|The following is from Chapters 3-4, Aviation and the Environment: FAA's Role in Major Airport Noise Programs. U.S. General Accounting Office, GAO/RCED-00-98 (April 2000). See the full report for figures, which are not reproduced here. [The link is to a file in Adobe "pdf" format.]|
Methods for measuring airport-related noise assess noise either from a single takeoff or landing or from the cumulative average noise that nearby communities are exposed to over time. Required by law to select a single method for measuring the impact of airport-related noise on communities, FAA chose a method that measures community exposure levels and that gives greater weight to the impact of flights occurring during the nighttime.
While subsequent studies have confirmed that this method best meets the statutory requirement that FAA establish a single system for determining the exposure of people to airport-related noise, a federal interagency committee addressing airport-related noise issues found that supplemental information, such as measures of noise from a single aircraft takeoff or landing, is also useful in explaining the noise that people are likely to hear. In addition, experts and community groups believe FAA’s chosen method provides insufficient information because it does not effectively convey to people what they can actually expect to hear in any given area.
Measuring Sound and Its Effects
To understand the methods used to measure noise, it is necessary to have some understanding of how sound is measured and how it affects humans. Some basic concepts include (1) sound waves and their measurement in decibels, (2) human ability to hear the entire range of sounds made, and (3) noise as a source of interference in people’s activities.
First, sound radiates in "waves" from its source and decreases in loudness the further the listener is from the source. [FN 1] As sound radiates from its source, it forms a sphere of sound energy. Sound waves exert sound pressure, commonly called a "sound level" or "noise level," that is measured in decibels. [FN 2] The higher the number of decibels, the louder the sound appears to someone hearing it. But because decibel levels are measured logarithmically, an increase of only 10 decibels -- for example, from 50 decibels to 60 decibels -- doubles the loudness that people believe they hear. [FN 3] Continuing the increase from 60 to 70 decibels would again double the perceived loudness of the sound. Which sounds are considered to be noise, however, is subjective.
[FN 2] A decibel is a unit of sound pressure used to measure noise.
[FN 3] An increase of 3 decibels represents a doubling of sound energy, but an increase of 10 decibels corresponds to the perception by people that the sound level has doubled.
Second, while the human ear can hear a broad range of sounds, it cannot hear all sounds. Sounds with very low pitches (low frequencies) and sounds with extremely high pitches (high frequencies) are generally outside the hearing range of humans. Because of this, environmental noise is usually measured in "A-weighted" decibels. The A-weighted decibel unit focuses on those sounds the human ear hears most clearly and deemphasizes those sounds that humans generally do not hear as clearly. Table 2 illustrates the typical sound levels of some common events.
|Sound level in|
|Rock band (indoors)||108-114|
|Dishwasher on rinse cycle at 10 feet||60|
|Bird calls (outdoors)||44|
|SOURCE: Federal Interagency Review of Selected Airport Noise Analysis Issues (Federal Interagency Committee on Noise; August 1992).|
Finally, the impact of noise on communities is usually analyzed or described in terms of the extent to which it annoys people. Annoyance refers to the degree to which noise interferes with activities such as sleep, relaxation, speech, television, school, and business operations. While it is difficult to predict how an individual might respond to, or be affected by, various sounds or noises, some studies indicate that it is possible to estimate what proportion of a population group will be "highly annoyed" by various sound levels created by transportation activities. The findings of a 1978 study that related transportation noise exposure to annoyance in communities has become the generally accepted model for assessing the effects of long-term noise exposure on communities. [FN 4] According to this study, when sound exposure levels are measured by a method that assigns additional weight to sounds occurring between 10 p.m. and 7 a.m., and those sound levels exceed 65 decibels, individuals report a noticeable increase in annoyance.
Measures of Noise Identify Noise Levels of Single Aircraft Operations and Community Exposure
Methods for measuring airport-related noise provide different kinds of information. First, airport-related noise can be measured from single events -- such as an individual aircraft’s takeoff or landing -- or as the cumulative average level of noise that communities near airports are exposed to over time. Principal methods for measuring cumulative average noise levels identify geographic areas exposed to the same noise levels but apply different weights to flights occurring during different times of the day.
Single-Event Measures Provide Short-term Information
The noise from a single takeoff or landing usually starts when the sound can be heard above the background noise; it reaches a maximum sound level and then recedes until the sound is hidden below the background noise level. One of two measures of the noise from a single takeoff or landing is commonly used: (1) the Maximum Sound Level method, which identifies the maximum sound level produced by the event, and (2) the Sound Exposure Level method, which measures the total sound energy that a listener is exposed to during a single event.
The Maximum Sound Level method is usually expressed in A-weighted decibels when measuring aircraft events. It does not provide any information, however, about the duration of the event or the amount of sound energy produced.
In contrast, the Sound Exposure Level method measures all of the sound energy from the duration of a takeoff or landing to produce the sound level that a person is exposed to from that event. Thus, this method reflects both the intensity and the duration of the sound that the takeoff or landing produces. For aircraft events, this method also usually uses A-weighted decibels. Because this method measures the cumulative sound energy averaged over a single second of time, the sound exposure level for an event that lasts longer than one second will be higher than the maximum sound level for that same event. Also, two events can have the same maximum sound level but different sound exposure levels. The event that lasts the longest will have a higher decibel measure than the shorter event, even though both may have the same maximum sound level.
To compare the different kinds of information these methods provide, FAA calculated maximum sound levels and sound exposure levels for single aircraft takeoffs and landings using an airport model that we designed. [FN 5] The results illustrate the different concepts embodied in the two measures of single events. Figure 6 illustrates the measures produced by both methods at one-half mile from the runway and at 1-mile intervals from the runway, for both approach and takeoff operations, for the Boeing 747 and C140 aircraft included in our model. Similar figures for the four other aircraft in our model appear in appendix VI. [FN 6]
|Figure 6: Single-Event Noise Levels Using the Maximum Sound Level Method and the Sound Exposure Level Method, Approach and Takeoff -- Boeing 747 and C140 Aircraft.|
[FN 6] A third method for examining single events, known as the Third Octave Band Sound Pressure Level method, separates the noise from a single event into about 30 segments covering the full range of noise generated, including the low and high frequency sounds that the human ear generally does not hear as well. Because it covers the full range of sound, this method does not use A-weighted decibels. However, according to a noise expert, A-weighted sound levels can be, and often are, computed from one-third octave band sound levels. FAA’s Integrated Noise Model does not produce measures for this method.
Noise in Communities Is Measured in Terms of Overall Exposure
The level of noise from airports that nearby communities are exposed to depends on several factors, including the types of aircraft using the airport, the overall number of takeoffs and landings, the time of day those aircraft operations occur, the runways that are used, weather conditions, and airport-specific flight procedures that affect the noise produced by a takeoff or landing. There are two approaches to measuring community exposure to noise: (1) identifying geographic areas on a map that are exposed to the same noise levels or (2) determining the length of time that a specific geographic area is exposed to particular noise levels.
The three main methods for measuring airport-related noise levels that nearby communities are exposed to include (1) the Equivalent Sound Level method; (2) the Day-Night Sound Level method; and (3) the Community Noise Equivalent Level method. These methods provide long-term, or cumulative, measures of exposure to noise. For each method, the key factors that determine the noise exposure level affecting a community are the types of aircraft using the airport, the number and type of engines on an aircraft, the number of takeoffs and landings that occur during an average day, [FN 8] and the time of day during which those aircraft operations occur. The measures are generally presented in the form of "noise contours" on maps -- lines around an airport that connect all the areas exposed to the same average sound level. A series of contours are drawn, usually at 5-decibel decrements from the airport, to produce a map that looks similar to a land elevation map. All three methods incorporate both the intensity of sounds produced by single events and the average frequency of those events.
The second method -- the Day-Night Sound Level -- is the same as the Equivalent Sound Level method for a 24-hour period, but it gives greater weight to flights occurring during the nighttime -- between 10 p.m. and 7 a.m. Additional weight is given to nighttime flights because they are more likely to interrupt sleep, relaxation, or other activities and because the background noise level during those hours is lower. To reflect that greater impact, the Day-Night Sound Level method equates 1 nighttime aircraft operation to 10 equivalent daytime operations. This effectively adds 10 decibels to the noise produced by each takeoff or landing that occurs during those nighttime hours. That is, the noise impact of each single nighttime takeoff or landing is reflected in the noise exposure level as if it were 10 daytime takeoffs or landings. For example, if eight takeoffs and eight landings occur between 7 a.m. and 10 p.m., they are reflected in the noise exposure level as 16 aircraft operations. If those same eight takeoffs and eight landings all occur between 10 p.m. and 7 a.m., they are reflected in the noise exposure levels as the equivalent of 160 aircraft operations.
Finally, the Community Noise Equivalent Level modifies the Day-Night Sound Level method by adding additional weight to flights occurring between the evening hours of 7 p.m. and 10 p.m. to account for an assumption that greater interference with activities may be occurring during the early evening than during the daytime. [FN 9] The second and third methods are considered to have only small differences.
|Figure 7: Noise Contours for Sound Equivalent Level, Day-Night Sound Level, and Community Noise Equivalent Level Measurement Methods, at 75, 70, 65, 60, and 55 A-weighted Decibels.|
In the first scenario, our model illustrated the effect of assigning additional weight to flights occurring during different times of the day. In this scenario, FAA calculated the noise exposure levels for seven different flight schedules. [FN 12] All three methods produced the exact same contours when all flights occurred during the day because no method applies additional weighting to daytime flights. However, when all flights occurred during the nighttime, both the Day-Night Sound Level and the Community Noise Equivalent Level produced contours that quadrupled the size of the areas exposed to the different noise levels. [FN 13] Table 3 illustrates the impact of changing flight schedules.
[FN 13] Scheduling all flights at night produces the same contours for both the Day-Night Sound Level method and the Community Noise Equivalent Level method because there are no flights scheduled during the evening, when the latter method applies additional weighting to flights.
|Method||All daytime flights||All evening flights||All nightime flights|
|Equivalent Sound Level||Areas exposed to noise (a)||No change||No change|
|Day-Night Sound Level||Areas exposed to noise (a)||No change||More than quadrupled size of areas|
|Community Noise Equivalent||Areas exposed to noise (a)||Doubled size of areas exposed to noise||More than quadrupled size of areas|
|(a) When all flights occurred during the day, all three methods produced the exact same size areas exposed to the various noise levels.|
[FN 15] The size of the geographic areas affected by the Community Noise Equivalent Level method at each contour level ranged from 3.6 percent to 4.2 percent greater than the areas affected by the Day-Night Sound Level method.
Two other measurement methods can provide additional kinds of information about the noise exposure of a community. The Time-Above method can identify how much time during a designated time period -- such as a day -- the noise exposure levels will exceed a specified decibel level. The sound level must be specified -- for example, 60 decibels. This method can then determine the length of time during a 24-hour period that noise levels will exceed 60 decibels.
To illustrate the Time-Above method, our model produced data on how many minutes in a 24-hour day the noise levels would be above 60 and 80 decibels at points one-half mile from each end of the runway and at 1-mile increments from the runway for both approach and takeoff operations. Table 4 illustrates the measures.
|Minutes per day|
above noise level--takeoff
|Minutes per day|
above noise level--landing
Another variation of this kind of information is the Lpercent method, which identifies the noise level exceeded for a portion of a time period. The portion must be specified -- for example, only 15 percent of a day. This approach might determine, then, that for 15 percent of the day, the noise level exceeded 60 decibels -- that is, for the rest of the day the noise level was at or below 60 decibels. FAA’s Integrated Noise Model does not produce measures using this method. Neither the Time-Above method nor this method identifies the time of day the higher noise levels will occur.