Aircraft Noise: A Toolkit for Managing Community Expectations (2009)

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National aircraft noise policy and public perception are often different in the view of what conditions constitute an adverse noise impact. The EPA and FAA have adopted the DNL as the defining metric for the description of aircraft noise impacts. The Federal Interagency Committee on Urban Noise (FICUN) (152) found in 1980 that DNL is the best descriptor of community impact. The finding was reconfirmed by the subsequent Federal Interagency Committee on Noise (FICON) (8) in 1992. Several studies of public reaction to noise have found the metric to best correlate to the number of persons highly annoyed by transporta- tion noise. Figure 7-1 (150) reproduces the Schultz and Fidell Curves, which relate the per- centage of persons who consider themselves to be highly annoyed by noise to the DNL decibel level. Although DNL takes into account every aircraft noise event experienced by the airport neigh- bor, it can be experienced only in cumulative terms. Many airports responding to the surveys and interviews conducted for this assessment, as well as the authors’ experience on numerous 14 CFR Part 150 and EIS evaluations, have indicated that individuals respond more positively to single event noise levels that they directly experience. As airport managers communicate with neighbors and the general public on noise issues, it is appropriate that they be acquainted with the variety of noise measurements and metrics available to respond to public comment. In dis- cussions with the public, it is essential to use graphic representations to demonstrate the con- cepts of noise. Several examples are provided for the more common of the metrics used in the United States. For complex situations, the reader also may consider various demonstration tools that have been developed to convey some of the complexities of noise and its associated metrics. Among these tools are the FAA’s designated computational model for noise evaluations, the Inte- grated Noise Model (INM) (http://www.faa.gov/about/office_org/headquarters_offices/aep/ models/inm_model/) (153), the Interactive Sound Information System (ISIS) (http://www. noisemanagement.com) (120) and the Noise Model Simulation (NMSIM) (http://www.wylelabs. com/products/acousticsoftwareproducts/nmsim.html) (154). A number of derivative graphical interfaces are commercially available from aviation and acoustic consultants to display the results of noise simulations. There are three types of noise metrics: those that express noise cumulatively as a function of total energy experienced over a set period of time, those that express the noise levels experi- enced during a discreet aircraft operation, and those that are a hybrid of the other two. The follow- ing sections will address each and how they might be used in describing the patterns of current or changing aircraft noise levels to the public. The tools identified throughout this document provide illustrative examples of ways each metric is used, including several animations devel- oped to aid public understanding. Table 7-1 provides a summarization of the utility of various metrics that are discussed in subsequent paragraphs for functions that are subject to public scrutiny. 111 C H A P T E R 7 Noise Metrics and Community Response

112 Aircraft Noise: A Toolkit for Managing Community Expectations Figure 7-1. Prediction curve from Fidell compared with original Schultz Curve (adapted from Fidell, Barber, 1991, as presented in ACRP Synthesis S02-01 Report, Effects of Aircraft Noise: Research Update on Selected Topics (145)). Applicable Noise Metric Type in United States HybridSingle EventCummulativeApplication DNL, CNELEIS/EA/Part 150 evaluations Comprehensive land use planning Noise abatement planning range distance, altitude NA, TA DNL, CNEL, LegSound insulation Lmax DNL, CNELSection 303(c) or 4(f) evaluations TA ambient, TAUDDNLNational Parks evaluations Vibration assessments SELSleep disturbance LmaxSpeech disturbance Public involvement and clarification programs DNL, CNEL range distance, altitude NA, TA DNL – Day-Night Average Sound Level CNEL – Community Noise Equivalent Level Leq – Equivalent Sound Level Lmax – Maximum Noise Level SEL – Sound Exposure Level NA – Number of Events Above (a user defined threshold) TA – Time Above (a user defined threshold) TAUD – Time Audible Lmax, SEL, Slant C-weighted SEL Lmax, SEL, Slant Lmax, SEL, AltitudeDNL, CNEL DNL, CNEL Table 7-1. Utility of available noise metrics for public communication programs. Cumulative Noise Metrics All cumulative noise metrics are expressions of the total amount of acoustic energy that is present. Most express a 1-second average of the noise energy that occurs during a selected period of time. Based on the needs of the governmental jurisdiction where each is used, they often are adjusted to weigh more heavily periods of the day that are considered to be more noise-sensitive. Those that the airport manager in the United States should become most familiar are the DNL

or CNEL in California and Leq, as described in the following sections. Other cumulative metrics such as the Day-Evening-Night Level (Lden) used in Europe, the Australian Noise Exposure Fore- cast (ANEF), the Weighted Equivalent Continuous Perceived Noise Level (WECPNL) used pre- dominantly in Asia, and the Noise Exposure Forecast (NEF) used in Canada, will occasionally be raised by the public in discussions of noise issues, but are not accepted as primary metrics in the United States. Occasionally these metrics may be used in complex studies for supplemental analyses. They are described in the toolkit. FAA evaluations of the environmental effect of airport development or air traffic modifica- tions require an assessment of the change of noise level that will occur. Consequently, the best examples of metric usage often are maps displaying areas of expected change. Throughout this chapter, examples of the key metrics and ways that change may be graphically illustrated for pub- lic consumption will be provided. Day-Night Average Sound Level (DNL or Ldn) DNL considers all the noise energy that occurs during an average day of operation. The sum of the noise energy present between the hours of 10 p.m. and 7 a.m. is multiplied by 10 then added to the sum of the noise energy present between 7 a.m. and 10 p.m. (this equates logarith- mically to multiplying each nighttime operation by 10 before the energy of each noise event is summed). This penalty is applied in recognition of the increased sensitivity of people to opera- tions during the nighttime hours. The total is divided by the number of seconds in a 24-hour day (86,400) to obtain a 1-second average energy level. The logarithm of the average is multiplied by 10 to obtain the DNL level (see the glossary for formulae) . The metric is complex, the mathematics are obscure to the general public, and the measure cannot be directly heard by neighbors. Many members of the public complain because the DNL averages energy across every second of the day, rather than only those seconds when aircraft noise occurs. The FAA has adopted the DNL as the only noise metric required for consideration in airport projects, although it may be supplemented by other metrics to better explain characteristics of the noise exposure pattern. As the metric of choice, the 14 CFR Part 150 land use compatibility guidelines are based on thresholds of DNL. A DNL of 65 dBA has been identified as the thresh- old of significant noise impact above which efforts should be taken to mitigate noise levels. Computer-generated contours connecting locations of equal noise exposure are typically used in the selection of land use development criteria and plans in airport environs. Example of Metric Display for Public Information: • Change in Day-Night Average Sound Level (DNL) Combined Alternatives (Boston Overflight Noise Study, Phase 1) (155) • Comparative DNL Contours for Baseline and Alternative Conditions at Boston Logan Inter- national Airport (Boston Overflight Noise Study, Phase 1) (156) • Example of DNL computation Video: (prepared for ACRP 02-05 project) Community Noise Equivalent Level (CNEL) CNEL is similar to DNL and used for essentially the same purposes. Required by state law in California, the FAA has approved CNEL as an acceptable substitute for DNL in federally funded airport noise analyses there. In addition to the penalty applied to nighttime operations by DNL, CNEL also applies a penalty to operations that occur during the evening hours between 7 p.m. 7-5 7-4 7-3 7-2 7-1 Noise Metrics and Community Response 113

and 10 p.m. The evening penalty multiplies the sum of the evening energy by 4.77 before adding it to the sums of the daytime and nighttime energy levels. The evening penalty is equivalent to con- sidering each evening operation to have three times the effect of an equivalent daytime operation. The State of California mandates that CNEL noise exposure contours be made available for public land use planning in the same way that DNL contours are used at airports outside the state. Further, at several large airports, the contour patterns are regularly monitored to strive to meet a state requirement that no residences be located within the 65 CNEL contour. Where such incompatible uses remain, the airports must allow variances to the rule and demonstrate their attempts to reduce the overall impact of noise in neighboring communities through noise abate- ment or mitigation. Equivalent Sound Level (Leq) The Leq is the simplest and most flexible of the cumulative metrics. It does not apply penalty factors based on time of day, nor does it require consideration over a 24-hour period. The metric may be used by planners to assess the comparative noise effects of any number of events on peo- ple. For example, the Leq may be computed for 1-hour periods, for the nighttime hours, for school hours, for a peak period of operations, or any other duration desired. It may be used to compare the cumulative contribution of specific aircraft types, users or user groups to the total noise energy. It also may be used to reflect the combination of discrete aircraft events having different noise characteristics. The analytical use of the Leq metric may be widely varied in considering various noise abate- ment operational techniques. Leq levels allow a more in-depth assessment of the specific costs or benefits of flight actions by comparing, for example, the noise energy from aircraft flying along one departure track as compared to another. When mapped, the two resulting patterns may guide noise management programs to maximize the reduction of impacts on underlying populations. Single Event Noise Metrics Cumulative aircraft noise contours often are challenged by airport neighbors as not represent- ing what can be heard and measured every time an aircraft flies over their home. Long duration measurements and computer technology may indicate the contour patterns are accurate for the community, but they fail to capture the discrete nature of the single events that people actually identify and complain about. As louder Stage 2 aircraft were removed from the commercial operating fleet during the 1990s, cumulative noise contours shrank significantly from earlier sizes. Although the con- tour reduction could be attributed largely to the reduction of noise from individual aircraft, the number of actual operations has generally increased. As a consequence of this change, the public has become more vocal in demanding that the number and noise levels of single events be assessed in environmental evaluations. Several metrics are available to respond to this demand. Sound Exposure Level (SEL) The SEL is a mathematical expression of the noise energy present during an event or a period of time, normalized to a single second. Consequently it is always larger than any cumulative noise measurement of the same event that lasts longer than one second. It provides the noise analyst the ability to directly compare the acoustic energy generated by two separate events, 114 Aircraft Noise: A Toolkit for Managing Community Expectations

while clearly accounting for both their peak noise levels and durations. For example, the same operation may be considered along an existing and proposed flight track. The SEL of each oper- ation would be compared to provide insight into the prospective effects of changing location on the underlying population. Further, the SEL is the preferred metric for the evaluation of sleep disturbance, making it critical to the evaluation of noise abatement measures that are directed at night operations. Typically, the SEL is approximately 8-10 decibels louder than the peak noise level that occurs during a single event. The SEL is represented by a series of dark bars and compared to the measured noise levels, including Lmax, through the course of several single events in Figure 7-2. Example of Metric Display for Public Information: • Example of SEL computation Video: (prepared for ACRP 02-05 project) Maximum A-Weighted Sound Level (Lmax) The Lmax of an event represents the loudest decibel level achieved during that event. It also can be expressed over a period of time (hour, day, year, etc.). Similar to SEL, the metric is useful to compare operational alternatives for noise abatement. Unlike SEL, it is not subject to the com- pression of all noise energy during an event into a single second, but rather provides the instan- taneous peak noise level achieved. Consequently, it is the loudest aircraft noise level the public experiences, and often the one most cited in public comment and complaint. Example of metric display for public information: • Comparison of Sound – Outdoor and Indoor sound levels (157) • ORD Aircraft Noise Comparison (single event footprints) (158) • Loudest Aircraft Noise Events Site Report (Chicago O’Hare) (159) • Example of Lmax computation Video (prepared for ACRP 02-05 project) 7-10 7-9 7-8 7-7 7-6 Noise Metrics and Community Response 115 Figure 7-2. Comparative single event noise metrics compared to equivalent noise level.

Slant Range Distance/Altitude Another factor frequently cited by the public as a noise issue is the distance from the aircraft in flight to the listener on the ground (the slant/range distance) or the aircraft’s altitude. The slant/range distance represents the hypotenuse of a triangle formed by the aircraft altitude and the lateral distance from the ground location to the point of closest approach along an aircraft’s path of flight. Noise complaints recorded by those airports surveyed for this evaluation cited “too low” as a common source of public concern. Similarly, the public will often be aware of preferred flight patterns and complain that the air- craft was “right over my house” and frequently point to a location halfway above the horizon instead of straight up. The perception of location, coupled with distance, is often difficult to answer without the use of triangulation. An aircraft 45 degrees above the horizon is as far away horizontally as it is vertically and farther than either when measured in a straight line distance. Members of the public often are interested in the number of events that overfly an area at dif- ferent altitudes and may request that altitudes be raised to abate noise. As the distance between the noise source and the receiver is increased, the noise level will decrease by six decibels with every doubling of distance. Consequently, a noise event of 80 dBA of SEL at 3,000 feet overhead will be decreased to 74 dBA of SEL if the altitude of the aircraft is increased to 6,000 feet. It is often very difficult to increase aircraft altitudes by significant amounts at any course along a flight track without adding substantial length to the track prior to passing over, or in some cases, with- out significantly increasing the power setting used in flight (and the resulting noise level). However, if the same flight track were relocated laterally by one mile, the noise reduction would be the same because the distance between the source and the receiver would be increased to 6,000 feet. It is often much easier to relocate a flight track than to increase altitude enough to achieve a meaningful reduction in single event noise levels. Hybrid Metrics As information has become more available for individual aircraft types, as noise prediction tools have been improved, and as the public has become more sophisticated in their understand- ing of the effects and consequences of aircraft development actions, new applications of avail- able metrics have evolved to be more responsive to public interests. Several supplemental metrics have evolved in recent years that allow the noise analyst to develop information that may be more meaningful to members of the public in their understanding of aviation noise. While such sup- plemental metrics are not required for inclusion in any environmental evaluation, their avail- ability is well known and often advisable at locations where noise is a contentious issue. These metrics are typically formed from combinations of simpler single event metrics. Because they are developed from more detailed computer modeling and post-processing requirements than the more standard metrics described in earlier sections, they are more costly to produce. They include Number of Events Above, Time Above, and Audibility. Number of Events Above (NA) In recent years, the Number of Events Above metric has become popular as a means of com- paring the number of operations above various thresholds of loudness associated with opera- tional alternatives. The INM is capable of computing the SEL or Lmax of every operation that is included in its input data, at any location selected by the program’s user. The availability of this information will allow the noise analyst to sort output data to provide the numbers of daily operations that exceed selected noise thresholds. For example, the number of flights exceed- 116 Aircraft Noise: A Toolkit for Managing Community Expectations

ing 75 dbA of SEL may be computed for a single location for two separate alternative flight routes, and the results compared to identify which alternative provides more noise abatement. Because output information is available in detail, the number of operations may be determined for any combination of noise level, type of operation, and if desired, aircraft type or group. The information provides the public with a level of detail that it often has demanded to assist in understanding of the potential effects of proposed airport development actions. It provides the planner the ability to design noise abatement and land management actions that more pre- cisely address the impacts perceived as meaningful by the public to those identified as significant by regulation. Traditionally, the alternatives are compared through mapping of change in the Numbers of Events Above, a threshold from a baseline condition. Example of Metric Display for Public Information: • Examples of Number of Events Above illustrations (160) • Change in Number-of-Events Above SEL 60 (Annual Average Day) Combined Alternatives (161) • Example of Number of Events Above Video (prepared for ACRP 02-05 project) Time Above The Time Above metric has been available for many years, but has rarely been used as a plan- ning tool for noise abatement or land use mitigation. Output information is computed by the noise model to indicate the amount of time noise exceeds a user-selected threshold. This infor- mation is helpful in evaluations of sleep or speech disturbance when the thresholds are set at lev- els that correlate to the significant effect levels of those activities. One substantial drawback to use of Time Above information is that simultaneous events heard within the same time span are added to the total independently, resulting in overestimates of the total Time Above. This is par- ticularly problematic at threshold levels below 80 dBA near an airport, and at levels below 70 dBA along departure paths. At very low thresholds, the total TA for a day may exceed the number of minutes in a day – a result that does not further the analysis of the noise conditions. Time Above results may be presented as either the minutes per day or the percentage of the day that the threshold of interest is exceeded. Example of Metric Display for Public Information: • Example of Time Above Video (prepared for ACRP 02-05 project) A metric of interest in the evaluation of national parks and other similar properties of signif- icant national interest (monuments, wilderness areas, etc.) is the Time Above Ambient (TAA) noise levels. The calculation of this metric requires knowledge of ambient noise levels at specific locations on the surface, or a digital map of the distribution of non-aircraft ambient noise across the area of interest. The metric is typically applied in areas of very low ambient levels where nat- ural quiet is an essential component of the environment. Consequently, the TAA levels tend to be greatly overestimated by the INM because simultaneous events at low noise levels will be added independently, rather than considered together. Audibility is a metric developed for the National Park Service that is applied in national parks and other park areas where natural quiet is an important part of the visitor experience. The metric is similar to Time Above in that it considers the amount of time each aircraft event is audible. However, audibility considers such exceedances within each of the twenty-four 1/3 octave bands of the frequency spectrum associated with aircraft noise. This measure also is subject to over- estimation through the addition of simultaneous events to a single total. 7-14 7-13 7-12 7-11 Noise Metrics and Community Response 117

Example of Metric Display for Public Information: • Cumulative Aviation Noise Percent of Time Above Natural Ambient with Existing Airport (St. George UT EIS) (162) • Relative Change in Project Noise – 2020 Airport Only – Percent Time Audible %TAnatural (St. George UT EIS) (163) Best Applications Based upon the results of surveys and interviews conducted for this analysis, as well as litera- ture available in the field and author experience, several noise metrics may be identified as best practices in communicating the complexities of aircraft noise to the public. Measures that pro- vide greater understanding of the effects a proposed change may have on the level of exposure are useful in focusing attention on reality rather than feared unknown consequences. The most effective measures identified by this evaluation to achieve this purpose include: Day-Night Average Sound Level (DNL). Although DNL is not well-understood by the general public, it has been found to be the best indicator of a high degree of annoyance by several differ- ent evaluations over a long period of time. Further, the FAA has adopted DNL as its required metric in all noise evaluations for which the agency provides funding assistance. It is the only metric for which widely accepted guidance is available for land use compatibility determina- tions. Consequently, DNL is the essential metric for evaluation during noise abatement and envi- ronmental planning processes involving aircraft noise exposure assessments. Sound Exposure Level (SEL). Comparisons of SELs between existing and proposed alterna- tive conditions are able to provide the public with greater insight into what changes may occur in the future, without actually experiencing that change. The greater the array of information available to assist the public in understanding the potential changes, the better able the airport is to communicate fact rather than public fear of the unknown. Comparison of single event sound levels is one of the better tools to communicate the extremes of anticipated noise events. Number of Events Above (NA). While this metric is relatively expensive to develop, it is flex- ible enough to allow the analyst to distinguish the effects of specific noise abatement actions on specific categories of aircraft. Hence, evaluations of cost and benefit may be enhanced through greater detail. Its use to project the anticipated number of events above a threshold of public interest has been widely used to assist communities to relate the effects of alternatives using what they hear (single events), rather than relying on analysts to tell them that the DNL doesn’t indi- cate they will be adversely effected. This is the single metric that responds to differences in num- bers of overflights in a way every individual can understand. Every community is unique, yet every community is interested in the same thing – its quality of life. The use of the NA metric allows the individual to better relate to how any proposed development action or change of operational characteristic may impact that quality. Time Above (TA). The TA metric is best applied to evaluations of proposed changes to facil- ities or operational characteristics on schools during the school hours. When combined with an Lmax threshold equivalent to the exterior to interior attenuation plus the threshold of interest, the data may be useful in identifying those schools that should be considered for additional noise mitigation. The public’s interest in the measure is usually restricted to the secondary interest in the effect of aircraft noise on the quality of life. Therefore, it may be developed to provide addi- tional understanding of existing and potential aircraft noise patterns. 7-16 7-15 118 Aircraft Noise: A Toolkit for Managing Community Expectations