3
Metrics for Assessing Environmental Noise

Selecting a metric for assessment of environmental noise is no simple task, because it must reflect the impact on people. No single metric can describe all responses in all situations.1 Context, expectations, and people’s experiences and circumstances all affect their responses. Hence, levels of community response (e.g., annoyance) may vary from community to community, just as individual responses vary from person to person, even if noise levels do not change. However, one consistent finding has been that changes in noise exposure do affect individual and community responses and that increases in man-made noise usually have a negative impact. This is illustrated by the Schultz curves later in this chapter.

Thus, it is important to understand which characteristics of noise elicit a negative response and how exposure to noise with those characteristics affects people’s lives. The metric chosen or developed for measuring community noise must reflect this human response and must be taken into account in making policy decisions.

Fifty years ago, when noise metrics were developed, the choices were based on simpler calculations and technologies and the acoustical quantities that could be predicted by sound propagation models used at the time. Although much more sophisticated measurements can be made today, many still consider these “older” metrics valid and continue to use them. However, with modern instruments (see Appendix E), much more accurate measurements and predictions can now be made of people’s reactions to noise.

A meaningful metric, or set of metrics, translates sound pressure-time history measurements into a prediction of the effects of noise, such as annoyance, sleep disturbance, changes in health, interference with understanding speech, and ability to learn. Ideally, this translation should be based on context, expectations, and personal situations and preferences, in addition to noise information, and should account for a distribution of responses, including responses of vulnerable populations, such as children. Unfortunately, a holistic model of community response is still beyond present capabilities.

One fundamental issue that must be considered in the choice of an environmental noise metric(s) is the purpose for which the metric will be used:

  • to implement public policy on noise immission from one or more sources

  • to provide information about noise exposures in a form understandable to the public

  • to assess a noise situation in terms of noise control engineering

The metrics to accomplish these purposes may differ, but all three relate directly to the impact of noise on the community. For example, a metric to inform decisions about noise control engineering strategies should result in reducing the noise impact, which would then be reflected in the policy metric(s) and the public information metric(s).

As new research results become available and accessible, they should influence the choice of metrics for the three purposes listed above. The results of such research may result in complex calculations that include many variables and may better quantify individual reactions to sound. Some modern procedures, such as calculation of loudness, are more complex than earlier methods, but available computational procedures make the results widely available.

Much of this chapter recounts the evolution of noise metrics and their applications to public policy. This history includes criteria originally used by the U.S. Environmental Protection Agency (EPA) to select a noise metric and the rationale Europeans have used for using a curve passed through highly variable data to determine what percentage of a population is “highly annoyed” by a given noise. In ad-

1

This chapter considers only metrics related to the effects of environmental/community noise on people. The effects of noise on wild and domestic animals and on sensitive historical structures are not considered, even though these effects are often considered in environmental noise impact analyses.



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3 Metrics for Assessing Environmental Noise Selecting a metric for assessment of environmental noise is on context, expectations, and personal situations and prefer- no simple task, because it must reflect the impact on people. ences, in addition to noise information, and should account No single metric can describe all responses in all situations.1 for a distribution of responses, including responses of Context, expectations, and people’s experiences and circum- vulnerable populations, such as children. Unfortunately, a stances all affect their responses. Hence, levels of community holistic model of community response is still beyond pres- response (e.g., annoyance) may vary from community to ent capabilities. community, just as individual responses vary from person One fundamental issue that must be considered in the to person, even if noise levels do not change. However, one choice of an environmental noise metric(s) is the purpose consistent finding has been that changes in noise exposure do for which the metric will be used: affect individual and community responses and that increases in man-made noise usually have a negative impact. This is • to implement public policy on noise immission from illustrated by the Schultz curves later in this chapter. one or more sources Thus, it is important to understand which characteristics • to provide information about noise exposures in a form of noise elicit a negative response and how exposure to noise understandable to the public with those characteristics affects people’s lives. The metric • to assess a noise situation in terms of noise control chosen or developed for measuring community noise must engineering reflect this human response and must be taken into account in making policy decisions. The metrics to accomplish these purposes may differ, but all Fifty years ago, when noise metrics were developed, the three relate directly to the impact of noise on the community. choices were based on simpler calculations and technolo- For example, a metric to inform decisions about noise control gies and the acoustical quantities that could be predicted by engineering strategies should result in reducing the noise sound propagation models used at the time. Although much impact, which would then be reflected in the policy metric(s) more sophisticated measurements can be made today, many and the public information metric(s). still consider these “older” metrics valid and continue to use As new research results become available and accessible, them. However, with modern instruments (see Appendix E), they should influence the choice of metrics for the three much more accurate measurements and predictions can now purposes listed above. The results of such research may be made of people’s reactions to noise. result in complex calculations that include many variables A meaningful metric, or set of metrics, translates sound and may better quantify individual reactions to sound. Some pressure-time history measurements into a prediction of modern procedures, such as calculation of loudness, are more the effects of noise, such as annoyance, sleep disturbance, complex than earlier methods, but available computational changes in health, interference with understanding speech, procedures make the results widely available. and ability to learn. Ideally, this translation should be based Much of this chapter recounts the evolution of noise metrics and their applications to public policy. This history includes criteria originally used by the U.S. Environmental 1This chapter considers only metrics related to the effects of environmen- Protection Agency (EPA) to select a noise metric and the tal/community noise on people. The effects of noise on wild and domestic rationale Europeans have used for using a curve passed animals and on sensitive historical structures are not considered, even through highly variable data to determine what percentage though these effects are often considered in environmental noise impact of a population is “highly annoyed” by a given noise. In ad- analyses. 

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0 TECHNOLOGY FOR A QUIETER AMERICA dition, alternative metrics are described that may be easier and Zwicker’s work on loudness were standardized by the for the public to understand than the day-night average sound International Organization for Standardization (ISO, 1975). level (DNL).2 Later work by Brian Glasberg and Brian Moore in the United Kingdom was the basis for the American National Standard on computation of loudness (ANSI, 2007). LOuDNESS AND A-WEIgHTINg Over the years, A-weighted levels were found to cor- Arguably the modern history of noise metrics began in respond reasonably well to human response, especially for the 1930s with the search for a way to describe the loudness noise spectra in typical offices. Single-number methods of of sound. This led to the definition of weighting networks rating noise in offices and other building spaces were also for sound-level meters and, because of limitations on the developed, including so-called noise rating curves (NR capabilities of calculating sound pressure levels at that time, curves—a curve tangent method of obtaining a single num- a single frequency-weighted value—either A-weighted or ber from an octave band spectrum) and ratings based on loudness and A-weighting.3 C-weighted—came into common usage. METRICS FOR MEASuRINg COMMuNITy REACTION Loudness TO NOISE In an early attempt to determine the loudness of sound (using discrete-frequency tones), Fletcher and Munson One early attempt to develop a metric for forecasting (1933) found that the loudness of a tone depends on both community response to noise was made by Stevens et al. its amplitude and its frequency. Knowing this dependence, (1955). Unlike the DNL, this metric included nonacousti- they were able to develop a set of equal-loudness curves. In cal factors as well as noise levels and yielded a “composite modern terms the unit of loudness is the phon. For example, noise rating.” This rating was then plotted against a scale of a 1,000-Hz tone with a sound pressure level of 40 dB has a community responses—vigorous community action, threats loudness of 40 phon. At this loudness level the sound pres- of community action, widespread complaints, sporadic sure level of tones between 1,000 and about 5,000 Hz is complaints, and no observed reaction. A few case studies generally lower than 40 dB, and the sound pressure level of showed a reasonable correlation between the measurement tones below 1,000 Hz and above about 5,000 Hz is higher and response but with considerable scatter. Community noise than 40 dB. levels were determined by measuring the average octave The sound-level meter was standardized in the early band levels in the community averaged in space and time. 1930s when microphones and electronic circuits were being A curve tangent method was used to reduce the octave band developed. Ideally, the standard sound-level meter would data to a single-number rating. have a single-number description of the sound at a given point in space. The best description at the time came from Day-Night Average Sound Level the studies by Fletcher and Munson, who clearly showed that the shape of the equal-loudness curve was dependent on After EPA established the Office of Noise Abatement both the amplitude and the frequency of sound. Thus, using and Control and after passage of the Noise Control Act of the linear electronic circuits of the time, a few curves had to 1972, EPA was faced with the task of developing a metric be selected based on the amplitude of the sound. One of the for community noise with the following characteristics curves selected, which is very close to the 40-phon curve, (EPA, 1974): was designated as “A-weighting.” Another, which was nearly independent of frequency, was designated as “C-weighting.” 1. The measure should be applicable to the evaluation of A third curve, the “B-weighting” curve, which fell between pervasive long-term noise in various defined areas and the A and C curves, has long since fallen out of favor. under various conditions over long periods of time. A-weighting and C-weighting are still used today, although 2. The measure should correlate well with known effects the shape of the curves has changed somewhat to provide a of the noise environment on the individual and the standardized mathematical description in terms of poles and public. zeros of a transmission network. 3. The measure should be simple, practical, and accurate. Work on improving the calculation of loudness based on In principle, it should be useful for planning as well as measurement of the spectrum of sound continued. The best- for enforcement or monitoring purposes. known early work in the United States was by S. S. Stevens 4. The required measurement equipment, with standard- and in Germany by Eberhard Zwicker. Stevens’s Mark VI ized characteristics, should be commercially available. 3A-weighting is less useful for measuring human response to sound when 2 Forinformation on how other countries measure noise in quiet areas, the spectrum has a large low-frequency component, when high-amplitude see Appendix B. More information on communication with the public can peaks in the spectrum are in the 2- to 4-kHz range, and when the sound is be found in Chapter 10. tonal or impulsive.

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 METRICS FOR ASSESSING ENVIRONMENTAL NOISE 5. The measure should be closely related to existing and the selection of a two-period (day-night) metric rather methods currently in use. than a three-period metric (day-evening-night) was based on 6. The single measure of noise at a given location should community reaction studies at the time and tests that showed be predictable, within an acceptable tolerance, from little difference between a two-period and a three-period k nowledge of the physical events producing the metric. Thus, the DNL (A-frequency weighting for both day- noise. time and nighttime levels and a 10-dB increase in measuring 7. The measure should lend itself to small, simple moni- system gain at night) came into being for the evaluation of tors that can be left unattended in public areas for long community noise. periods of time. In the United States, DNL and the percentage of per- sons highly annoyed (discussed in the next section) are EPA also published its rationale for choosing A-weighting widely used, especially by the Federal Aviation Adminis- and for leaving open the possibility of using a different met- tration (FAA). The Federal Highway Administration uses ric in the future (EPA, 1974; von Gierke, 1975): A-weighting and the average sound pressure level during the busiest traffic hour as a measure of community impact. With respect to both simplicity and adequacy for char- The difference between C-weighted and A-weighted levels acterizing human response, a frequency-weighted sound is used as an indication of the low-frequency content of the level should be used for the evaluation of environmental sound, and the sound exposure level (see Appendix A) is noise. Several frequency weightings have been proposed for used to evaluate sounds of finite duration—for example, an general use in the assessment of response to noise, differing aircraft flyover. primarily in the way sounds at frequencies between 1000 and 4000 Hz are evaluated. The A-weighting, standardized Day-evening-night sound level is widely used in Europe. in current sound level meter specifications, has been widely In some countries, Lday and Lnight, (average A-weighted used for transportation and community noise description. sound pressure levels) are used in addition to or instead of a For many noises, the A-weighted sound level has been DNL-type metric. None of these metrics takes into account found to correlate as well with human response as more the time of night when the noise occurs, even though noise complex measures, such as the calculated perceived noise appears to cause greater sleep disturbance at the beginning level or the loudness level derived from spectral analysis. and end of the night. However, psychoacoustic research indicates that, at least for Several issues have arisen from the use of DNL and some noise signals, a different frequency weighting which the percentage of persons highly annoyed: no one actually increases the sensitivity to the 1000–4000 Hz region is more “hears” a DNL; there is a high variability from study to study reliable. Various forms of this alternative weighting function around a nominal Schultz curve; and in many situations have been proposed; they will be referred to here as the type “D-weightings.” None of these alternative weightings [have] “highly annoyed” is not an appropriate measure of human progressed in acceptance to the point where a standard has response. Although the percent highly annoyed and DNL been approved for commercially available instrumentation. approach has been widely endorsed, variability around a It is concluded that a frequency-weighted sound pres- nominal Schultz curve is troubling, and there are reports that sure level is the most reasonable choice for describing the this approach is not sufficient to predict community response magnitude of environmental noise. In order to use available (Fidell, 2002). Attitudinal and personal variables impact standardized instrumentation for direct measurement, the people’s responses and are, to some extent, the reason for A-frequency weighting is the only suitable choice at this scatter (Fields, 1993; Flindell and Stallen, 1999; Miedema time. The indication that a type D-weighting might ulti- and Vos, 1999). mately be more suitable than the A-weighting for evaluating As shown in Figure 3-1, some researchers (Miedema and the integrated effects of noise on people suggests that at Oudshoorn, 2001) have found in their analyses of survey such time as a type D-weighting becomes standardized and available in commercial instrumentation, its value as the results that the nominal Schultz curve appears to depend weighting for environmental noise should be considered to on the noise source (e.g., aircraft, road traffic, rail traffic). determine if a change from the A-weighting is warranted. In addition, DNL is a relatively insensitive measure of sleep disturbance and thus is not an appropriate metric for The decision to add 10 dB4 in measuring nighttime levels predicting awakenings in sleep disturbance studies. Finally, A-weighting is not the best weighting for measuring noises 4A number of metrics have been developed to take into account day- with unusual spectra (e.g., excessive high- or low-frequency time versus nighttime operations around airports. These include Noise noise or noise that has unusual peaks in its spectrum). For Exposure Forecast, Community Noise Equivalent Level, and Noise and sounds with levels that evolve over time, the most appropriate Number Index. The EPA rationale for selecting a 10-dB nighttime penalty (EPA, 1974) is as follows: “Methods for accounting for the differences in interference or annoyance between daytime/nighttime exposures have been employed in a number of different noise assessment methods around the world. The weightings applied to the nondaytime periods differ slightly 10 dB for the nighttime weighting made in Section 2 was predicated on its among the different countries but most of them weight night activities on extensive prior usage, together with an examination of the diurnal variation the order of 10 dB; the evening weighting if used is 5 dB. The choice of in environmental noise.”

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 TECHNOLOGY FOR A QUIETER AMERICA 100 Aircraft 90 Percentage Highly Annoyed New USAF logistic curve 80 Schultz curve (1978) 70 Percentage Highly Annoyed Fidell et al. (1989) 60 Road 50 Traffic 40 30 Railroads 20 10 0 Day-Night Average Sound Level (dBA) FIGURE 3-1 Variability in survey results. ▼ = road traffic. ★ = air traffic. ♦ = rail traffic. Curves are the results of fits to data associ- ated with different modes igure_3-1.eps F of transportation. Source: Adapted from Day-Night Average Sound Level (dB) Schomer (2005) and Fidell and vector axis labels bitmap with Silvati (2004). FIGURE 3-2 Three versions of a Schultz curve. ■ = the U.S. Air Force logistic curve. ✚ = the curve proposed by Schultz (1978). Figure_3-2.eps ❋ = a curve by Fidell et al. (1991). Source: Reprinted from Finegold et al. (1994). with vector axis labels & key labels bitmap weighting should change with the level; typically, however, only one weighting is used. Percentage of Persons Highly Annoyed Consultants and other professionals are often asked to The next major event in the selection of a noise metric study community noise issues and recommend remedial was a study by Schultz (1978) of surveys of community action. Predictions of community response should not be reaction to noise. Schultz went back to original data to es- based only on variations of the Schultz curve. It has been timate the percentage of the population “highly annoyed” known for many years (Stevens et al., 1955) that nonacousti- as a function of DNL. Even at that time, it was recognized cal factors influence community reaction to noise. Thus, at that, for a variety of reasons, there was considerable scatter a minimum, temporal and spectral variations must also be in the data. Nevertheless, Schultz proposed that a single taken into account. curve (the Schultz curve) drawn through the data should be Based on work by EPA, Schomer (2002) proposed used as a measure of community response. Later studies led modifications to DNL to account for tonality, impulsiveness, to modifications of the Schultz curve (Fidell et al., 1991; background noise, type of community, and other factors. Finegold et al., 1994). In the latter study, three curves were Schomer also showed how this modified approach could compared (see Figure 3-2), and a U.S. Air Force logistic be used to reduce variances in the survey data on which the curve was defined Schultz curve is based. The Federal Interagency Committee on Noise (FICON, %HA = 100/[1 + exp(11.13 – 0.14Ldn)] (1) 1992) endorsed the use of percent highly annoyed and DNL as metrics for assessing community noise around airports The scatter in the highly annoyed response, compared and recommended that the equation above be accepted as to scatter in the average curve, was presented by Miedema showing the definitive relationship between percent highly and Vos (1998) and has been commented on by several sub- annoyed and DNL (see also Finegold and Finegold, 2002). sequent researchers (e.g., Schomer, 2005). The first problem Response curves for community annoyance have now been with scatter is that it causes great uncertainty in the predic- standardized nationally (ANSI, 2005) and internationally tion of community reaction. A second problem is that com- (ISO, 2003). munity reaction (percent highly annoyed) appears to depend on the source of the noise; for example, responses to aircraft ALTERNATIVE METRICS noise, road traffic noise, and rail noise vary, even if the noises have the same DNL (see Figure 3-1). The question that must The science of measuring environmental noise has pro- be answered is whether the variability in response is due to gressed rapidly in the past decade as computer technology has the nature of the noise source or reflects how the metric is come on line to provide rapid data acquisition and analysis in calculated. small portable packages. The end result has been a revolution

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 METRICS FOR ASSESSING ENVIRONMENTAL NOISE in the type and complexity of measurements and calculations explain to the public than a metric that uses the phon (which that can be made in analyzing environmental noise. This uses a logarithmic scale) as a unit of loudness. For sounds section provides a more detailed description of presently in the midfrequency range, an increase in A-weighted level used metrics and a variety of alternative metrics that are well of 10 dB corresponds to a doubling of loudness. within the capabilities of modern instrumentation.5 Speech Interference A Different Frequency Weighting Standard methods of calculating speech interference are An alternative to A-weighting (i.e., D-weighting) could available, and the values may be translated into effects that be considered. As noted earlier, this weighting was consid- are easier for the public to understand than DNL. For ex- ered by the EPA in 1974 but rejected because there was no ample, the difficulty of communicating over a given distance standard shape for the curve. between speaker and listener may be quantified in terms of percentage of speech likely to be understood. Speech inter- ference can be affected by the fact that hearing loss increases Perceived Noise Level with age and usually starts at high frequencies. Thus, the Community reaction to noise from jet planes led to ability to distinguish consonants that have high-frequency important events in the development of noise metrics. The content such as “s” and “th” is diminished. problem, which arose in 1956, is described in an autobiog- raphy by Beranek (2008). According to measurements made Nighttime Sleep Disturbance with a standard sound-level meter, the noise produced by a Boeing 707 jet airplane and that by a propeller airplane In Night Noise Guidelines for Europe, published by the (Super Constellation) were equal. However, subjective test- World Health Organization (WHO, 2007), sleep disturbance ing showed that the 707 was considered much noisier; by is related to the nighttime level designated as Lnight, although subjective measures, the A-weighted sound pressure levels researchers also use indoor LAmax and indoor A-weighted of the 707 would have to be significantly reduced to be con- sound exposure level (ASEL) when investigating the rela- sidered as noisy as the Super Constellation. This early test of tionship between awakenings and noise. The temporal pat- the usefulness of A-weighted levels in judging noisiness led tern of noise at night, however, is known to influence sleep to further evaluations of the relative noisiness of propeller- disturbance. This problem is addressed to some extent in a driven and jet airplanes and the development of the concept new American National Standard (ANSI, 2008), in which of “perceived noisiness” (Kryter, 1960; Kryter and Pearsons, terms such as the likelihood of awaking, are used; the new 1962, 1963).6 standard may be more understandable to the public than Perceived noise level (PNL) was used in the development the day-night average level or the nighttime level used in of specifications of noise emissions from airplanes for regu- Europe. latory purposes in 1969 and is still used to certify airplanes today. When the perceived noise-level metric was adopted, METRICS FOR COMMuNICATINg WITH THE PuBLIC it was possible to compute it only with a large amount of equipment. Today, it can be done with a handheld sound- An often-cited shortcoming of DNL is that the public does level meter. D-weighting simplifies the PNL calculation, but not understand what it means. Over the years, various people neither PNL nor D-weighting solves the decibel issue, which have advocated using supplemental metrics that describe relates to explaining noise to the public. noise in ways that are more understandable to the majority of people (FICAN, 2002). Metrics used to supplement DNL include time above (a certain level), number of events above Loudness a given value of the ASEL, number of loud events above a Historically, the method of calculating PNL was simi- certain ASEL in a given period, and single-event descrip- lar to the method of calculating loudness. Today, several tors such as LAmax and ASEL. Most advocate using a group methods can be used to calculate loudness, all of them with of metrics to give a fuller picture of the potential impact of a handheld sound-level meter. Loudness that exceeds some the exposure and explaining that these measures supplement agreed-on value a given percentage of the time also can be metrics such as DNL. The same argument can be made for calculated. On a linear scale (as opposed to a logarithmic using a group of metrics when addressing other measure- scale), a doubling of the value of the calculated value cor- ments or predicting a variety of impacts (Eagan, 2007), such responds to a doubling of the loudness. This may be easier to as the number of occurrences of speech interference; when noise levels inside buildings exceed recommended levels for a particular activity, such as learning in schools (ANSI, 5 For a description of the instruments, see Appendix E. 2002); or the likelihood of being awakened based on pre- 6 For a general assessment of human reaction to aircraft noise, see Beranek dicted indoor single-event metrics (ANSI, 2008). et al., 1959.

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4 TECHNOLOGY FOR A QUIETER AMERICA INTERNATIONAL ACTIVITIES RELATED TO NOISE The number of events has been recognized as an important METRICS factor in noise exposure, and it is included in metrics that are or have been used to predict annoyance; alternatives to DNL, The International Commission for the Biological Effects such as the Noise Exposure Forecast (NEF) system used in of Noise holds meetings at five-year intervals. In 2008 the Canada (Transport Canada, 2005) and elsewhere; and the meeting was held in the United States, but most of the par- Noise and Number Index (NNI) that was used prior to 1990 ticipants came from other countries, as did the presenters. in the United Kingdom. The NEF metric is based on effective Truls Gjestland of Norway presented a summary report perceived noise level as well as the number of events; hence on research in the past five years related to the effects of it takes into account some of the impact of tonal components community noise, specifically annoyance. Although some and impulsiveness on annoyance. NNI is also based on a very research has been done in Japan, he said, not many signifi- basic loudness measure, perceived noise level in decibels cant projects had been undertaken. However, he noted that at and number of events. Analysis of data from a study at U.K. least three different versions of the Schultz curve had been airports in 1982 and another study in 2005 showed that the developed, all of them based generally on the same datasets relationship between annoyance and A-weighted equivalent (Gjestland, 2008). Around the same time Lawrence Finegold level had changed. However, by combining a measure of of the United States presented a review of major noise-related average noise exposure with the number of events, it was policy efforts around the world during the same time period possible to develop a metric that worked consistently for (Finegold et al., 2008). both studies (ANASE, 2007). European Activities NOISE METRICS FOR RuRAL/NATuRALLy QuIET AREAS In 1996 the European Union (EU) published The Green Paper, which established new noise programs that are used to Neither day-night average sound level nor percent highly address noise issues today (EC, 1996). European Directives annoyed is an appropriate metric for measuring noise in have been issued concerning noise emissions from consumer naturally quiet areas. Because of the logarithmic nature of the products, and an EU Environmental Noise Directive (END) decibel, short-duration sounds of high amplitude compared in 2002 led to the development of noise-mapping and, in with background noise can significantly increase the day- a few cases, action plans that require noise metrics (EC, night level, even though the sound remains at the background 2002a). Related activities include the HARMONOISE and level most of the time. As for percent highly annoyed, this is IMAGINE projects (http://www.imagine-project.org/). hardly the best measure of satisfaction for areas where quiet and solitude are valued. In addition, it can be difficult to European Metrics (Indicators) measure very low sound pressure levels. A-weighted levels of 40 dB are at the upper end of the range, and lower levels can A-frequency weighting for determining sound levels that be at or even below the levels measurable with conventional have been standardized in the United States and interna- sound-level meters. tionally is widely used in Europe. However, as discussed Nevertheless, some quantification of noise impact is elsewhere in this chapter, frequency weighting alone is not clearly needed in these areas as a basis for establishing enough to define a metric. A Working Group (WG1) that public policy, which usually means regulatory action. The produced a report in 2000, Position Paper on EU Noise In- classic definition of noise is “unwanted sound,” so the source dicators, in support of future European noise policies, identi- of sound must be identified, either as part of the natural fied five criteria for selecting an indicator: validity, practical soundscape or not. Thus, simple metrics like sound pressure applicability, transparency, enforceability, and consistency. level are clearly not appropriate. For example, an airplane Although this report was not an official EU document, the overflight may have a much lower sound pressure level and metrics recommended therein are now widely used (EC, shorter duration than sound from a rushing stream, but the 2000). former is considered noise and the latter is considered sound. WG1 recommended that two indicators, both based on The method of assessment of the noise environment should A-frequency weighting, be used for reporting data on noise also take into account the likely long-term impact on animals exposure. These indicators were designated LEU and LEUN that use, for example, very low level sounds (perhaps inau- but today they are widely known as the day-evening-night dible or unnoticed by people) to locate prey or predators.7 sound level, DENL, and the equivalent sound pressure level during the eight-hour nighttime period, Lnight. The group ex- plained, and questioned, the rationale for using 5 dB as level weighting for the evening period and 10 dB for nighttime. Nighttime was nominally designated as eight hours, from 11:00 p.m. to 7:00 a.m.; daytime, 12 hours; and evening, 7 See Chapter 2 and Appendix B for more on noise metrics in quiet areas.

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 METRICS FOR ASSESSING ENVIRONMENTAL NOISE four hours (with some variation, depending on the country). case of environmental noise and use the same curve irrespec- tive of the population in the situation evaluated. For general purposes, the long-term average A-weighted sound pressure level, LAeq, was used. The WG1 report also recognized that the character of noise (impulsive, tonal, etc.) Nighttime Effects may affect human response. Thus, corrections to the metrics In 2004, WG2 produced Position Paper on Dose-Effect may be necessary, and A-frequency weighting may not be Relationships for Nighttime Noise, again not an official appropriate for measuring low-frequency noise. EU document. In this paper the metric used was Lnight, as The WG1 report was also the basis for metrics specified defined above as the measure for sleep disturbance. Based in the 2002 END that led to noise mapping. The directive on questionnaires, curves similar to Schultz curves were also suggests supplemental metrics based on the WG1 report developed, the ordinate being the percent highly disturbed (EC, 2002a). and the abscissa being the nighttime noise level. An effort was made to relate single events to the nighttime sound level Dose-Effect Relationships (Exposure-Response (EC, 2004). Relationships) Another Working Group (WG2) on Health and Socio- Annoyance and the Microstructure of Noise Exposure Economic Aspects of Noise also produced a report, again Several studies have been published, mostly in connection not official EU policy. In Position Paper on Dose-Response with the EU-funded SILENCE project (www.silence-ip.org), Relationships Between Transportation Noise and Annoyance on the importance of the microstructure of a noise exposure (EC, 2002b), the group recommended that the percent highly situation. The argument is that equivalent levels do not “tell annoyed (%HA) be used as a measure of community re- the full story.” Different traffic noise situations with the same sponse to noise. Updated and modified Schultz curves, based equivalent level may be assessed differently with respect to on the work of Miedema and colleagues (e.g., Miedema annoyance. This is important information for decisions about and Oudshoorn, 2001) for the %HA as a function of day- how to reduce the negative impact of road noise through evening-night sound level, are used to measure road traffic, traffic management measures. Laboratory experiments have rail traffic, and aircraft noise. WG2 also acknowledged the provided several examples: variability from study to study in the mean values in Schultz curves (e.g., Gjestland, 2008) but still supported the use of • An even flow of traffic causes the same annoyance “norm” curves: as when vehicles are clustered, but an even flow is Substantial deviations from the predicted percentage [of] more damaging to mental performance than clustered annoyed persons must be expected for limited groups at traffic. individual sites because random factors, individual and local • A large difference between equivalent level and Lmax circumstances and study characteristics affect the noise an- is more annoying than a small difference. noyance. However, in many cases the prediction on the basis • Trams should receive a 3-dB “bonus” over buses. of a “norm” curve that is valid for the entire population is • Different noises from a rail yard at equal equivalent a more suitable basis for policy than the actual annoyance levels may have a subjective difference of as much as of a particular individual or group. For example, a “norm” curve is useful when exposure limits for dwellings and noise 5 dB. abatement measures are discussed. Equity and consistency require that limits and abatement measures do not depend Recommendations for Future Research in Europe on the particularities of the persons and their actual circum- stances. For similar reasons, a “norm” curve also can be used Research for a Quieter Europe in 00, a report pro- to estimate the number of annoyed persons in the vicinity of duced under the auspices of the CALM Network (2007), an airport, road, or railway when different scenarios concern- provides a strategy for future noise research in the EU. The ing, e.g., extension of these activities or emission reductions report includes an excellent review of EU activities related are to be compared. That the norm curve does not take local to noise and covers a wide variety of future needs, includ- circumstances or reactions to a change in exposure itself into ing noise emissions from various sources and the need for account, is considered to be an advantage for many purposes. perception-based research into the effects of noise. There is Equity and consistency of policy would not be served if in each case the actual annoyance is taken as the (only) basis for one short section on metrics (indicators). these evaluations. The use of “norm curves” or “norm thresh- olds,” which are valid for the entire population (or a particular European Versus Japanese Results on Transportation sensitive subgroup), is common practice when exposures to Noise other environmental pollutants, such as air pollutants or radia- tion, are evaluated. There they are used for the evaluation of A recent Japanese study by Yano et al. (2007) compared an individual situation, irrespective of the population in that the effects of transportation noise in Japan with the EU situation. It is recommended to take the same approach in the

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6 TECHNOLOGY FOR A QUIETER AMERICA results. The effects of road traffic noise are similar, but the way, might affect them. DNL has also been a mechanism by effects of railway noise were quite different (see Figure 3-3). which people could be protected and systematically helped to The authors suggest that the differences may be attributable address problems with environmental noise exposure fairly to the proximity of Japanese homes to railroad tracks (where and equitably. they are subject to vibration as well as noise). Differences in DNL also has some drawbacks. First, there is a great deal the construction of homes may also be a factor. of variability from study to study in the percentage of the Japanese data for aircraft noise are based on one dataset population believed to be “highly annoyed” as a function of of 410 responses around Kumamoto, a small airport, and DNL, which predicts only part of a community’s response may not be representative of noise around Japanese airports to noise. Efforts to develop metrics that can provide a more in general (Yano et al., 2007). There was also an active anti- definitive assessment of community impact are still a topic noise group near Kumamoto airport. However, considering for research and policy debate. the scatter from study to study (e.g., Yano et al., 2007), the Many limitations of a DNL-type metric based on the results of the Kumamoto study may be representative. average A-weighted sound pressure level used to assess environmental noise have been noted: SuMMARy FINDINgS AND RECOMMENDATIONS • DNL is insensitive to the impact of very loud, isolated events. Established and New Environmental Noise Metrics • Fewer loud events can have the same DNL as many Use of the DNL metric has helped policy makers, road quieter events; thus, the impacts of very different planners, airport managers, the public, and others understand soundscapes are described as equal. potential noise impacts on communities and has helped guide • DNL is insensitive to the time when an event occurs noise mitigation efforts around airports, roadways, and rail (e.g., noise early in the night causes different sleep systems. However, DNL has both strengths and weaknesses disturbance than noise early in the morning). as a measure of noise. • The only strong argument for using night and evening The strengths of DNL are that it has become familiar weightings in DNL is based on the fact that average over time, its calculation has been standardized, through nighttime ambient levels are lower than those during experience it has become well understood, and it is now the day. embedded in software used for planning. DNL has made it • Other metrics such as speech interference level and possible to communicate evaluations of noise to the public nighttime levels provide a better measure of annoyance to provide people with a better understanding of how noise with speech interference and conscious awakenings. policy decisions are made and how changes in transportation • DNL is an outdoor noise measure that may not reflect systems, or choosing to live near an airport or a busy high- differences between outdoor sounds and the same sounds heard indoors. • A-weighting does not reflect the results of research studies in psychoacoustics over the past 40 years. • DNL does not take into account other sound character- istics (e.g., tonality and rate of loudness onset) that can Air traffic Road traffic Rail traffic influence annoyance and sleep disturbance levels. Yano et al. Percentage Highly Annoyed Miedema & Vos Although DNL has limitations, it has served as the major environmental noise metric since the early 1970s. Despite the variability in community response, it is clear that the percentage of the population highly annoyed for a DNL of 65 dB is considerably greater than the corresponding percent- age for a DNL of 55 dB. This supports the findings of EPA in the 1970s (EPA, 1974) that a DNL of 55 dB is the level necessary to protect the public health and welfare with an adequate margin of safety. When new metrics are developed and values selected as a matter of public policy, the goal should be to protect a larger fraction of the population than is protected under the value now widely used—the DNL = 65 dB criterion. Many steps Day-Night Average Sound Level (dBA) would have to be taken before a different metric (or set of FIGURE 3-3 Comparison of the present dose-response curves metrics) could be recommended to policymakers. Changing with results from Miedema and Vos. Source: Adapted from Yano to another metric would entail significant effort and cost et al. (2007). Figure_3-3.eps bitmap with vector axis labels

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7 METRICS FOR ASSESSING ENVIRONMENTAL NOISE (e.g., in conducting surveys and development of databases) sounds people hear. A more holistic model of annoyance and would be of limited value unless the new metric offers is also needed that incorporates situational variables that significant benefits over DNL, most importantly in provid- can be used to generate predictions for overall response, as ing a more transparent and definitive connection between well as responses of vulnerable populations (e.g., elderly noise level and annoyance or other effects on people’s lives. people, sick people, children, and noise-sensitive individu- Unfortunately, because of a lack of “real-world” data to test als). International cooperation in this effort will facilitate the performance of metrics, it is difficult to establish their the development of national and international standards for advantages and disadvantages. The situation with respect to calculating metrics and should include open-source code to DNL has been recognized by the FAA, and two meetings facilitate broad implementation of the metrics. Certain mea - have been held—one in August 2009 and one in December sures should be taken to facilitate this development: 2009—to discuss a “roadmap” to improve the situation re- garding noise metrics. 1. The international noise control engineering community A set of metrics, rather than a single metric, to describe should develop an open, collaborative data-sharing different types of outcomes of environmental noise (e.g., environment in which researchers can deposit and number of interruptions of speech, learning impairment in access data from community noise surveys (e.g., data schools, number of additional awakenings) would provide a from surveys of acoustic, environmental, community, multidimensional picture of noise impact and may be the best and transportation systems to support comparisons of approach to informing the public. Supplementary metrics metrics and predictions by models). could make possible predictions of noise from transporta- 2. Policy agencies should conduct extensive surveys tion in sufficient detail to enable the development of noise around at least six U.S. airports to generate high- maps. quality data to populate the database. These surveys When communicating with the public, it might be useful should serve as models of good survey practices, to translate metric values into words (e.g., categories such including data recording and archiving to ensure that as no observed reaction, sporadic complaints, widespread they are useful for future studies. complaints, threats of community action, vigorous commu- nity action) that can be more easily understood than DNL Noise Metrics for Quiet Environments and other numerical metrics. The ability to predict direct health effects of noise (e.g., The impact of man-made noise in national parks and hypertension, speech interference, cognitive impairment, other quiet environments is another parameter that is not sleep disturbance) and the relationship between these effects well modeled by the metrics used to assess the impact of and annoyance requires further study in order to develop new noise around airports or roads. Detection of the sound metrics that account for health effects. and distinguishing between man-made and natural sounds are important because human reactions to man-made and Recommendation 3-1: The federal government (e.g., natural sounds differ. If one goal of the national parks is to agencies of the U.S. Department of Transportation with preserve places of natural beauty, then the natural sound - responsibilities related to noise and the U.S. Department of scape of a park, which is an aspect of its beauty, should Housing and Urban Development) should adopt as a goal also be preserved. the 1974 recommendation of the Environmental Protection In addition, predicting the impact of noise on wildlife in Agency (EPA, 1974) to limit the day-night average sound national parks may require a different kind of metric that level (DNL) to 55 decibels (dB) to protect the public health reflects animals’ hearing systems. Preserving wildlife is es- and welfare. Currently, DNL (DENL in Europe), the ac- sential to preserving the ecostructure of a park. But wildlife cepted metric for characterizing the impact of community preservation will require that animals’ hearing also be pre- noise, shows that a large proportion of the population is served and protected, because many animals depend on their highly annoyed at a DNL of 65 dB or higher. hearing to hunt and to detect potential predators. The U.S. Department of the Interior should fund the development of Recommendation 3-2: R elevant agencies of the fed - metrics to support noise management decisions in national eral government (e.g., agencies of the U.S. Department of parks and other quiet environments. 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