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42 The entries in the following bibliography are not intended to be comprehensive, but rather to summarize interpretations of findings of some of the better known studies of the annoyance of helicopter noise. They exclude studies intended mostly to measure helicopter noise emissions and some laboratory studies of rotor noise whose findings have little direct bearing on the design of social surveys of the annoyance of helicopter noise. Although preference was given to annotating peer-reviewed studies, a number of technical reports are annotated as well. Ahuja, K., M. Benne, M. Rivamonte, R. Funk, J. Hsu, and C. Stancil, Operation Heli-STARâHelicopter Noise Annoyance Near Dekalb Peachtree Airport, Georgia Tech Research Institute Report A5146-110/2, DOT/FAA/ND-97/11, 1997. This report describes the conduct of a small scale social survey of the annoyance of a temporary increase in helicopter noise exposure near a regional airport at the time of the 1996 Olympics in Atlanta, Georgia. Only 142 interviews were completed with neighborhood residents living in households with noise exposures in the range 56 â¤ Ldn â¤ 64 dB. Table 4.5 of this report notes the following observations of the noise exposure and percentage of respondents highly annoyed by helicopter noise. The numbers of interviews in the noise exposure catego- ries shown here were as few as ten in some cases: DNL, dB % highly annoyed by helicopter noise 56 20.7 61 39.4 62 13.5 63 23.1 64 43.8 The small number of interviews led to confidence intervals for the percentages of the highly annoyed that were too wide to infer any systematic relationship between helicopter noise exposure and annoyance prevalence rates. Figure B1 plots the field observations, along with an a priori dosage-response relation- ship described by Fidell et al. (2011). Although the findings of this study are suggestive at best, they provide weak support for the hypothesis that people are (about 3 dB) less tolerant of helicopter noise than of fixed-wing aircraft noise. Atkins, C., P. Brooker, and J. Critchley, 1982 Helicopter Disturbance Study: Main Report, Civil Aviation Authority/Department of Transport/British Airports Authority, 1983. The authors report the results of a large-scale field study that intended to evaluate the attitudinal dif- ferences between fixed- and rotary-wing aircraft. Six interviewing areas were chosen with differing pro- portions of the two aircraft types, from none to exclusive. Areas near military installations were avoided in the belief that attitudes near such installations might differ from those of the general population. Each potential site received considerable pre-study qualification, including site visits to some and consultations with air traffic control and airport personnel. Exclusive helicopter exposure was found in areas where aircraft served North Sea oil platforms and helicopter passenger service. Interviews were conducted in person. Interview areas were sized to encompass cumulative exposure ranges no greater than 5 dB. (All respondents within such areas were assumed to receive the same dose.) Questionnaire completion rates across interviewing areas ranged from 61% to 82%. Continuous sound level measurements were conducted for 10 or more days in each area. The measurements were largely unattended except in areas where varying source contributions or complex flight procedures were anticipated. The survey instrument was quite lengthy, as it sought information about a large number of variables that might relate to respondent attitudes. The main questionnaire item about bother or annoyance used a four-point category scale. This question was asked only of those respondents who responded positively that they heard aircraft noise in an earlier question. An average of 30% of respondents expressed fear that APPENDIX B Annotated Bibliography
43 an overhead aircraft might crash. The attitudinal response of bother or annoyance to aircraft noise was found to be positively correlated with crash fear: âOn the whole, residents who feared a crash were more annoyed by aircraft noise than those who did not.â The authors noted that the scatter of dose-response points about their trend line exhibited greater scatter than expected by chance alone. This scatter was somewhat reduced when the respondent socio-economic group factored into the analysis of the scatter was measurably reduced. Some neighborhoods differed markedly in the age of the population; however, no age effect was found in the dose-response analysis. Austin-Bergstrom International Airport, âHelicopter Air Taxi Analysis, 2013 Formula 1,â City of Austin, Jan. 20, 2014. The city of Austin prepared a report on the helicopter operations and associated air traffic and noise associated with the year 2013 Formula 1 Gran Prix held in Austin. The report is interesting for those looking for information on the effect of a major sporting event in terms of helicopter demand and opera- tions, how preferred routes (Fly Friendly Corridors) were established and used and how coordination was handled between the airport and the event organizer. The 2013 event generated three noise complaints, while the 2012 event had generated 116 complaints. Flight track maps are provided for the 2012 event and the 2013 event, and the result of the Fly Friendly Corridors is clearly seen as confining the tracks to a desired route over less sensitive areas. Edwards, B., Psychoacoustic Testing of Modulated Blade Spacing for Main Rotors, NASA Contractor Report 2002-211651, 2002. Edwards reports the results of laboratory studies on the annoyance of noise created by a simulated 5-bladed main rotor with unevenly spaced rotors. Forty subjects assigned numeric ratings to the annoyance of vari- ous simulated blade configurations and 40 provided paired comparison ratings. Edwards concludes that âNo strong subjective differences among the predicted helicopter test sounds were found in either test . . . ,â and that A-weighted measures of helicopter rotor noise are ânot strongly indicative of subjective response.â Federal Aviation Administration, Report to Congress: Nonmilitary Helicopter Urban Noise Study, Report of the Federal Aviation Administration to the United States Congress Pursuant to Section 747 of the Wendell H. Ford Aviation Investment and Reform Act for the 21st Century (AIR-21), Washington, D.C., 2004. Human Response FAAâs review of the technical literature on the annoyance of helicopter noise in its Report to Congress cites eight (mostly laboratory) studies supporting the imposition of a blade slap âpenaltyâ on A-weighted measurements of helicopter noise, and seven suggesting that such a penalty is not justified. The FAA FIGURE B1 Field observation and a priori dosage-response relationships. Source: Fidell et al. (2011). Helicopter Annoyance (per Table 4.5, Ahuja et al.)
44 report also cites two studies of âheightened reactionâ to helicopter noiseâpresumably not associated with blade slapâby Schomer (1983) and Atkins et al. (1983). Despite the inconsistency and ambiguity of these findings, the report repeats the common assertion that âHelicopter noise may be more noticeable because of its periodic impulsive characteristic.â The report also cites âthe possible phenomena (sic) of âvirtual noiseââ (see annotation for Leverton 2014), which it suggests may be the result of attitudes and beliefs about the necessity of helicopter operations and fear of crashes. The FAA report also includes brief discussions in Sections 3.5.5 through 3.5.8 of contentions that âHelicopter noise is more annoying than fixed-wing aircraft noiseâ; that âHelicopter sounds may be more readily noticeable than other soundsâ; that attitudes such as fear of danger, beliefs about the importance of the noise source, and invasions of privacy may influence the annoyance of helicopter noise; and that rotary-wing flight capabilities such as prolonged hovering and proximity to residences may also heighten the annoyance of helicopter noise. The primary conclusion of FAAâs Report to Congress is that âmodels for characterizing the human response to helicopter noise should be pursued.â The report also includes a wide range of recommenda- tions, including some that are reflected in the current effort. For example, FAA recommends study of ânon-acoustical effects,â among which it includes vibration and rattle, and âvirtual noise,â as described informally by Leverton (2014) and systematically by Fidell et al. (2011). The report also suggests that unique characteristics of helicopter noise emissions (notably including blade slap) may heighten commu- nity annoyance with helicopters; that evaluation of noise metrics other than DNL should be undertaken; and that âoperational alternatives that mitigate noise should be examined.â The latter specifically include higher altitude flight and route planning to avoid noise-sensitive areas. Noise Impact Mitigation Despite technological improvements that have allowed aircraft noise in general to decrease, the noise generated by nonmilitary helicopters has increasingly become a point of concern, particularly in densely populated communities. To address this, FAA carried out a study that investigated the impacts of helicop- ter noise and offered recommendations as to how to reduce those effects on the quality of life, all on the behalf of the Secretary of Transportation, who was in turn operating under a mandate of the United States Congress. FAA began by reviewing the available literature relevant to determining what effects noise could have on people. The major consequences regarding health were identified in a report by the World Health Orga- nization (WHO), and these included hearing impairment; interference with speech communication or per- formance; general physiological, cardiovascular, behavioral, and mental health effects; sleep disturbance; and annoyance. FAA also took into consideration the possibility of helicopters in particular causing more annoyance resulting from impulsivity, lower frequency (which in turn can cause vibrations and rattles), or general heightened awareness of its noise. The next step taken by FAA was to seek public input through Federal Register notices as well as two public workshops in Washington, D.C. Together, these forms of outreach allowed for the compilation of comments from a variety of parties; private citizens, elected officials, civic group representatives; and the helicopter industry. In terms of the types of operations that sparked the most concern, Electronic News Gathering (ENG) and sightseeing were seen as causing the most problems with the least justification. These comments were then divided into two categories: operational and nonoperational issues. The most frequently indicated operational issues included a minimum altitude in Above Ground Level (AGL) or maximum Sound Pressure Level (SPL) for overflight and hover, the improvement of operational routes and routing design guidelines through more in-depth analysis of noise-sensitive areas, a limit on hover duration time, the retirement of the noisiest helicopters, and the marking of helicopters with vis- ible identification. Also among the operational issues and suggestions mentioned were controlling the frequency of helicopter operations (sometimes by operation type), restricting the time frame of helicopter operations, limiting the operations of helicopters on heliport or airport property, general noise abatement procedures, noise certification limit stringency, and the implementation of noise reduction technology; facing the problem at its source. In addition, a large number of issues fit into the nonoperational category. The effectiveness of voluntary noise mitigation programs that ENG flights should pool their operations more often, the exemption of public service helicopter operations from limits, and the improvement of VFR corridors and IFR access for heli- copters were all nonoperational concerns. The empowerment of local municipalities with airspace control, the inclusion of military helicopter impact, the need for a socio-acoustic study relating medical and health effects, flight tracking and noise monitoring systems, the utilization of differential Global Positioning Systems (dGPS), and a better helicopter noise metric were brought up as well.
45 Through the assistance of the Volpe Center, FAA then acquired measurements of helicopter noise at the urban center of New York City, chosen in order to quantify the noise levels in a densely populated metropolitan community. Among the locations of focus were the New Jersey Liberty State Park and the vicinity of downtown heliports. Data were collected using microphones, a video-based tracking system, camera-based photo scaling methods, and laser range-firing devices before they were analyzed. Their mea- surements matched those in the New York City Master Plan Report, as well as suggested that there should be a +2 dB adjustment for urbanization. Through modelling by the FAAâs INM, these data also showed that flying at higher altitudes reduced the noise inhabitants were exposed to; supporting the validity of the voluntary helicopter industry recommendation of operating at higher altitudes. Taking into account the information gleaned through the study, FAA came up with a set of conclu- sions and recommendations. They believed that additional socio-acoustic studies about the annoyance effects of low-frequency and impulsive noise specific to helicopters could improve the measurement and evaluation of helicopter noise and lead to better mitigation. Operational mitigation measures that do not affect safety could also be beneficial, such as higher altitudes, better route planning, the incor- poration of more precise technologies such as dGPS into helicopter approaches and departures, and advocating noise abatement among both operators and the ATC. FAA further suggested that communi- ties and operators should come up with voluntary agreements for noise mitigation and that, in general, public service helicopters such as law enforcement, firefighters, and EMS should be exempt from any restrictions relating to noise. Federal Aviation Administration (FAA), âThe New York North Shore Helicopter Route, Final Rule,â 14 CFR Part 93, U.S. Department of Transportation, Washington, D.C., July 26, 2012. FAA published a mandatory helicopter route in response to noise complaints associated with helicopter operations in the vicinity of Long Island. This rule is of interest to the reader because it contains a detailed history of the noise issue, the complex process of converting the existing voluntary route to a mandatory route, and the associated analyses. The rule includes a discussion of the benefit of route and the economic cost to operators of implementing the route. FAA identified five reasons that the situation in Long Island was unique enough to warrant a mandatory route: 1. Because Long Island is surrounded by water, it was possible to develop a route that took helicop- ters a short distance off the shoreline. Thus, the North Shore Helicopter Route does not negatively impact other communities and operators can use the route without significant additional costs. 2. There are disproportionately more multi-engine helicopters flying in Long Island than the national averages (approximately 65% versus 10%â15% nationally.) This allows for greater use of the off-shore route. 3. There are visual waypoints along the route that allow pilots to fly along the route with no additional equipment during good weather. 4. The helicopter traffic along the north shore of Long Island is largely homogenous, in that it is primarily point-to-point transit between New York City and the residential communities along the northern and eastern shores of Long Island. 5. The population corridor along the north shore of Long Island is significant and, coupled with the number of airports/heliports on the island, FAA found it reasonable to develop a route to mitigate noise impacts. In addition to the rule, FAA published the Long Island North Shore Helicopter Route Environmental Study that describes the route, number of operations, and noise levels along the route. The route was to sunset in 2 years from initiation unless FAA chose to extend the time. FAA extended the time for another 2 years in 2014. Federal Aviation Administration (FAA), Report on the Los Angeles Helicopter Noise Initiative, May 31, 2013, FAA, Washington, D.C. FAA, in response to a request from members of the California Congressional Delegation, formed the Los Angeles Helicopter Noise initiative in response to long-term citizen concerns over helicopter noise in the Los Angeles area. This report documents the work done as part of public workshops, with suggestions from private citizens, elected officials, civic group representatives, and the helicopter industry. The report states that âthere is no single remedy that can be implemented on a large-scale basis throughout the Los Angeles Basin. The airspace over Southern California is among the most congested and complex in the world. For safety reasons, helicopter traffic must be separated by altitude from higher-performing and faster-moving fixed-wing aircraft. The density of land use in the area, as well as the complexity and diversity of airspace users present challenges to identifying optimal helicopter routes that are safe, efficient, and serve noise abatement purposes.â
46 FAA grouped the approaches for addressing the noise issue that were gathered from all stakeholders into 10 groups and the report presents technical discussions relative to each approach: â¢ Ensure Safety of Helicopter Operations â¢ Establish Noise Abatement Helicopter Routes â¢ Keep Helicopters at Higher Altitudes â¢ Limit Hovering â¢ Reduce Helicopter Source Noise â¢ Reduce Flights by Electronic News Gathering (ENG) Operations â¢ Restrict Helicopter Flights â¢ Charge Fees for Helicopter Operations â¢ Improve Information on Helicopter Operations and Noise Abatement Practices â¢ Establish a Forum for Addressing Helicopter Noise Issues. FAA committed to undertaking and supporting the following actions: â¢ Evaluate existing helicopter routes to identify feasible modifications that could lessen impacts on residential areas and noise-sensitive landmarks. Any new routes intended to provide noise relief will be evaluated to avoid simply shifting noise from one residential neighborhood to another. Safety Risk Management studies would be required to ensure that helicopters can transition airspace safely and efficiently. â¢ Analyze whether helicopters could safely fly at higher altitudes in certain areas along helicopter routes and at specific identified areas of concern. Any proposed altitude changes would be required to go through an FAA Safety Risk Management Panel prior to adoption. â¢ Develop and promote effective practices for helicopter hovering and electronic news gathering. Hover times are site-specific and event-specific. FAA will continue to issue Advisory Notices to Airmen (NOTAMs) for large events and encourage helicopter operators and news organizations to employ practices that reduce noise. â¢ Conduct outreach to helicopter pilots to increase awareness of noise-sensitive areas and events. A collaborative effort among FAA, pilot groups, and communities has identified noise âhot spotsâ within the Los Angeles Basin. FAA seeks to increase pilotsâ situational awareness of noise problems on the ground and of community issues with noise. â¢ Explore a more comprehensive noise complaint system. A centralized system that provides a single repository for helicopter noise complaints in Los Angeles County may be more advantageous than current individual systems, with differing geographic and jurisdictional coverage. FAA will support the assessment of the prospects for developing such a system with homeownersâ associations and operator groups. â¢ Continue the collaborative engagement between community representatives and helicopter opera- tors, with interaction with FAA. A significant positive result of the Los Angeles Helicopter Noise Initiative is that community representatives and helicopter operators plan to meet regularly, with input from FAA, to identify specific noise-sensitive locations and helicopter operating practices that contribute to noise concerns. The group is committed to identifying measures that will provide noise relief without degrading safety or eroding business opportunities. The report goes on to describe federal efforts on the national level to reduce helicopter noise includ- ing sponsoring research on aircraft noise. FAA is currently creating a research roadmap to identify new areas of aircraft noise research, including helicopters, and will be preparing additional studies pending availability of funding and resources. FAA is also in the process of rulemaking to implement a Stage 3 helicopter noise standard in the United States. The Stage 3 helicopter noise standard will apply to all new helicopters types certified after the implementation date of the rule. As older helicopters are retired and new helicopters are purchased, the percentage of quieter Stage 3 helicopters in the U.S. fleet will increase. The report concludes that âthe most satisfactory and widely accepted noise abatement measures are those that are collectively discussed by engaged stakeholders and FAA at the local level and are supported by local consensus. As explained in the conclusion of the report, a federal regulatory process is not well suited to the helicopter noise situation in Los Angeles and could reduce com- munity and other stakeholder involvement, as well as delay other remedies for an indefinite period of timeâ and that âthe FAA recommends the engagement of a robust local process and is prepared to support such a process to pursue remedies that reduce helicopter noise, are responsive to community quality-of-life and economic interests, and are consistent with National Airspace System safety and efficiency.â Generally the report is an excellent resource for understanding how FAA manages airspace and sepa- rates helicopters from fixed-wing aircraft and gives a detailed explanation of how the airspace is structured and why helicopters are kept at altitudes below fixed-wing aircraft. Graphical presentation of helicopter
47 routes in the vicinity of specific airports illustrates the problem of separating helicopters from fixed-wing aircraft. Specific areas of concerns such as the Hollywood Bowl and Getty Center, as well as communities where helicopters are an issue are described. Fidell, S. and R. Horonjeff, âDetectability and Annoyance of Repetitive Impulse Sounds,â Proceedings of the 37th Annual Forum, American Helicopter Society, New Orleans, La., 1981, pp. 515â521. The audibility of low-frequency rotor noise is of concern not only in residential settings, but also in military applications (where the element of surprise can be mission-critical) and in airspace subject to spe- cial federal aviation regulations intended to protect natural quiet. In such applications, the main concern is prediction of the audibility of wave trains of repetitive acoustic impulses, rather than of individual impulses. Fidell and Horonjeff (1981) demonstrated that over a range of observation intervals (0.25 to 2.00 s) and repetition rates (5 Hz to 40 Hz, corresponding to the range of fundamental and harmonics of blade passage rates of present interest) the audibility of impulse wave trains is very closely predictable from the audibility of a single impulse. Under highly controlled listening conditions, participants determined when impulse wave trains of varying repetition rate and observation interval duration were just audible in white noise. The impulse was a 1000 Hz sinusoid. Test participants also listened for a single impulse randomly placed within a 500 ms observation interval. Equation 1 shows a derived relationship between the energy ratio of a wave train divided by single impulse (left side of equation) and the repetition rate and observation interval (right side). 10 log 10 log 5 log 8 log 1.5 Eq. 110 0 10 0 10 10) )( ( ) )( (â = + +E N E N RR Dri si where: Eri/N0 = signal energy to noise power density ratio of impulse wave train, Esi/N0 = signal energy to noise power density ratio of a single impulse, RR = impulse repetition rate (Hz), and D = observation interval (seconds). Figure B2 shows the resulting clustering of data points (each an average over all test subjects) when the energy ratio is plotted against repetition rate and the energy ratios have been adjusted for the duration term, 8 Log10(D) in Equation 1. The tight fit of the data points about the line (Â±0.3 dB) suggests a strong predictive relationship between repetition rate and observation interval (all for the same waveform) and the energy ratio of the wave train FIGURE B2 Observed relative signal-to-noise ratios (10 log10[En/N0] - 10 log10[Esi/N0]) of equally detectable impulse wave trains as a function of impulse repetition rate collapsed over observation interval duration by 8 log10[D].
48 and single impulse. The positive slope of about 1.5 dB per doubling of repetition rate (or 5 dB/decade) indicates that greater signal energy is needed at increasing repetition rates to maintain constant detection performance, and that these slopes are effectively independent of observation interval duration over the investigated range. Fields, J. and A. Powell, âCommunity Reactions to Helicopter Noise: Results from an Experimental Study,â Journal of the Acoustical Society of America, Vol. 82, No. 2, 1987, pp. 479â492. Noting the characteristically small numbers of helicopter overflights in many residential exposure settings, Fields and Powell focus on âthe applicability of the equivalent energy assumptions about the relative importance of noise level and number of noise events.â They devised a controlled listening field study in which the same 330 respondents were paid $40 to complete repeated interviews on the evenings of 22 days about their annoyance with late morning and early afternoon weekday helicopter noise. The study area, in close proximity to an army helicopter training base, was a strip 500 m long, con- taining 861 dwellings, in a âquiet, well-maintained, middle-class suburban areaâ with high military employment. The residents were thoroughly habituated to helicopter overflight noise. Large percentages of respondents considered helicopters âvery importantâ (64%), believed that âpilots or other authoritiesâ could not do anything to reduce helicopter noise (62%), and were not afraid that a helicopter might crash nearby (67%). The daily interview lasted only about 4 minutes and was confined to determining the times at which respondents were at home during the day, what noise sources they heard, and how annoyed they were by them. Noise measurements were limited to those made at one fixed site at the end of the exposure area, and two roving mobile sites. Fields and Powell found that respondentsâ annoyance ratings of helicopter noise increased with both number and level of noise exposure. The average annoyance scores were almost all below 4 on a 10-point scale indicating that few, if any, respondents were highly annoyed by helicopter noise in the target popula- tion. They also found only minor differences in annoyance scores for long-term exposure to more or less impulsive noise: âannoyance, in general, was slightly higherâ for exposure to more impulsive (UH-1H). Correlations between noise exposure levels and annoyance scores accounted for less than 10% of the variance in the relationship. Helicopter Association International, âFly Neighborly Guide,â Third Edition, 2009 [Online]. Available: http://www.rotor.com/Resources/NoiseAbatementTrainingCD.aspx. The Helicopter Association International (HAI) has published a number of materials that are of inter- est for managing helicopter noise. Included is a downloadable CD (see URL), which includes the âFly Neighborly Guide.â This document is of sufficient relevance to this synthesis that it is included here as Appendix C. The guide is divided into eight chapters that include general information and background materials, how helicopters generate sound, guidelines for helicopter operators on noise abatement, how to operate helicopters quietly, pilot training issues, guidelines for helicopter operators (companies) on noise abate- ment, managing public acceptance, and the âFly Neighborly ProgramâWhat Can be Achieved?â The guide is written for helicopter operators not airport operators; however, much of the material is useful and informative both to the airport operator and the general public. The training program is designed to help helicopter operators: â¢ Recognize the impact operations have on noise â¢ Understand the dangers of not addressing noise concerns â¢ Recognize the main noise generators on a helicopter â¢ Recognize which noise sources dominate each helicopter flight regime â¢ Recognize the effect that distance has on sound â¢ State the effect of temperature, humidity, and wind on sound â¢ Recognize the impact of terrain on sound â¢ Recognize the steps manufactures have taken to reduce helicopter noise â¢ Recognize new design features being examined for future noise reduction â¢ Recognize the need for noise abatement â¢ Recognize how pilot attitude factors into noise abatement â¢ Relate general guidelines for reducing helicopter noise â¢ Recognize the role of associations in establishing and enforcing noise abatement procedures. The HAI website also includes specific guidance on noise abatement procedures for specific aircraft and can be found at: http://www.rotor.com/Resources/NoiseAbatementProcedures.aspx.
49 It is interesting to note that the guidelines generally recommend that helicopters stay 1,000 ft above noise-sensitive areas. The following aircraft are included in the noise abatement procedures document and, while it appears comprehensive, it is from 2009 and should be updated as some of the guidance for some of the aircraft listed is quite general and not as aircraft-specific as that done for other aircraft in the list: Agusta A109A, A109A II, and A109C Bell Helicopter 204, 205, 212, UH-1, AH-1 Series Helicopters 206A, 206B, 206B-3, 206L, 206L-1, 206L-3, 206L-4, and 206LT 427, 429 407 430 Boeing 234 and CH-47A [Note: Some information also applies to 107 and CH-46.] Enstrom F28F and 280FX Airbus Helicopters (Eurocopter) EC120, EC130, AS350BA, AS350B2, AS350B3, AS355F1, F2, N, NP EC135T1,T2,T2+, EC135P1, P2, P2+, BK117B1, C1, C2, EC145, BO105 all models SA 365N (formally Aerospatiale) MD Helicopters MD500N, MD500D, and MD500E (formerly McDonnell Douglas) MD530F PLUS (formerly McDonnell Douglas) MD 600N (formerly McDonnell Douglas) MD 900 (formerly McDonnell Douglas) Robinson R22 R44 Rogerson Hiller UH12 and RH1100 Schweizer 300C Sikorsky S-76A/A+/A++/B/C/C+ S-92 Leverton, J., âHelicopter Noise: What Is the Problem?â Vertiflite, Vol. 60, No. 2, March/April 2014, pp. 12â15. (See also Leverton and Pike 2007, 2009.) The standard measure of adverse public reaction to transportation noise exposure is the prevalence of a consequential degree of noise-induced annoyance (FICON 1992; ISON 1996-1). Leverton (2014) asserts that vigorous adverse community reaction to helicopter noise âis a little difficult to understand because most helicopters generate less noise than the noise certification standards [ for fixed-wing aircraft]. . . .âi He infers from this observation that âthere appears to be something different about the way in which helicopters are perceived.â Leverton expands the concept of âsomething differentâ about the perception of helicopter noise into the concept of âvirtual noise.â However, he offers somewhat contradictory definitions of virtual noise. On the one hand, Leverton states that virtual noise is nonacoustic in nature. This is a plausible belief, since the annoyance of an unwanted noise intrusion is, after all, a property of an unwilling listener, not of a noise source per se. A sound level meter measures sound pressures, not annoyance. Absent a reliable dosage- response relationship, useful inferences cannot be drawn from noise levels alone about the prevalence of annoyance with transportation noise in noise-exposed communities. On the other hand, Leverton believes that even though virtual noise is not directly related âeither to the absolute level or to the character of the noise generated by helicopters,â it is nonetheless âtriggered by the direct acoustic signal.â As Leverton puts it, âVirtual noise is dependent on a wide range of inputs, but is triggered initially by any distinctive feature of the acoustic signature and, to a far lesser extent, the absolute noise level.â In other words, adverse community reaction to helicopter noise is conditioned on i This assertion tacitly assumes that compliance with ICAO standards for fixed-wing aircraft noise certification precludes vigorous adverse reactions in aircraft noise-exposed communities near airports. ICAOâs recommendations are merely consensus standards for noise levels that may not be exceeded by aircraft offered for sale in those member states that chose to adopt ICAOâs recommendations. ICAOâs noise certification standards are not intended to, and do not in fact, preclude adverse community reaction to aircraft noise exposure. Indeed, it is commonplace for communities near airports served by large fleets of ICAO-compliant aircraft to vigorously oppose continued, unmitigated airport operation and expansion.
50 two sets of factors other than the conventionally measured, A-weighted, acoustic energy of helicopter noise emissions. The first component of virtual noise is the noticeability of distinctive features of heli- copter noise emissions, such as high-speed impulsive noise (HSI), tail rotor noise (TR), main rotor/tail rotor interaction noise (TRI), and blade/vortex interaction noise (BVI). In Levertonâs view, the second component of âvirtual noiseâ is entirely nonacoustic. Levertonâs concept of virtual noise has several limitations. First, it does not consider the possibility that certain characteristics of helicopter noise could be highly annoying at levels that do not control a helicop- terâs total A-weighted noise emissions. Second, it does not clearly distinguish between the influences of acoustic and nonacoustic factors on the annoyance of helicopter noise, nor offer any quantitative guidance about the relationships between them. Third, it does not provide any operational definition or methods of quantifying the nonacoustic aspects of virtual noise. The major contribution of this publication is that it reinforces the notion that factors other than those that can be measured with a sound level meter may somehow affect the annoyance of helicopters. Magliozzi, B., F. Metzger, W. Bausch, and R. King, A Comprehensive Review of Helicopter Noise Literature, FAA-RD-75-79, 1975. The âcomprehensive reviewâ of Magliozzi et al. is more of a summary of early field measurements of helicopter noise than a critical review. It focuses more on noise emissions and noise control concerns than on the subjective effects of helicopter noise on individuals or communities, and includes little novel analy- sis. Some of the reasoning is specious, as for example, when the authors conclude âSpectrum analyses of helicopter noise show that the main rotor, tail rotor, and engine sources contribute significantly to annoy- ance.â Merely because rotating noise sources contribute conspicuously to a spectrogram does not mean that they are âsignificantâ sources of annoyance. Likewise, Magliozzi et al. simply repeat othersâ views that a need for âa new noise unitâ for measuring helicopter noise is required, and assert that a âmodification of the Day-Night Noise Level (sic) . . . shows promiseâ for assessing community acceptance of helicopter noise. Maryland Aviation Administration and Metropolitan Washington Council of Governments, âRegional Helicopter System Plan For the Maryland and Metropolitan Washington Area,â June 2005. The Maryland Aviation Administration and Metropolitan Washington Council of Governments con- tracted for a helicopter systems plan for the Washington D.C. area. The report is in the form of an aviation system plan that presents aviation forecasts and facility requirements for anticipated helicopter demand in the D.C. area. The report also includes a section on environmental impact including noise. The noise sec- tion includes background material on helicopter noise, measuring helicopter noise, and modeling helicop- ter noise. The report goes into detail and existing and preferred routes for helicopters and noise contours for those routes. The noise mitigation section discusses a number of regulatory issues that describe what is under federal authority, the limits of what the state can do, and local land use controls. The report lists the Helicopter Fly Neighborly Program, published by the Helicopter Association International, suggestions for managing helicopter noise: Routes and Airspeeds: â¢ Fly highest practical altitude â¢ Routes to heliport/helistop should fly over least populated area â¢ Follow major thoroughfares or railway beds â¢ Avoid flying over densely populated areas â¢ If flying over populated areas, use a 95 knot cruise speed â¢ Select a final approach route avoiding noise-sensitive areas. Approach and Landing: â¢ Use one of two procedures when commencing an approach: â Establish a 500 ft/minute rate of descent â Reduce airspeed and increase descent to 800 ft/minute. â¢ Hold rate of descent to less than 200 ft/minute while reducing airspeed to 57 knots â¢ Increase rate of descent to 800 ft/minute â¢ Use convenient airspeed between 50 and 80 knots and an 800 ft/minute descent on glide slope â¢ Reduce airspeed to 60 knots when approaching the flare â¢ Execute a normal flare. Takeoff â¢ Use a higher rate of climb to reduce overall area exposed to noise.
51 Miller, N., âTechnical Memorandum, Review of Studies that Address Effects of Helicopter Noise,â HMMH, Feb. 2015 (for the Town of East Hampton) [Online]. Available: http://www.htoplanning. com/docs/Town%20Documents/150203%20HMMH%20Memorandum%20re_%20Review%20 of%20Studies%20that%20Address%20Effects%20of%20Helicopter%20Noise.PDF. This technical memorandum was developed for the town of East Hampton and describes the back- ground on noise and human annoyance response to aircraft noise. The memorandum goes in to some detail showing where previous studies have shown that people are more highly annoyed by helicopter noise of the same level of other sources of noise. The report goes through current theory on noise annoyance in detail. The following summary of current noise annoyance theory is a quoted from the report: â¢ âExcept for Luz, all studies reviewed were focused on annoyance reactions and associated variables that could affect annoyance. â¢ Annoyance may be correlated with Leg, DNL. â¢ Some adjustment to SEL-based metrics may be appropriate if surveyed helicopter noise annoyance is to be predicted in terms and with metrics used to estimate the annoyance of fixed-wing aircraft noise. â¢ Annoyance reactions (e.g., complaints), although not the degree of annoyance may be triggered by noticing the event rather than by the loudness of an event. â¢ Helicopter noise may contain aspects that increase probability of noticing: â Low-frequency modulation of broad-band noise and â Slow travel speed and relatively low and constant altitudes that may lend to long audibility of approaches. Fear reactions to approaching sounds may be endemic to humans. â¢ SEL or SEL-based metrics are not likely the entire answer as far as complaints are concerned; they may depend upon noticeability as well.â The town of East Hampton has included on the town website a number of reports, studies, and pro- posals for managing helicopter noise. These may be of interest to the reader and is available here: http:// ehamptonny.gov/HtmlPages/AirportInterimNoiseAnalysis.html. Molino, J.A., Should Helicopter Noise Be Measured Differently from Other Aircraft Noise?â A Review of the Psychoacoustic Literature, NASA Contractor Report 3609, 1982. Molinoâs review describes the many differences between fixed and rotary-wing aircraft noise, but pays most attention to the impulsive nature of helicopter blade-vortex interaction noise (âblade slapâ). He reviewed 34 studies of the noisiness of helicopter blade slap, many of which were non-peer reviewed conference papers or technical reports, which yielded conflicting if not contradictory findings. His con- clusion that âthere is apparently no need to measure helicopter noise any differently from other aircraft noiseâ is based largely on the lack of consistent empirical findings about the âexcessiveâ (with respect to the annoyance of fixed-wing aircraft noise) annoyance of impulsiveness per se. The zeitgeist of the early 1980s, particularly ISOâs attempts to recommend noise metrics appropriate for certification of helicopter noise, appears to have influenced Molinoâs analyses. Several national heli- copter industries had proposed methods for assessing the annoyance of helicopter noise. Each dispropor- tionately penalized the noise emissions of competitorsâ products. AÃ©rospatiale, for example, proposed a âcorrectionâ to helicopter noise that heavily penalized even slight short-term temporal variation in noise levels. âCorrectionsâ proposed by British sources, on the other hand, heavily penalized tonal components of helicopter noise, such as those produced by Sud Aviationâs (subsequently AÃ©rospatiale, Eurocopter, and now Airbus Helicopters) high-speed, ducted fan (âFenestronâ) tail rotor. Molinoâs report goes into considerable detail about the acoustic characteristics of helicopter noise emis- sions, and into variability in noise emissions associated with various helicopter types and operating condi- tions. He notes that relationships between operating mode, engine power, and airspeed in helicopters are not as straightforward as they are for fixed-wing aircraft. For example, Molino observes that unlike fixed-wing aircraft, âhelicopters generally produce a minimum sound level at some intermediate airspeed, with higher sound levels at lower and higher airspeeds.â He also observes that âfor the same airspeed, helicopters often exhibit different sound spectra for approach versus level flight.â The psychoacoustic research reviewed by Molino consists mostly of 1970s-era studies, with a smatter- ing of earlier and later studies. A major part of Molinoâs review addresses the methodological advantages and disadvantages of varying forms of signal presentation, listening contexts, and annoyance rating scales for controlled listening tests. He ultimately speculates (1) that âthe source of . . . [discrepancies among empirical findings] . . . may lie in the methodologies and approaches selected by the experimenters,â rather than in bona fide differences in the annoyance of helicopter noise; and (2) that inadequate experimen- tal treatment of the complexity of helicopter noise may obscure the annoyance of helicopter noise. For
52 example, Molino notes âThe presence of blade slap, in and of itself recognized as contributing to increased annoyance, produces changes in other acoustic parameters that can compensate for or account for the increased annoyance cause by the presence of blade slap.â Molino concludes from the contradictory and inconclusive nature of the findings of laboratory studies about the annoyance of helicopter noise that âthere is apparently no need to measure helicopter noise any differently from other aircraft noise.â The logic and universality of Molinoâs conclusion are open to ques- tion, given the limited nature of comparisons that Molino describes among the findings of different forms of laboratory studies of the annoyance of helicopter noise. Another major limitation of Molinoâs review is that he confines his review to the direct annoyance of airborne acoustic energy produced by helicopters, and ignores the potential contributions to annoyance of secondary emissions (audible rattle and sensible vibration) produced by helicopter flight operations inside residences. To the extent that any excess annoyance of helicopter noise is related to the annoyance of sec- ondary emissions, Molinoâs conclusion about the sufficiency of A-weighted measurements is premature. More, S.R., Aircraft Noise Characteristics and Metrics, Purdue University Doctoral Thesis and Report No. PARTNER-COE-2011-004, West Lafayette, Ind., 2011. Moreâs thesis reports the findings of laboratory studies of second-order effects, such as âsharpnessâ (spectral balance of low- and high-frequency energy), tonality (presence of prominent tones), slow fluc- tuations in loudness (fluctuation âstrengthâ), and âroughnessâ (rapid fluctuations in loudness) on abso- lute judgments of the annoyance of single-event, fixed-wing aircraft noise presentations. (The reported work does not address the effects of rattle and vibration or the annoyance of cumulative noise exposure.) Although Moreâs interests did not specifically extend to the annoyance of helicopter noise, some of the factors that he studied are more characteristic of complex rotary-wing noise emissions than those of sim- pler, broadband fixed-wing aircraft. The laboratory judgments failed to demonstrate any clear contributions of sharpness, roughness, and fluctuation strength to judgments of the annoyance of aircraft noise. Loudness remained the major deter- minant of judged annoyance, with a clear contribution of tonality. Munch, C. and R. King, Community Acceptance of Helicopter Noise: Criteria and Application, National Aeronautics and Space Administration, NASA-CR-132430, 1974. Because assumptions made by the authors have not withstood the passage of time, the reasoning in this 40-year old studyâdating from the era prior to FICONâs recognition of the prevalence of a consequential degree of annoyance as a preferred measure of adverse impact of transportation noiseâis largely irrel- evant to modern analyses of the effects of helicopter noise exposure on communities. For example, the authors loosely define âcommunity noise acceptance criteriaâ in terms of âa noise exposure acceptable to the average member of the community.â Further, they interpret EPAâs recommen- dation of a DNL of 60 dB as a level consistent with ârequirements for human compatibility in the areas of annoyance, speech interference, and hearing damage riskâ as a basis for regulating aircraft noise. They also assume that A-weighted noise levels 2 dB lower than ambient levels are completely acceptable and that ambient noise levels in inhabited places will decrease âover the years as a result of stricter controls on noise sources other than aircraft.â Neither assumption is correct. The audibility of aircraft noise cannot be reliably predicted from A-weighted noise levels, and Schomer et al. (2011) has shown that the slope of the relationship between population density and cumulative noise exposure has remained unchanged for 40-odd years. The authors also report an informal study of the noticeability of blade slap, from which they estimate that notice of blade slap occurs at a crest factor of 13 dB. This figure is little greater than the crest factor of many urban ambient noise environments. Although the authors repeatedly emphasize that understanding of the annoyance of blade slap is âsketchy,â âinadequate,â âvery limited,â âinconsistent,â etc., they none- theless conclude that a âpenaltyâ is required to account for the annoyance of repetitive impulsive aircraft noise. The magnitude of the recommended penalty in units of Perceived Noise Level is 4 to 6 dB, or 8 to 13 dB in A-weighted units. Namba, S., S. Kuwana, and M. Koyasu, âThe Measurement of Temporal Stream by Hearing by Continuous JudgmentsâIn the Case of the Evaluation of Helicopter Noise,â Journal of the Acousti- cal Society of Japan, Vol. 14, No. 5, 1993. Namba et al. suggest that the practice of calculating equivalent energy metrics for time-varying envi- ronmental noises (such as those produced in the course of helicopter flight operations) can misestimate
53 their annoyance because they do not take into consideration the temporal context of noise intrusions.ii They propose instead a method of continuous judgment, such that the annoyance of helicopter and other â. . . fluctuating sounds [can be measured] by pressing a key on a response box . . . ,â in real time. The authors found marked differences in the momentary annoyance of helicopter takeoffs, overflights, and landings. Ollerhead, J., Laboratory Studies of Scales for Measuring Helicopter Noise, NASA Contractor Report 3610, 1982. Ollerhead solicited absolute judgments from scores of test subjects of the annoyance of tape-recorded helicopter sounds presented both over headphones and by loudspeaker in a series of laboratory studies. A set of preliminary investigations was conducted to pilot-test the annoyance rating and signal presentation methods. A set of âmainâ tests followed, in which six undergraduates at a time rated the annoyance of the sounds of 89 helicopters (mostly level flyovers) and 30 fixed-wing aircraft heard through headphones. The headphone presentation results were generally replicated in subsequent free-field testing at NASA LaRC. Ollerhead concludes that tone-corrected effective (that is, duration-adjusted) Perceived Noise Level predicts the annoyance of helicopter noise better than does A-weighted sound pressure level, and that any putative effects of impulsiveness per se may be equally attributed to increases in helicopter noise level and duration. Ollerhead, J.B., Rotorcraft Noise, Loughborough University of Technology, Leicestershire, England, 1985. Ollerheadâs review addresses âsubjective impactâ (individual and community response to exposure to helicopter noise), mechanisms of helicopter noise generation, and potential helicopter noise control mea- sures, with greater emphasis accorded to the latter two topics.iii Like most other review articles, Ollerheadâs dwells at length on differences between rotary- and fixed-wing noise emissions. Among other salient differences, Ollerhead notes that unlike fixed-wing aircraft, âhelicopters are usually confined to low alti- tudes,â and that âmany helicopters radiate maximum noise in a forward direction,â so that âan approaching helicopter can often be heard for as long as five minutes.â Ollerheadâs review of subjective impacts of helicopter noise consists in large part of re-statements attributed to Molino (1982). Like Molino, Ollerhead draws attention to contradictory findings and to apparent discrepancies between the findings of field studies and laboratory studies. Ollerhead notes, for example, that his own 1971 finding âthat the very long attention-arresting sound of an approaching heli- copter did not affect annoyance responses in the laboratory experimentsâ conflicts with âhearsay evidence of complainants near heliports that [duration of audibility] may be a particular source of aggravation to people at home.â Pater, L. and R. Yousefi, âHangars as Noise Barriers for Helicopter Noise,â National Conference on Noise Control Engineering, Williamsburg, Va., NOISE-CON, 1993, pp. 241â246. In their effort to reduce the noise effects of military activities on communities in the vicinity of army airfields, the U.S. Army Construction Engineering Research Laboratories studied the effectiveness of the expedient use of potential large noise barriers such as hangars to shield the community from engine run-up or in ground effect hover noise. A UH-1H helicopter was measured at two locations, one with and one without a hangar separating it from the microphones, and in six orientations relative to a marker. Shielded and unshielded microphones were placed at various distances and heights to keep track of sound level using the Leq sound metric, and noise samples were recorded as well. In the hangar measurements, the helicopter was 49 ft from the near- est side of the hangar. Both flat and A-weighting were used to analyze the data to take into account both possible indoor and outdoor experiences of helicopter noise. The data show that helicopter noise levels were clearly higher than the ambient noise and this allows the experimenters to measure the actual insertion loss, in addi- tion to theoretical calculations. These measurements and calculations were compared with one another. Researchers concluded that for helicopter operations close to or on the ground, large physical barriers such as buildings can help mitigate noise. ii The influence of meaning on annoyance judgments was also demonstrated by Fidell et al. (2002b), who solicited annoyance judgments under highly controlled listening conditions to sounds with identical duration and power spectra, but differing phase spectra. Large differences were documented between meaningful sounds and the same sounds with scrambled phase spectra. iii For example, Ollerheadâs conclusions include no mention of the subjective impact of helicopter noise.
54 Pater, L., R. Yousefi, and J. Burnett, âMeasurement of the Effect of Helicopter Landing Approach on Community Noise Level,â 24th International Congress and Exposition on Noise Control Engi- neering, Newport Beach, Calif., INTER-NOISE, 1995, pp. 767â770. As more measures are implemented to reduce the noise impact of helicopter landings on nearby com- munities, it has become a subject of debate what the most effective procedures are. Pater et al. decided to measure whether noise-reducing benefits were achieved when a UH-H1 helicopter followed Fly Neigh- borly landing procedures. In this study, aircraft pilots were asked to fly what is essentially a missed approach in accordance with the Fly Neighborly descent, beginning at an altitude of 300 m. The pilot tried to maintain constant airspeed and glide slope with help of visual approach slope indicator (VASI), until 15 m, where they pulled out of landing and turned off noise instruments. A range of landing approach speeds and glide slopes were used, these being 75â185 km per hour and 2â10 degrees, respectively. Measurements were taken from both on aboard the helicopter and on the ground, the latter being from 16 sound measurement sites over 4 square kilometers of flat and sandy land that had the landing site as its center. Researchers kept track of data in ASEL, SEL, MXA, and MX noise metrics, as well as recorded meteorological data, specifically humidity, temperature, barometric pressure, wind speed, and wind direction. For purposes of comparison, normalized data from each of the 16 sites were averaged to get a single num- ber descriptor for each helicopter run. This paper considered unweighted noise data because A-weighting does not sufficiently factor in low frequency, and based on this noise metric a steeper glide slopes lowers noise level. The data were A-weighted as well, to predict community response, and the results suggested that the combination of a speed of 120 km/h or slower and a 2â5 degree glide slope made for the quiet- est landings. However, measurements made on the helicopter found that 3 degree glide slope landings were the loudest, suggesting that data collected from on board are not indicative of the actual community exposure. As described in the paper, it was very difficult for the pilots to fly these procedures for landing. Another finding was that blade slap occurred intermittently during most flights as a result of the blade-vortex interaction (BVI) caused by the pilot holding glide slope constant when there were thermal updrafts, ulti- mately resulting in an increase of the noise over standard arrivals. Although perhaps not widely known, it is this researcherâs understanding that the original blade-slap regions and quiet regions were developed by descents from say 2,500â2,000 ft AGL and that the measurements were primarily internal to the aircraft, casting doubt onto the effectiveness of Fly Neighborly procedures. Unfortunately, this InterNoise paper is the only publication I know of that was made about these mea- surements. To my knowledge, they have never been replicated and, also, this was the first real test of the Fly Neighborly procedure. Basically what they found was only if the pilot went slow enough and shallow enough, would you get any quieting, but the basic procedure did not work. Regardless of what the actual best practices may be, the total 7 dB data spread reveals what a big difference varying landing procedures could make in terms of noise reduction. Patterson, J., B. Mozo, P. Schomer, and R. Camp, Subjective Ratings of Annoyance Produced by Rotary-Wing Aircraft Noise, USAARL Report No. 77-12, Bioacoustics Division, U.S. Army Aero- medical Research Laboratory, Fort Rucker, Ala., May 1977. Patterson et al. describe an outdoor noisiness magnitude estimation test in which a panel of 25 audio- metrically screened participants rated the sounds of actual rotary-wing aircraft passbys relative to that of a fixed-wing C-47 propeller driven aircraft. The goals of the study were fourfold with regard to determining a metric that would best predict subjective annoyance: (1) Which spectral weighting function(s) are most appropriate?, (2) What type of temporal integration should be used?, (3) Is an impulsive blade slap cor- rection factor necessary?, and (4) Do present fixed-wing annoyance predictors underestimate annoyance from rotary-wind aircraft? To evoke differing spectral and temporal characteristics the listening test involved nine different rotary- wing aircraft each flying six different flight maneuvers: (1) level flyover, (2) nap-of-the-earth, (3) ascent, (4) decent, (5) left turn, and (6) right turn. During each passby the sound pressure level signature was FM-recorded on magnetic tape for subsequent analysis into one-third octave bands. Observers recorded their noisiness rating relative to the C-47 at the end of each passby. In the subsequent analysis five broadband frequency-weighted metrics were considered, A-weighted sound level, B-weighted sound level, C-weighted sound level, and tone-corrected perceived noise level (per FAR Part 36). For each, four different temporal treatments were examined: the maximum sound level, the peak sound level, the average sound level over the passby, and the time-integrated level over the passby. The Pearson product moment correlations (r), relating noisiness to all frequency weightings
55 and temporal considerations, are shown in Figure B3. This figure plots the correlations in four groups of differing temporal considerations. Within each group the four different frequency weightings are shown. The figure reveals that the A-weighted and D-weighted sound levels and the tone-corrected perceived noise level all performed equally well as noisiness predictors regardless of the time integration method employed. The dashed horizontal line plots the average value of all the coefficients for these metrics (0.81). In addition, the figure shows that B-weighted and C-weighted sound levels performed demonstrably more poorly. It is interesting to note, however, that the maximum level was a better predictor of annoyance for both the C-weighted sound level and tone-corrected perceived noise level than was a temporal integration of these measures. These correlations notwithstanding, the authors found that on average the rotary-wing aircraft were rated an equivalent of 2 dB more annoying than the fixed-wing C-47. This difference repre- sents only about one-third of the scatter in sound level observed for any given relative annoyance rating, but is probably significantly different from zero (not determined by the authors). The authors note that the similar performance of the A, D, and tone-corrected metrics was largely the result of the high correlation between the metrics themselves. The correlations (r) were largely indepen- dent of temporal consideration and ranged from 0.91 to 0.98. The authors thus concluded that âThe high correlation among these predictors of annoyance makes any attempt to show the superiority of one over another unlikely to succeed.â The authors also explored two measures of impulsivity to determine whether either improved the cor- relation. These were (1) the crest factor (peak minus rms), and (2) a novel adjunct to crest factor that mea- sured the rms level between blade slaps and subtracted this value from the peak level. No improvement was found using crest factor. However, some modest improvement was found using the second method, but the authors concluded the method was too cumbersome to be used in practice. Powell conducted two controlled-listening studies in which 91 test participants located both indoors and outdoors judged the noisiness of 72 helicopter and propeller-driven, fixed-wing aircraft flybys. After noting the âvery diverseâ character of helicopter noise, Powell comments on the inconclusiveness of studies intended to ascertain whether an impulsiveness correction is useful for predicting the noisiness of helicopter noise. One purpose of the current investigation was to determine whether highly impulsive helicopter overflights are judged to be noisier than less impulsive helicopter overflights at constant EPNL values. The other purpose was to determine the utility of ISOâs then recent suggestion of an impulsiveness correction to EPNL. Powellâs findings were counterintuitive, and in direct contrast to the common assumption (cf. Sternfeld and Doyle 1978) that the impulsiveness of helicopter noise accounts for much of its annoyance. Powell found that âat equal effective perceived noise levels (EPNL), the more impulsive helicopter was judged less noisy than the less impulsive helicopter.â Powell also found that ISOâs proposed impulsiveness correction, based on measurements of A-weighted crest factors, failed to improve the ability of EPNL to predict helicopter noisiness judgments. Powell concluded that â. . . some characteristic [of helicopter FIGURE B3 Subjective noisiness correlations with four frequency weighting functions and four temporal integration measures. Source: Powell (1981).
56 noise] related to impulsiveness is perceivable by subjects but is not accounted for by either EPNL or [ISOâs] proposed impulsiveness correction.â Schomer, P.D., B.D. Hoover, and L.R. Wagner, Human Response to Helicopter Noise: A Test of A-weighting, USACERL Technical Report N-91/13, U.S. Army Corps of Engineers, 1991; and Schomer, P.D. and R.D. Neathammer, âThe Role of Helicopter Noise-Induced Vibration and Rattle in Human Response,â Journal of the Acoustical Society of America, Vol. 81, No. 4, 1987, pp. 966â976. Schomer et al. (1991) describe this study as a continuation of a field study (âjury testâ) conducted by Schomer and Neathammer (1987). The former study solicited individual paired-comparison judgments of the annoyance of helicopter flybys with respect to a single broadband noise from groups of paid test participants seated in a house, a tent, and a mobile home. Schomer and Neathammer (1987) concluded that A-weighted measurements of helicopter flyby noise did not adequately predict differences in annoyance between the flyby noise and the control signal, and that the level of secondary emissions (helicopter- induced rattle) in the listening environment influenced the annoyance judgments. The annoyance judg- ments were solicited in a field setting rather than in a laboratory because âthe very low-frequency sounds, the rattles, and the vibrations characteristic of helicopter noise would be too hard to simulate realistically in a laboratory. . . .â Neither A-weighted nor C-weighted measurements of helicopter noise were able to predict offsets between objective measurements of sound levels produced by helicopter flybys and the comparison sounds when heard at subjectively equally annoying levels. The differences between A-weighted and C-weighted levels of helicopters and equally annoying broadband noise varied from 10 dB (for helicopters with two bladed main rotors) to 8 dB for helicopters with greater numbers of rotor blades. In other words, Schomer et al. (1987, 1991) found that that exposure to helicopter noise depended in part on its impulsive characteristics (blade passage frequency and/or repetition rate) and the rattle induced by repetitive impulsive signals in residences. This finding directly contradicts Molinoâs interpretation a decade earlier of the (largely laboratory-based) research findings that âthere is apparently no need to measure helicopter noise any differently from other aircraft noise.â Note, however, that the Schomer et al. studies included no direct comparisons of the annoyance of exposure to rotary- and fixed-wing aircraft sounds. Because these studies included no direct empirical comparisons of helicopter noise with fixed- wing aircraft noise, they do not clarify whether the observed âexcessâ (that is, greater than A-weighted) annoyance of helicopter noise also holds with respect to fixed-wing aircraft noise.iv Schomer, P. and L. Wagner, âOn the Contribution of Noticeability of Environmental Sounds to Noise Annoyance,â Noise Control Engineering Journal, Vol. 44, No. 6, 1995a, pp. 294â305. Schomer and Wagner provided modest numbers of paid volunteers at three locations with portable (palm-top) computers to self-report prompt annoyance judgments for naturally occurring outdoor noises that they noticed while at home. The computers administered a brief questionnaire that asked respondents to identify the source of the annoying sound (e.g., rotary- or fixed-wing aircraft) and their degree of annoy- ance with it). Unattended outdoor noise measurements were made at locations near the test participantsâ homes. The authors analyzed both the per-event annoyance ratings and the rate of notice of noise events. They found only minor differences in the per-event annoyance ratings of fixed- and rotary-wing aircraft noise of comparable A-weighted sound exposure levels. For some of the test participants, the annoyance ratings varied little with sound exposure levels. Mere detection of noise events appeared sufficient to annoy these participants. However, the authors also found that the rate of notice of helicopter noise was three times as great as the rate of notice of fixed-wing aircraft noise. They speculate that the greater rate of notice of helicopter noise was the result of the âdistinct sound characterâ of rotary-wing aircraft. Because the participants were exposed to notably fewer helicopter than fixed-wing overflights, it is also possible that they were less habituated to helicopter noise than to fixed-wing aircraft noise. Sternfeld, H. and L.B. Doyle, Evaluation of the Annoyance Due to Helicopter Rotor Noise, NASA Contractor Report 3001, NASA Langley Research Center Contract NAS1-14192, 1978. Sternfeld and Doyle conducted controlled (laboratory environment) listening tests in which 25 volun- teer listeners adjusted the annoyance of three degrees of rotor impulsiveness, heard at four blade passage iv It is possible, for example, that rattle and vibration produced by fixed-wing aircraft at the relatively short ranges of the controlled helicopter flybys would also have created âexcessâ annoyance.
57 (repetition) rates, to the annoyance of a single broadband noise. As with virtually all other publications in this research area, Sternfeld and Doyle characterize helicopter noise as âunusually complex.â They assert, however, without further elaboration, that âIt is the more impulsive types of rotor noise which are responsible for most of the noise complaints against helicopters.â Sternfeld and Doyle made no effort to match the annoyance of broadband noise with that of fixed-wing aircraft noise. The experimentation conducted by Sternfeld and Doyle was premised on the assumption that main rotor impulsiveness controls the annoyance of helicopter noise. The authors therefore made no effort to study the potential contributions of other sources of helicopter noise to annoyance judgments. Sounds pre- sented to test participants for annoyance judgments were reproduced by headphones, rather than in free- field settings, and consisted entirely of synthesized signals. On the continuum of compromise between face validity and precision of control, the work of Sternfeld and Doyle sacrifices nearly all claims to face validity to a desire for very high precision of control of signal presentation. The authors concluded that their findings permit designers of helicopter rotor systems âto trade off rotor design parametersâ to minimize their annoyance, but note certain limitations of the generalizability and practicality of their findings. They were also puzzled (1) by an âapparent inconsistence that when different rotor sounds were adjusted to be equally annoying as a broadband reference sound, subsequent subjective ratins of the rotor sounds were not equal to each other, or to the broadband reference soundâ; and (2) about âthe apparent relative insensitivity to the rotor blade passage period.â They conjecture that headphone presentation of signals for annoyance judgments deprived test participants of the sensations of high-level, near-infrasonic harmonics on body surfaces. Sternfeld, H., R. Spencer, and P. Ziegenbein, Evaluation of the Impact of Noise Metrics on Tiltrotor Aircraft Design, NASA Contractor Report 198240, 1995. Sternfeld et al. (1995) introduce their indoor, controlled listening study of the judged annoyance of simulated rotor noise by re-capping the inappropriateness of the A-weighting network as applied to rotary- wing aircraft noise, which characteristically includes large amounts of low-frequency, if not infrasonic, acoustic energy associated with the fundamental blade passage frequency of a main rotor and its harmonics. Although the work is motivated by concerns about noise produced by a hovering tiltrotor, the arguments apply generally to other rotary-wing aircraft. Forty test subjects rated the annoyance of 145 outdoor and 145 indoor simulated rotor noise sounds. The sounds varied in A-weighted and overall sound pressure level from 72 to 96 dB, and in fundamental blade passage rates from 15 to 35 Hz. The spectra and presentation levels of the test sounds were arranged such that the overall sound pressure levels of the test sounds always exceeded A-weighted levels by 6 dB. Sounds intended to represent indoor listening conditions were accompanied by a projection of an indoor scene, whereas sounds intended to represent outdoor listening conditions were accompanied by a projec- tion of an outdoor scene. Sternfeld et al. concluded that A-weighted measurements of the sounds rated by the test subjects were inferior predictors of the annoyance ratings because they were insufficiently sensitive to low-frequency rotor harmonics. They also concluded that: 1. A combination of A-weighted and overall sound pressure level measurements provided improved prediction of the annoyance ratings; 2. Annoyance predictions based on a combination of the two metrics were at least as good as, if not superior to, predictions made from the Stevens Mark VII method of predicting perceived sound levels; and 3. Including blade passage frequency as a predictor of annoyance judgments improves matters yet further. The differences in correlations between predicted and observed ratings for the various prediction schemes were quite small in some cases. For example, adding blade passage frequency to perceived level increased the variance accounted for in outdoor judgments by only 2%, from R2 = 0.87 to R2 = 0.89. Con- sidering the marginal size of many of the observed differences, and that the ISO standard for low-frequency equal loudness curves has changed since the conduct of the Sternfeld et al. analyses, the authorsâ conclu- sions are best regarded as suggestive rather than definitive. Sutherland, L. and R. Burke, Annoyance, Loudness, and Measurement of Repetitive Type Noise Sources, EPA 550/8-79-103, 1979. This report evaluated âsubjective and objective aspects of moderate levels of noise from impulsive sources,â such as truck-mounted garbage compactors, drop hammers, two-stroke motorcycle engines, and rock drills. The report specifically excludes consideration of high energy impulses (sonic booms, weapons
58 fire, and quarry blasting), and treats helicopter blade slap as a special case. Sutherland and Burkeâs sum- mary of early findings about the annoyance of blade slap may be paraphrased as follows: â¢ The mean observed blade slap correction or penalty factor was 3.3 Â± 2.7 dB for 11 (laboratory) studies that measured this quantity directly. However, three of these 11 studies found essentially a zero or nega- tive correction. The maximum correction for moderate blade slap (i.e., crest level of 10 to 15 dB) was about 6 dB. The maximum correction for severe blade slap (i.e., crest level about 20 dB) was 13 dB, comparable to the values measured for a variety of nonhelicopter sounds. â¢ The methods proposed (by ICAO in the late 1970s) to objectively compute a blade slap correc- tion factor do not appear to agree consistently with the correction factors measured subjectively to account for annoyance of blade slap. â¢ Improved results are obtained if (ICAOâs proposed methods) are modified to account for variations in the frequency of the blade slap. Adjustments of 2 dB (for a blade slap repetition rate of 10 Hz) to 7 dB (for a blade slap rate of 30 Hz) might be appropriate. [These findings are discussed in the annotation for the Fidell and Horonjeff (1981) findings.] The dependency on repetition rates in this frequency range suggests that a blade slap âcorrection factorâ may arise from inherent errors in per- ceived noise level computations for signals with significant energy below 50 Hz. The latter inference is not fully consistent with the observations of Fidell and Horonjeff (1981). â¢ ICAOâs proposed methods for predicting a subjective correction factor depend on some means of measuring the relative impulsiveness. These methods vary from a simple measurement of the crest level of A-weighted noise levels to more complex procedures involving sampling the detected signal (e.g., instantaneous A-weighted level) at a high rate (~5000 Hz) and computing a measure of mean square fluctuation level from these samples. Transportation Research Board, Annual Meeting Presentations, 2014 There were three presentations made at the 2014 TRB Annual Meeting that addressed helicopter noise. These presentations can be found on the TRB website as referenced here: Pagnano, G., âClean Sky Green Rotorcraft Projectâ [Online]. Available: http://onlinepubs.trb.org/ onlinepubs/sp/airport/ACRP11-03-S02-13NASARotaryWingResearch.pdf. This presentation describes the European Union efforts in managing helicopter noise. It is generally quite technical regarding helicopter and tilt rotor noise generation and efforts to design quieter aircraft. The efforts on managing helicopter noise in terms of operational procedures in the presentation described the following: FRIENDCOPTER: noise abatement procedures, improved engine integration, active rotor blades with distributed actuation for lower noise, and reduced power loss. OPTIMAL: airport approach procedures specific to rotorcraft for noise abatement and integration in the general ATM. NICETRIP: developing the ERICA tilt rotor aircraft and previous projects addressing noise impact and power demand. Gorton, S., âOverview of Rotary Wing Research in NASAâ [Online]. Available: http://onlinepubs. trb.org/onlinepubs/sp/airport/ACRP11-03-S02-13NASARotaryWingResearch.pdf. This presentation describes the NASA program to reduce noise associated with helicopters. These include a description of which facilities are involved in NASA helicopter research and the programs. Much of the material describes very technical programs on researching low noise helicopter design, but also lists a project on human response to helicopter noise. In general, the NASA presentation describes its goals as Vision: â¢ Improve capabilities, performance, and acceptance of existing and future rotorcraft configurations for civil and dual-use military missions! â¢ Explore and develop new capabilities for rotorcraft use as commercial transportation in national airspace. Scope: â¢ Conventional and nonconventional light, medium, heavy, and ultraheavy rotorcraft. â¢ Technologies that address performance, noise, efficiency, safety, passenger acceptance, and affordability.
59 Brentner, K., âRotor Source Noise Prediction and the Challenges of Rotor Noise Abatement,â Penn- sylvania State University, 2014 [Online]. Available: http://onlinepubs.trb.org/onlinepubs/sp/airport/ ACRP11-03-S02-13TRB2014-RotorNoisePredictionandAbatement-Brentner.pdf. This presentation provides background information on helicopter noise sources similar to Appendix A1 and then goes on to describe technical details of helicopter noise generation and the methods to try to reduce rotor noise. The presentation does summarize best efforts to manage helicopter noise and includes the following recommendations: â¢ Fly higher! â¢ Avoid over flying neighborhoods, outlying residential areas, and noise-sensitive areas. â¢ Follow high ambient noise routes such as major roadways or highways to mask the sound of the helicopter. â¢ Fly at an altitude that is as high as practical. â¢ Identify noise-sensitive areas and adjust routes to avoid them to the extent possible. â¢ Fly normal cruising speed or slower and observe low-noise speed and descent recommendations per the manufacturers recommendations. â¢ Avoid sharp maneuvers, use takeoff and descent profiles consistent with the Pilot Operating Hand- book, and vary the route since repetition contributes to annoyance. â¢ Avoid late night/early morning flights.