constructed on at least one side of the common wall to augment sound isolation.
Wood-frame buildings almost always need some concrete cover for structural decks to ensure the isolation of impact sounds. The thicker the concrete, the better the sound isolation, but also the more seismic restraint required in the building structure. Much work has been done with 1.5-inch concrete thickness on wood decks (Warnock and Birta, 2000), but current design conventions, motivated by cost and desires to maximize headroom, have reduced the thickness to 0.75 inches or less, which reduces the sound isolation of the floor/ceiling interface.
To meet the most common building code requirement of a minimum impact insulation class (IIC) rating of 50 requires multiple layers of gypsum wallboard on a resilient ceiling suspension system, as well as sound-absorptive material added to the floor/ceiling cavity and either carpeting or a hard finish in a resilient interlayer for the floor finish. The most common resilient suspension system is the resilient channel, of which only a single-leg type with long slots has been shown to provide reliable sound isolation (Lilly, 2002). However, newer elastomeric suspension elements used in lieu of resilient channels are now known to provide better and more consistent sound isolation than most available resilient channels (Kinetics, 2009; PAC International, 2009).
Resilient underlayers generally provide the much-needed increase in impact sound isolation when nearly all hard-finished floor systems are used in either wood or concrete buildings. Many manufacturers seized the need for resilient underlayers as an opportunity for using their products that were designed for other purposes. For example, foundation drainage and soil erosion control mats are being used as resilient underlayers in buildings. Even though this is effective in most cases, the multifamily housing industry would benefit greatly from a more systematic development of floor/ceiling assemblies and products that act together by design.
Sound isolation performances stipulated in building codes often fall short of occupants’ expectations. Although building design teams often realize this and set higher design goals, their principal sources of guidance are collections of sound transmission class (STC) and IIC test reports, which are often conducted under early standards with products of unknown performance characteristics. The result is a wide range of measured ratings for ostensibly identical sound isolation assemblies.
This problem has been largely remedied for sound isolation wall assemblies by a large body of test reports published by the National Research Council (NRC) of Canada, which has also published floor/ceiling STC and IIC ratings for a variety of assemblies. For a summary of the NRC of Canada activities related to sound transmission in buildings, see Quirt (2009). There is more variation in floor/ceiling assem-
blies than common-wall assemblies, and it would be helpful if the NRC of Canada provided data on the wide range of floor/ceiling assemblies in the built environment.
The housing industry would also benefit from the development of theory and testing to characterize improvements to the design of acoustical materials such as the dynamic stiffness of resilient underlayers and how this information can be used to evaluate the IIC rating of floor/ceiling assemblies. Recent advances in the incorporation of damping into panelized building materials such as drywall should also be rated to provide a better understanding of how damping works in building sound isolation systems; this would also encourage product development. New material concepts should also be explored, such as distributed absorbers composed of heavy lumped masses embedded in a lossy sheet binder, which has been shown to improve the sound isolation of low-frequency airborne noise, and nanogels that offer high sound absorption and partial translucency.
American National Standard S12.60, Acoustical Performance, Criteria, Design Requirements, and Guidelines for Schools is the first widely used standard for acoustical conditions in classrooms (available online at http://asastore.aip.org). This standard establishes limits for sound isolation between spaces; background sound produced by mechanical, electrical, and plumbing equipment and systems; and reverberation time.
For the most part, sound isolation and reverberation control methods and materials are well known. However, this is not true for in-room unitary HVAC (heating, ventilation, and air-conditioning) units or classroom ventilation units. Currently, sound produced by these units exceeds the American National Standards Institute (ANSI) recommended maximum sound pressure level of 35 dB(A), thus requiring the use of central air distribution using air handlers, air heating and cooling methods, and air distribution terminal units. The cost of these systems, according to manufacturers, school building owners, and designers, is considerably higher than the cost of typical classroom ventilation units.
Arguments by classroom equipment manufacturers to exclude or significantly raise permitted sound levels in order to permit the use of noisier conventional units have not persuaded the standards and education communities; the standard has not been modified. Nevertheless, the cost of school buildings and the need for flexibility are important issues. Hence, quiet design concepts for classroom ventilation units should be investigated. So far, manufacturers have had only limited success in developing units that are comparable in cost to more conventional central system equipment.
Certain manufacturers of electroacoustical products (microphones, loudspeakers, etc.) have argued that their systems can be used in place of more expensive architectural solutions to background sound, sound isolation, and reverberation. Most of these are one-way systems; the teacher speaks into a microphone and students wear hearing assistance devices. These systems generally do not work for student-to-student communication or student-to-teacher communication. The use of electroacoustical solutions to architectural acoustics problems is hotly disputed in the architectural acoustics profession. However, there may be a place for electroacoustical devices in classrooms, particularly for hearing-disabled students or those who have different learning styles.
GREEN ACOUSTICAL DESIGN
The importance of Leadership in Energy and Environmental Design (LEED) certification5 for newly constructed buildings and for the reuse/rehabilitation of existing buildings is rapidly becoming a focal point of building design. Whereas only two years ago little attention was paid to green design, including acoustics in the green design of buildings, it is now being addressed in some cases, notably in classrooms and hospitals. Up to now, acoustics has played a minor role in the LEED rating of a building, although significant contributions to LEED ratings have been possible through high-recycled-content products, such as acoustical ceilings, duct silencer fill, and the use of acoustical products produced near project sites. It is expected that the availability of green acoustical products will increase over time.
Green factors affect all building systems, which in turn affect the acoustics of a building. In a post-occupancy survey of building acoustics (see Muehleisen, 2009), it was found that bad acoustics was at the top of the list of undesirable features (acoustics, thermal comfort, air quality, lighting, etc.) for all buildings and was considered even more undesirable for green buildings. Some features of green buildings that are considered important for reasons other than acoustics include more use of natural lighting, natural ventilating systems, use of hard interior surfaces, maximum use of windows (especially when they must open), and the lack of conventional (porous) acoustical materials. All of these features tend to degrade the acoustical quality of workspaces.
Some green features include lower partial-height partitions, which may be used to extend natural light farther into an open-plan building space. However, this can reduce speech privacy between workstations. Another feature is the use of green materials that, in many cases, absorb less sound than conventional materials. However, this can result in an excessively reverberant environment and reflections from the ceiling can compromise speech privacy in open-plan offices. Natural ventilating systems are considered to be desirable in green buildings, but they can transmit noise throughout a building. The ability to open windows is considered desirable but can result in transportation noise entering a building and being transmitted through the ventilation system. Lack of conventional acoustical materials in buildings can affect
speech privacy, as mentioned above, and can also affect speech intelligibility in conference rooms.
Electronic sound masking, widely used in open- and closed-plan offices since the 1960s, is now necessary as a means of maximizing speech privacy. But, although electronic sound masking can go a long way toward ensuring acceptable speech privacy, it is usually not a sufficient solution. Green solutions to office workstation partition height and sound absorption will have to be developed. The requirement for more natural ventilation, including opening of windows, just adds to the challenge.
Razavi (2009) has reported on some acoustical improvements in green building ventilation systems, but the noise control engineering and architectural acoustics community face a major challenge in integrating good acoustical conditions into green buildings. The Green Guide for Health Care and the Green Guide for Schools establish design objectives for acoustical building characteristics, including reverberation, sound isolation, and ambient sound in building spaces (http://www.gghc.org; http://www.buildgreenschools.org). LEED points6 are added if these objectives are met using green materials and methods.
AIR DISTRIBUTION SYSTEMS
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has been the pioneer and sole standard bearer in the development of standards and estimation methods for sound produced by air distribution systems. Much to the organization’s credit, it has funded most of the research that is now the exclusive basis for estimating and evaluating sound produced by building ventilation systems.
Designers of building mechanical systems rely on the ASHRAE Guide (ASHRAE, 2007), a handbook updated every five years that covers all aspects of the design of building mechanical systems (thermal and ventilation). The ASHRAE Guide includes a chapter on design goals for sound produced by building mechanical systems. The design goals are divided into noise criteria, room criteria ratings, and A-weighted sound pressure levels. The algorithms for estimating sound in building spaces are based on work by Reynolds and Bledsoe (1989).
Little progress has been made since 1989, when the algorithms were published, despite efforts by TC 2.6 (the ASHRAE committee on sound and vibration) to improve the situation. In fact, it has been generally agreed that the previously used general method of estimating the sound power level of ventilation fans should be dropped from the Guide because of its unreliability.
It would be beneficial if industry and academia formed a partnership to study the acoustical literature, produce some additional theory and testing, and include new information in the ASHRAE Guide. This would reinforce the tools used by the mechanical engineering profession to address sound produced by new green mechanical systems, such as numerous small fans operating in parallel in lieu of a single large fan, new concepts in passive induction units that replace fan-powered terminal units, and the development of new, quiet classroom ventilators (discussed above).
The rapid increase in multifamily urban dwellings is likely to increase demand for public entertainment venues, both inside and outside buildings, particularly small venues that can nurture a sense of community. Small venues can provide opportunities for the performing arts in intimate, attractive performance spaces. Entertainment in the broadest sense includes music, cinema, and theater but also dining and parks.
The proliferation of small entertainment venues would open the door to commercial opportunities in lighting, sound system equipment, and computer-controlled software, all of which have been addressed in the marketplace. However, the proximity of entertainment venues to living spaces, and community annoyance from sound that sometimes results, can be a significant challenge. Rather than prohibiting such proximity, communities and developers should be guided by planning guidelines and codes that protect residences with only minimal compromises in performance or entertainment. Conflicts that arise between performers and the public were discussed and resolved in a decision by the U.S. Supreme Court in 1989 (Ward, 1989).
FINDINGS AND RECOMMENDATIONS
More people are probably affected by noise inside buildings—such as sound transmission in multifamily buildings, noise (and reverberation) in classrooms, noise in residences from road, rail, and air traffic, and noise in hospitals—than in any other environment. Clearly, trade associations and professional societies will play important roles in the design and construction of quieter interior spaces.
Recommendation 5-5: The acoustics and noise control communities should actively promote the inclusion of noise criteria in requirements for Leadership in Energy and Environmental Design (LEED) certification of buildings, not only to improve the noise environment but also to ensure that the acoustical environment is not degraded. Design standards (e.g., building codes) must be improved to ensure that good acoustical practices are followed in the construction of buildings.
Recommendation 5-6: The National Institutes of Health and/or the Facilities Guidelines Institute should fund the de-
velopment of improved materials for hospital environments, where traditionally used materials may harbor and promote the growth of bacteria and other harmful biological agents.
MODELING, SIMULATION, AND DATA MANAGEMENT
Perhaps the greatest change in technologies for noise reduction has occurred because of increased computational power, which has changed the way products are designed, tested, and analyzed. We now have tools for defining and manipulating structures and mechanisms, for modeling and simulation, for laboratory measurements on prototypes, and for processing and interpreting voluminous amounts of data.
Mechanism analysis programs compute the motions and forces of gears, cams and followers, cranks, and sliders that are the sources of audible energy in many products. It is said that a sewing machine contains more interesting mechanisms per dollar than any other product. The forces that these mechanisms place on their supports lead to vibrations in the product structure that are analyzed using finite element analysis. These vibrations in turn cause radiated sound, analyzed by boundary element analysis.
The computer also has just as important a role in the experimental testing that is part of the product engineering process. Accelerometer arrays allow the measurement and display of the natural modes of structural vibration, and postprocessing using modal analysis programs is used to test the validity of both the measured modes and those computed using finite element analysis. Microphone arrays allow the quantification and display of the radiation of sound from the product using software for acoustic intensity and acoustical holography.
As discussed below, the existence of these technologies does not mean that product companies are able to take advantage of them. Cost—in terms of the acquisition of the software/hardware and the commitment to the training and retention of specialized personnel—can be a problem, particularly to smaller companies. Making these new methods more affordable and available to companies is a challenge to be met.
MODELING AND SIMULATION
Traditional modeling for sound has been based on “canonical problems” representing different aspects of a sound source. Simple examples include radiation from bending waves on a plate to estimate sound from machine or equipment housing and a simple monopole source of sound to represent the radiation of sound from the unsilenced inlet of a compressor. These models can be useful aids to understanding, but they cannot deal with all aspects of design.
Some modeling procedures are oriented toward describing and analyzing mechanisms. These models, which can compute motions and forces attributable to cams, followers, and other components, enable computation of forces at supporting points, combined with structural finite element analysis, to predict vibrations of the structure. Other software packages can use information about structure and vibration to compute radiated sound. Although at one time these capabilities were available only in distinct packages, software companies today offer them as an integrated package.
In some products, airflow and heat transfer, accompanied by noise from fans and airflow, represent a different kind of interaction between mechanisms, product geometry, and sound production. Progress toward an integrated procedure is not as advanced as in the example cited above, but there is little doubt that integration will be achieved in the near future.
DATA MANAGEMENT AND ANALYSIS
Microphone arrays are now commonly used to characterize radiation from a structure. The analysis of these data can take the form of acoustic intensity or acoustical near-field holography. Data rates are typically 50 kilobits per second for 24-bit words; thus, a 10-second recording is 1.2 megabits of data and a 100-microphone array will generate 120 megabits of data per experiment. This kind of data collection can be done with modern (even ordinary) computers, but keeping track of all of these data for later processing can be a challenge. Generating the intensity and/or hologram graphs for N channels of data may require as many as N(N − 1)/2 cross spectra for these data records of a simple 10-second experiment that will be repeated many times.
Similar issues arise in collecting and processing vibration data to correlate with acoustical data. Accelerometers are the most widely used sensors, but new scanning, three-axis laser vibrometers are increasingly being used. The latter have signal processing, in the form of cross spectra between channels, “built in” to the system. A laser vibrometer channel is much more expensive than an accelerometer channel, but in some situations being able to analyze data without physical contact between the sensor and the structure or the airflow can be valuable.
MAKING PRODUCTS QUIETER AND SELLING QUIET
U.S.-made white goods (major household appliances such as refrigerators, dishwashers, and cookers), health care devices, personal care products, and other products are mostly sold on the domestic market; the export sector is relatively small. In addition, foreign competitors are moving into U.S. markets and challenging U.S. companies abroad. The sound and sound quality of products is important for market acceptance, and technology for improving sound and/or producing quieter products is important for maintaining U.S. competitiveness. (See Chapter 6.)
Although the current economic situation may slow reductions in product noise, there is little doubt that consumers have concluded that quieter products are better built and have “real quality” and not just better “sound quality.” On the other hand, the market has not favored developments that result in increased prices. Thus many consumer products become commodities with different manufacturers meeting the same price points and offering very similar products.
In some product sectors, however, consumers are willing to pay more for products with extra features or materials. For example, new countertop cookers and refrigerators with brushed steel exteriors and countertops made of granite have become status symbols and statements of achievement. Kitchens are becoming gathering places where these products are displayed. These upscale products (made both in the United States and abroad) are generally quieter and have profit margins sufficient to support extra engineering and manufacturing costs. But these products, although growing, remain a smaller part of the market. There is still a need to make the technology for better noise control more available in the manufacturing environment where cost constraints are very important.
PRODUCT SOUND QUALITY
Metrics for product sound are important for controlling noise exposure, measuring customer satisfaction, and guiding design. The acceptability of the sound of a product is influenced by user expectations, context, and signal content or information. Unfortunately, noise control professionals have labeled product sound as “product noise,” implying that any sound from a product is undesirable. Perhaps as a reaction to the notion of product sound as product noise, the most attention has been paid to metrics, such as A-weighted sound pressure level, that measure noise exposure, annoyance, and hearing impairment and reflect negative reactions to sound.
However, hearing scientists (psychologists and engineers) and product designers are aware that A-weighted sound level is an imperfect measure for predicting product sound acceptability. Recent work has focused on defining physical metrics that can select out certain sound signal features that are separately audible and are likely to be associated with positive or negative reactions to sound.
In some cases the link between metrics and design is very strong. Product engineers in the automotive industry can sit at a workstation, manipulate signals by filtering and other means, and decide that certain signal features (tones, modulation, and transients) should be changed to achieve a more desirable sound for the driver and passengers in a car. The “sound quality” programs used allow them to modify signals and process the resulting signals to determine changes in 20 to 30 physical metrics. The changes in values are an indication of how the sound should be evaluated as design changes are made. In this case the first evaluation is made by an engineer or a product designer.
Jury (listening panel) studies are a useful mechanism for designing for better sound quality. Listeners are presented with a group of sounds from real or virtual products and asked to rate them in terms of acceptability. The number of sounds, their order, the number of listeners, and the scaling of responses are all part of the experimental design. In a sense the jury is a measuring instrument, the output of which is a measure of sound quality. But to anticipate the effect of future design changes on sound quality, either the jury study must be repeated or a correlation must be found between physical metrics and the jury’s response.
Historically, acousticians have associated perceptual aspects of sound with individual physical metrics. Thus, the perception of loudness correlates well with the physical metric of “loudness.” A similar correlation between the perception of annoyance and the metric “noisiness” was developed for jet aircraft and later applied to other noise sources. But as the perceptions become more complex, involving expected, informative, and hedonistic dimensions, the correlation between any single physical metric and perception breaks down, and one is required to look for patterns of acceptability or sound quality of a product, and that correlation will be different for each product. This has been expressed as “a good lawnmower does not sound like a good washing machine.”
Physical metrics in use include tonality (the presence of tones in the signal), spectral balance (high-frequency versus low-frequency content), fluctuation strength (presence of modulation), and roughness (nonharmonic dissonant components) as well as loudness and noisiness. One sound quality program evaluates nearly 20 such physical metrics to form a profile of values to correlate with jury judgments of product sounds. Products for which such metrics profiles have been used to correlate with jury study judgments of sound quality include washing machines, dishwashers, vacuum cleaners, cookers, and room air cleaners.
The metrics profile that best correlates with good sound quality (or most acceptable) will be different for different products, but there are certain features of the sound that are undesirable for any product. Loudness, noisiness, tonality, and fluctuation strength are all undesirable if too strong. Modulation is an interesting example because it is very desirable in music as vibrato or tremolo but undesirable in a product sound. The reason seems to be that modulation captures our attention—desirable in music, undesirable in a product.
There is little cost to generating a profile of 20 or more metrics since this only requires running the same sound samples that are to be presented to a jury through the signal processing algorithm for each metric. Using a larger set of physical metrics can give some reassurance that nothing has been missed, but making sense of the profiles can be difficult. If the metrics profile for each sound is labeled with the jury evaluation for that sound, it is possible to combine the metrics into a smaller set of variables using the method of principal component analysis.
Manufacturers would like to have a single metric such as A-weighted sound level that would enable them to claim their products have better sound than their competitors and can also be used in product development. Organizations such as Consumers Union that routinely evaluate products for sound would also like such a metric. Unfortunately, the correlation between any single metric and sound quality and the outcome of jury studies has not been generally accepted by the acoustics community, so claims that one product has “better sound” than another cannot be supported by physical metrics, even though improvement in the sound quality of a particular product in a particular organization is possible. For more information on product sound quality, see Lyon (2000, 2004) and Lyon and Bowen (2007).
R&D IN SUPPORT OF QUIETER PRODUCTS
Sound is very important for some products (e.g., automobiles), and companies spend heavily in terms of facilities and personnel to make these products quiet and pleasing. But in the past 40 years or so, the price of an automobile has risen by a factor of more than 10, while the price of a dishwasher has risen by a factor of 3 to 4. One result is that while the automobile companies have developed large staffs and good facilities for sound, most appliance companies have not (with one notable exception). In typical appliance and health care products companies, engineers are “jacks of all trades,” working one day on problems of airflow or heat transfer and the next on product sound. Also, these engineers may have significant motivation to move around in a company where the path upward is through management and not technical expertise.
Another factor that affects nonautomotive producers is the pace of model changes. Appliances, health care, and personal care products go through much more frequent changes, so consumers will replace older products or choose to buy a newer product because of a desired feature. The effect of this is to compress development schedules and to limit the transfer of a new development (e.g., a quieter way to support a small motor) into the new model.
It would appear that simpler products such as a sleep apnea device should have noise issues that are simpler. But this product has a couple of brushless DC motors, a fan, an air pump, and valves, each of which produces audible sound in a device that is in someone’s bedroom at night. In addition, cost and utility constraints mean the enclosure is lightweight and stiff, a perfect construction for the efficient radiation of sound. The manufacturer probably buys the motors from a manufacturer in China and finds it impossible to convince his supplier to do the engineering to make the motor quieter.
There are other trends that are not helpful in terms of product sound. Design for manufacturing has a cachet that is attractive to industry because of lower assembly costs and easier model changes. One such method is “layering,” in which an assembly is achieved by placing components into the supporting structure in a sequence that minimizes the need for reconfiguring the assembly. When this method was applied to a popular electric mixer, its noisiness was significantly increased because of the increased tolerances in the drive train gearing inherent in this method of assembly.
The basic message is that issues of product sound are very complex and do not become simpler and easier to handle because a product is simpler and less costly. Indeed, the situation may be quite the opposite. But there are good tools for meeting the need. The question is: are they being used and, if not, why not?
TOOLS FOR QUIET PRODUCT DESIGN AND TESTING
Most companies now use computer-aided design (CAD) software to visualize their product designs and to anticipate problems of parts interference and fit before a prototype is built. These CAD programs can be interfaced with certain computer-aided engineering programs like finite element analysis for structural analysis (stiffness, resonant modes, mass distribution) or dynamic analysis for mechanism forces. But these programs (discussed above) while useful, are limited in their assistance in designing for quiet function.
For example, a fan can be analyzed using a computer fluid dynamics (CFD) program, which most likely does not reflect the actual flow environment of a typical product. Also, these programs are very expensive to run, and considerable expertise is needed to run them. Most consumer products companies will not make the investment in personnel or funds to have their products analyzed in this way. Some CFD providers will work with manufacturers on a consulting basis to provide such analyses, but the process remains expensive and the idealized calculations may not provide the information needed for design decisions.
Manufacturers are more likely to invest in experimental facilities than software for analysis for several reasons. First, the cost of experimental equipment has been coming down and its capabilities are increasing. Multichannel systems of microphones and vibration sensors (accelerometers) involving dozens of sensors are now commonplace, and the software to analyze the patterns of sound and vibration, such as acoustical near-field holography and modal analysis, is widely available. Also, experimental work is generally more relied on in product development than is analysis. The ability to keep engineers in place long enough to become proficient in the use of both hardware and software remains an issue but seems to be much less of an issue than for the analytic software.
Although the current economic situation may slow sound improvements, there seems little doubt that consumers have become convinced that quiet products are better built and have “real quality” and not just better “sound quality.” So
the issue of better sound as a marketing feature will not go away, and the need to support the industry in its attempts to meet this marketing and technical challenge will not go away. Thus the technology for better sound must be more available in the manufacturing environment where cost constraints are very important.
ACTIVE NOISE CONTROL
The most efficient and cost-effective way of reducing noise is to design equipment to produce less noise. If this strategy has been fully implemented and additional noise reduction is needed, add-on measures must be applied. Active noise control is one of these measures.
Most noise sources produce noise in a wide frequency range. Passive noise control measures (such as silencers, acoustic enclosures, wrappings, barriers, etc.) usually provide sufficient noise reduction at middle and high frequencies (approximately 200 Hz and above), and they are robust, reliable, and cost effective. However, they are ineffective at low frequencies (below about 200 Hz). At these low frequencies, active control becomes an alternative; it may be the only solution for frequencies below 100 Hz.
Noise sources such as gas turbines and large reciprocating compressors produce high levels of low-frequency noise. Almost without exception, the noise control of such sources requires a combination of both passive and active measures. The passive measures attenuate the mid and high frequencies, and the active measure attenuates the low frequencies.
There are four major active noise control strategies:
Reducing the sound radiation efficiency of the sound source by placing a secondary source (loudspeaker in an enclosure) in its immediate vicinity and driving it with an electric signal that produces the same magnitude but opposite phase fluctuating volume flow as the primary noise source. In this case the air volume pushed out of the primary source during the positive cycle fills the void generated by the receding volume of the secondary source and, conversely, the receding volume flow of the primary source is supplied by the outflow from the secondary source. This strategy, which reduces the radiation efficiency of the original source and effectively reduces the noise level at all locations, is sometimes referred to as “global” noise reduction.
Creating a limited “zone of silence” in the vicinity of the receiver (the person to be protected) by sensing the local sound pressures, driving the loudspeaker with an electric signal (located as near to the receiver as practicable) that produces a sound pressure of the same magnitude and opposite in phase as the primary signal. This is the only situation where “noise cancellation” is appropriate. This active noise control strategy, in almost all cases, is inferior to the first strategy because its effectiveness is limited to a single area. Because this strategy does not affect the sound power output of the primary source and creates a secondary source, the overall noise level is increased in locations where cancellation does not occur. A good practical application of this strategy are noise-canceling headphones, such as those manufactured by the Bose Corporation that achieve a significant reduction in sound pressure level in the ear canal.
Increasing the low-frequency sound attenuation of tuned dissipative silencers by placing actuators (loudspeakers) in the cavity behind the thin porous lining as described by Vér (2000). The sound pressure is sensed behind the porous lining by a microphone and entered into a control system that feeds the loudspeaker with a signal so that for a wide frequency band it produces (nearly) zero sound pressure immediately behind the porous liner. This condition maximizes the sound pressure gradient across the liner and consequently its ability to absorb sound. In a passive silencer this condition occurs only at single frequencies where the depth of the airspace is one-quarter the acoustic wavelength and at odd multiples of that frequency (frequency, f, and wavelength, λ, are related by f = c / λ, where c is the speed of sound).
When the noise is produced by the sound radiation of a structure exited to vibration by localized dynamic forces (such as the attachment points of the wing of an airplane to a ring frame), the most efficient way to obtain global noise reduction is to mount a shaker at the attachment point and feed it by a control system to produce nearly zero vibration (i.e., render nearly zero power input to the structure). Here, again, the noise that is attributable to the vibration force is reduced at all locations.
One early example of active control was the electronic sound absorber (Olson and May, 1953), which was a microphone, phase inverter, and loudspeaker that could be used to create a “zone of silence” around the head of a factory worker. At that time all of the circuits were analog, and phase shift through the system was critical. It was not until digital signal processing became feasible that applications began to be developed.
Active control of sound is effective only when the wavelength of the sound is long compared with the dimensions of the volume in which cancellation is desired. For example, the most successful application of the technology is in active headsets where cancellation of sound in the (small-volume) ear canal is desired. Another example is cancellation in the cabin of a turboprop commuter airplane, which requires a large number of microphones and loudspeakers and is only effective at low frequencies.
This limitation of cancellation to low frequencies also
has implications for sound perception, sound quality, and hazard to hearing. A listener may perceive the sound as lacking in low frequencies. Hence, it may sound “hissy.” The A-frequency weighting network already attenuates low-frequency sound, and therefore additional attenuation through active control may not produce a significant decrease in the A-weighted sound level. According to current standards, a small decrease in A-weighted sound level produces only a small decrease in the hazard to hearing.
APPLICATIONS OF ACTIVE CONTROL
Despite the complexity of active control and the above limitations, the technology has been applied in a number of cases. Some examples are given below. Active headsets provide noise reduction and both comfort and protection from hazardous noise for the user. The Federal Railroad Administration has demonstrated both active control in locomotive cabs and proof of principle for active control of exhaust stack noise from idling locomotives. Hansen (2005) developed an active control system to control sound propagation in the exhaust stack of a spray dryer unit in a dairy factory. Scheuren (2005) discussed a number of engineering applications of active control, including wind tunnel buffering, control of combustion burners, noise control in gas turbines, and modification of sound in the cabin of automobiles. Cancellation of the blade passage tone in a small axial flow fan was achieved by Sommerfeldt and Gee (2003) by using four small cancellation loudspeakers placed around the fan. There are a number of applications of active control in the aerospace industry; these have been described by Maier (2009). Gorman et al. (2004) produced noise reduction on the flight deck of an airplane, and Cabell et al. (2004) have shown how active control can be used to control chevrons and produce noise reduction of a jet engine exhaust. Finally, Fuller et al. (2009) reduced noise from a portable generator set by using active control.
Impediments to Commercial Development
Despite the long history of the development of active control technology and digital processing systems, there are few devices (except for active headsets) on the market today. Some of the barriers to commercial development are expense and reliability as well as the materials used and characteristics of transducers, amplifiers, and materials.
Active control systems are expensive to implement because of the required microphones (or accelerometers), loudspeakers (or force transducers), and electronic control systems. If a universal control system were to be developed, it would have to be versatile because the control algorithm will depend on the type of noise being canceled (e.g., a single-frequency tone, a tone in noise, or broadband noise). Reliability is also an issue in complex systems.
For high-intensity noise sources, high-powered amplifiers and special loudspeakers may be required. There is also the problem that the materials used for transducers (microphones, accelerometers, loudspeakers, force transducers) must, in many cases, withstand hostile environments. Examples are hot exhaust gases and turbulent flow.
There is a rich literature on active control of sound and vibration. This includes books (Hansen and Snyder, 1997; Nelson and Elliott, 1993), technical articles (Nelson and Elliott, 1993; Tichy, 1996), and conference proceedings papers (ACTIVE, 2009; Fuller, 2002).
Recommendation 5-7: Research agencies should fund university research on active noise control to address situations where the use of traditional noise-control materials is problematic or where they are not suitable for attenuating noise in the appropriate frequency range. Investigations into hybrid active-passive and adaptive-passive noise control systems and the development of low-cost microphones and loudspeakers that can be used in hostile environments should also be funded.
Active controls of sound and vibration have been under development for many years, but few products on the market have incorporated them, and many barriers must still be overcome.
In this chapter, technologies for controlling noise from a large variety of sources have been described. Clearly, aircraft noise control technology is much more advanced than technologies for addressing other noise sources, and the funds expended to reduce the noise of airplanes themselves as well as mitigation measures around airports are far greater than for other noise sources. Road traffic noise has been controlled mostly by constructing noise barriers, but work is being done on promising technologies for reducing noise generated by tire/road interaction. Technologies are available for reducing noise from rail-guided vehicles, and these will become more important as the nation develops light rail systems and high-speed trains. Technologies for the built environment will also become more important as building construction is driven by LEED certification and “green” principles.
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