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 1] The number of times the waves crest within one second is referred to as the frequency of the sound, and is expressed in cycles per second, called hertz. The general range of human hearing is between 20 to 20,000 hertz. However, the clearest range of human hearing is between 1,000 and 4,000 hertz.

[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.

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.

[FN 4] T.J. Schultz, "Synthesis of Social Surveys on Noise Annoyance," Journal of the Acoustical Society of America 64(2) (1978), pp. 377-405.

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]

[FN 5] FAA performed a variety of calculations based on input data we selected to compare single event and community exposure measurements methods in the following three contexts: (1) a single set of measures to illustrate the outputs of the different methods under the same airport conditions; (2) a series of measures using changing flight schedules to illustrate the impact of the time of day when flights occur; and (3) a series of measures using changing numbers of total aircraft operations to illustrate the impact of the number of takeoffs and landings. FAA calculated the measurements using its Integrated Noise Model (Version 6) -- FAA’s preferred computer model for measuring airport-related noise when conducting environmental impact or land use compatibility analyses. App. V describes the airport model.

[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.

[FN 7] Federal Agency Review of Selected Airport Noise Analysis Issues (Federal Interagency Committee On Noise; Aug. 1992).

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.

[FN 8] The operations profile for an average day is based on operations that occur during a calendar year period.

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.

[FN 9] According to FAA officials, the value of the added weight differs for airport calculations depending on whether it is decibels or "equivalent number of operations." The State of California, which uses the Community Noise Equivalent Level method, equates each evening flight to three 3 daytime flights. This results in added weight of 4.77 decibels for each flight between 7 p.m. and 10 p.m. The Community Noise Equivalent Level method may also be calculated by applying a 5-decibel penalty, which would equate each flight to 3.1623 daytime operations. FAA’s Integrated Noise Model uses the approach applied by California.

• 500 aircraft operations with an average sound exposure level of 87.4 decibels;
• 100 aircraft operations with an average sound exposure level of 94.4 decibels; and
• 50 aircraft operations with an average sound exposure level of 97.4 decibels.

[FN 10] See FAA Brochure entitle Aircraft Noise: How We Measure It and Assess Its Impact.

[FN 11] All three methods produced a similar proportional distribution of land areas exposed to the various noise levels. The area exposed to noise levels from 65 to 85 decibels is generally 20 percent or less of the total area within the 55 to 85 decibel noise exposure range. The portion exposed to 60 to 64 decibels is about 25 percent of the total area, while the portion exposed to 55 to 59 decibels is generally between 55 percent and 60 percent of the total area.

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 12] The following flight schedules were used: (1) all flights in the daytime (7 a.m. to 7 p.m.), (2) all flights in the evening (7 p.m. to 10 p.m.), (3) all flights in the nighttime (10 p.m. to 7 a.m.), (4) half of the flights in the daytime and half in the evening, (5) half of the flights in the evening and half in the nighttime, (6) half of the flights in the daytime and half in the nighttime, and (7) half of the flights during the daytime, one-fourth in the evening, and one-fourth in the nighttime.

[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.

[FN 14] We examined noise measures when total aircraft takeoffs and landings equaled 26 operations, 78 operations, 234 operations, 468 operations, 702 operations, 1,056 operations, and 1,586 operations. The different levels of total operations illustrate the impact of the growth in aircraft operations at an airport, and the differences in noise impacts of airports of different sizes, holding all other elements constant. Representatives of the Air Transport Association of America, Inc. noted that if quieter aircraft replace noisier aircraft, increasing the number of aircraft operations will not necessarily expand noise contours and may reduce them. The impact of the quieter aircraft on noise contours will depend, however, on the extent to which aircraft are replaced, the extent to which operations increase, and when those operations occur.

[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.

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.