All sounds come from a sound source. It takes energy to produce this sound, and this energy is transmitted through the air in sound waves, or tiny, quick oscillations of pressure just above and just below atmospheric pressure. These oscillations, or sound pressures, impinge on the ear, creating the sound that humans can hear. The denser the air, the better conductor of sound waves, and the less dense air is a poorer conductor of sound waves.
Much of the sound generated by an aircraft is the result of moving air. As air moves through a jet engine, past a propeller, or over a wing, it is deflected and accelerated resulting in turbulence as it mixes with the surrounding air. These changes in airflow can generate significant sound levels. There are many different terms used in the discussion of aircraft noise.
While these sound levels can be measured in many different ways, the Federal Aviation Administration (FAA) provides guidelines for the measurement and presentation of data. The following acoustic terms are defined in easier to understand terms to help visitors to better understand the information presented.
FAA Aircraft Stage Classifications
The FAA classifies aircraft into three stages for clarification: Stage 1, 2, and 3 in order from loudest to quietest. Noise levels for Stage definition of aircraft are measured at three points. These points are designed to measure noise levels for take-off, approach, and flyovers (sideline).
Furthermore, classification is also based on the number of engines. FAA uses FAR Part 36 for further details of Stage clarification and Noise Term Definitions.
A Stage 1 noise level means a take-off, flyover, or approach noise level is greater than the Stage 2 noise limits.
Stage 2 noise limits for airplanes regardless of the number of engines are as follows:
For Take-off: 108 Effective Perceived Noise (EPNdB) for maximum weights of 600,000 pounds or more, reduced by 5 EPNdB per halving of the 600,000 pounds maximum weight down to 93 EPNdB for maximum weights of 75,000 pounds and less.
For Sideline and Approach: 108 EPNdB for maximum weights of 600,000 pounds or more, reduced by 2 EPNdB per halving of the 600,000 pounds maximum weight down to 102 EPNdB for maximum weights of 75,000 pounds or less.
Stage 3 noise limits are as follows:
For Take-off: airplanes with more than 3 engines 106 EPNdB for maximum weights of 850,000 pounds or more, reduced by 4 EPNdB per halving of the 850,000 pounds maximum weight down to 89 EPNdB for maximum weights of 44,673 pounds or less.
For Take-off: airplanes with 3 engines 104 EPNdB for maximum weights of 850,000 pounds or more, reduced by 4 EPNdB per halving of the 850,000 pounds maximum weight down to 89 EPNdB for maximum weights of 63,177 pounds or less.
For Take-off: airplanes with fewer than 3 engines 101 EPNdB for maximum weights of 850,000 pounds or more, reduced by 4 EPNdB per halving of the 850,000 pounds maximum weight down to 89 EPNdB for maximum weights of 106,250 pounds or less.
For Sideline: regardless of the number of engines 103 EPNdB for maximum weights of 882,000 pounds or more, reduced by 2.56 EPNdB per halving of the 882,000 pounds maximum weight down to 94 EPNdB for maximum weights of 77,200 pounds or less.
For Approach: regardless of the number of engines 105 EPNdB for maximum weights of 617,300 pounds or more, reduced by 2.33 EPNdB per halving of the 617,300 pounds maximum weight down to 98 EPNdB for maximum weights of 77,200 pounds or less.
FAA Noise Regulations
All Airports are owned and operated by either city, county or regional public agencies and licensed by the Federal Aviation Administration (FAA) and governed by comprehensive regulations. A few of the Federal Aviation Regulations (FARs) pertinent to San Antonio International Airport noise program are under Title 14 of the U.S. Code of Federal Regulations.
The FAA publishes material of a policy, guidance or informational nature in documents called Advisory Circulars. If you’re interested in learning more about what advisory circulars are available, please contact the FAA publishing office at:
Airport Noise and Capacity Act of 1990
The Airport Noise and Capacity Act of 1990 was a pivotal piece of legislation because it recognized the need for a national aviation noise policy. A critical part of the statute was direction to eliminate the use of Stage 2 aircraft weighing more than 75,000 pounds in the contiguous U.S. after December 31, 1999.
The final revision and rules of this act were established in September 1991 as Part 91. Another important element of the noise policy is the Notice and Approval Airport Noise and Access Restrictions, Part 161. This FAR establishes a program for reviewing airport noise and access restrictions on the use of Stage 2 and Stage 3 aircraft.
Far Part 36
Part 36 sets noise standards that aircraft must meet to obtain type and airworthiness certificates for operation in the U.S. First created in 1977 for application to civil subsonic turbojets and large (over 12,500 pounds) propeller-driven aircraft,
the government has subsequently amended the regulation to address civil supersonic aircraft, small propeller aircraft (less than or equal to 12,500 pounds), and helicopters. Measurement results for individual aircraft types are provided in Advisory Circulars.
Part 77 covers objects affecting navigable airspace. Federal Regulation 49 CFR Part 77 establishes standards and notification requirements for objects affecting navigable airspace. It evaluates the effect of the construction or alteration on operating procedures and determines the potential hazardous effect of the proposed construction or alterations on air navigation. It identifies mitigating measures to enhance safe air navigation and charts new man-made or natural objects. FAR Part 77 allows the FAA to identify potential aeronautical hazards in advance thus preventing or minimizing the adverse impacts to the safe and efficient use of navigable airspace. Once the FAA as completed an aeronautical study, a determination is made regarding the impact to air navigation.
One of three responses is typically issued:
The subject construction did not exceed obstruction standards and marking/lighting is not required.
The proposed construction/alteration would be acceptable contingent upon implementing mitigating measures (marking and lighting, etc.)
The proposed construction/alteration is determined to be a hazard and is thus objectionable. The reasons for this determination are outlined to the proponent.
Part 77 determines if an object constitutes an obstruction to navigation if it is either 200 ft. above ground level or 200 ft. above the airport elevation (whichever is greater) up to 3 miles (for runway lengths > 3200 ft.) from the airport. This specification increases 100 ft. for every mile up to 500 ft. at 6 miles from the ARP (airport reference point).
An object constitutes an obstruction if
- It is 500 ft. or more above ground level at the object site.
- If penetrates an imaginary surface (a function of the precision of the runway)
- If penetrates the terminal obstacle clearance area (includes initial approach segment)
- If penetrates the enroute obstacle clearance area (includes turn and termination areas of federal airways)
Far Part 91
FAR Part 91 limits civil aircraft operations in the U.S. based on Part 36 certification status. The most important aspect of Part 91 is that it led to elimination and ongoing prohibition of all Stage 1 operations in the U.S. in civil subsonic turbojets over 75,000 pounds. The new Part 91 includes a phased schedule to achieve the 1999 Stage 2 operations elimination.
Part 91 sets forth final rules for the elimination of Stage 2 aircraft weighing more than 75,000 pounds in the contiguous United States after December 31, 1999. This revision passed in 1990, under the Airport Noise and Capacity Act of 1990.
Far Part 150
This regulation was made in response to the need of setting forth national standards for identifying noise and land use incompatibilities and developing programs to eliminate them. It was established under the Aviation Safety and Noise Abatement Act of 1979.
FAR Part 150 prescribes specific standards and systems for measuring noise by estimating the cumulative noise exposure using computer models. These standards classify the noise exposure (including instantaneous noise levels, single event noise levels, and cumulative exposure) and help coordinate noise compatibility program development with land use planning officials and other interested parties.
Documentation of the analytical process and development of any noise compatibility programs must be submitted to the FAA and allow for the public review process. The FAA will either approve or disapprove the submission.
Far Part 161
Part 61 establishes a program for reviewing airport noise and access restrictions on the use of Stage 2 and Stage 3 aircraft. The FAR requires that airport proprietors examine the impacts of a proposed noise or access restriction within an “airport noise study area”.
That area must include all property which lies within the Ldn 65 dB contours. Furthermore, in determining whether land use around an airport is compatible with airport noise, an airport proprietor must use the land use compatibility guidelines set forth in FAR Part 150
How the FAA Measures Aircraft Noise
Noise -Unwanted Sound
Noise is usually regarded as unwanted sound – sound that disturbs our routine activities or peace and quiet, and perhaps causes a feeling of annoyance. Which sounds are noise is obvious to each listener, and he or she has no need to measure it. It’s there, and it’s bothersome. But those who work to abate noise, to minimize its effects, or to develop quieter vehicles, need to measure noise. And that is not an easy task. Consider sounds typical of a suburban neighborhood on a “quiet” afternoon.
The sound levels are plotted in units of A-weighted decibels (abbreviated dB, or sometimes dBA), a logarithmic measure of the magnitude of a sound as the average person hears it.
The “A-weighting” accounts for the fact that humans do not hear low frequencies and high frequencies as well as they hear middle frequencies, and it corrects for the relative efficiency of the human ear at the different frequencies. A logarithmic measure is used in order to cover efficiently the wide range of sound magnitudes encountered daily.
In this example, the background, or residual sound level in the absence of any identifiable noise sources, is about 45 db. During roughly three-quarters of the time, the sound level is 50 dB or less. The highest sound level, caused by a nearby motorcycle, is 73 dB, while an aircraft generates a maximum sound level of about 68 dB. The question then becomes: how do we “measure” this variable community noise?
MAXIMUM SOUND LEVELS
One obvious way of describing this sound environment is to measure the maximum sound level – in this case, the nearby motorcycle at 73 dB. But the aircraft sound, although not as loud as the motorcycle, lasts longer. Studies have shown that human response to noise involves both the maximum level and its duration, so the maximum sound level alone is not sufficient to evaluate the effect of noise on people.
SOUND EXPOSURE LEVEL
A second way of describing this sound environment is to measure the sound exposure level (abbreviated SEL), which is the total sound energy of a single sound event and takes into account both its intensity and duration. One way to under-stand SEI, is to think of it as the sound level you would experience if all of the sound energy of a sound event occurred in one second. This normalization to a duration of one second allows the direct comparison of sounds of different durations. In sample time history, a motorcycle generates an SEL of about 77 dB, while an aircraft generates an SEL of about 81 dB.
EQUIVALENT SOUND LEVEL
The maximum sound levels and sound exposure levels measure individual sound events that may occur only once or may occur several times during the day in our neighborhood. The number of times, these events occur is also important in measuring the noise environment. One way to describe this factor might be to count the number of events per day for which the SELs exceed 80 dB, plus the number which exceed 75 dB, plus the number which exceed 70 dB, and so on. A more efficient way to describe both the number of such events and the sound exposure level of each is the time-average of the total sound energy over a specified period, referred to as the equivalent sound level (symbolized Leq). In the example shown in Figure 1, the time-average sound level is roughly 56 dB. This accounts for all of the sound energy during the sample period, and provides a single-number descriptor in terms of sound energy per second.
DAY- NIGHT AVERAGE SOUND LEVEL
One additional factor is also important in “measuring” a sound environment – the occurrence of sound events during nighttime. People are normally more sensitive to intrusive sound events at night, and the background sound levels are normally lower at night because of decreased human activity. Therefore a “penalty” may be added to sound levels which occur during night hours, to include these factors. By convention, a 10 dB penalty is added to sound levels occurring between 10:00 p.m. and 7:00 a.m. the following morning. The 24-hour average sound level, including this 10 dB penalty, is known as the day-night average sound level (abbreviated DNL). This 10 dB penalty means that one nighttime sound event is equivalent to 10 daytime events of the same level.
Annoyance is a summary measure of the general, adverse reactions of people to noises which disrupt their daily activities telephone conversations, TV/radio listening, sleep, or simple tranquility. Currently, the best measure of this reaction is the percentage of people who characterize themselves as – “highly annoyed” by long-term exposure to their noise environments. Extensive research has found that day-night average sound level correlates very well with community annoyance from most environmental noise sources.
Noise Compatibility Guidelines
Using this research, federal agencies have adopted certain guidelines for compatible land uses and environmental sound levels. Land use is normally determined by property zoning, such as residential, industrial, or commercial. Noise levels that are unacceptable for homes may be quite acceptable for stores or factories. The Federal Aviation Administration has issued these guidelines as part of its Airport Noise Compatibility Program, found in Part 150 of the Federal Aviation Regulations.
In general, most land uses are considered to be compatible with DNLs that do not exceed 65 dB, although Part 150 declares that “acceptable” sound levels should be subject to local conditions and community decisions. Nevertheless, a DNL of 65 dB is generally identified as the threshold level of aviation noise, and other sources of community noise, which are “significant”.
In adopting a threshold criterion for noise impact, we must keep several important factors in mind. First, a day-night average sound level below 65 dB does not mean that no one is annoyed by that level of noise from transportation sources. To the contrary, as shown in Figure 2, about 12 percent of people living with a DNL of 65 dB report themselves to be “highly annoyed”. About 3 percent are highly annoyed at a DNL of 55 dB. This is understandable, because the same research on noise effects has found that the physical amount of noise is only one element in feelings of annoyance with environmental noise. Activities which may be disrupted by noise events (study, conversation, listening to music, watching TV, solitude, etc.): beliefs that such noise could be better controlled; attitudes toward the noise maker, and personal fears regarding the source of the noise, are all important factors in people’s perception of annoyance. Additionally, a small percentage of people are simply more sensitive to noise than most other people, while a small percentage are little annoyed even at high noise levels. The combination of these factors causes different people to interpret sounds as “unwanted” noise in different ways. A measure of noise impact, such as day-night average sound level, provides a reliable indicator of overall community response, but does not tell how any single individual to respond.
As a result, there is probably no minimum level of transportation noise at which no one is annoyed. General guidelines for noise compatibility identify day-night average sound levels between 55 and 65 dB as “moderate exposure” and as generally acceptable for residential use. Above a DNL of 65 dB, these guidelines identify the noise impact as “significant”, and this designation is currently a factor in decisions to provide federal funds for mitigation projects.
Because DNL combines both the intensity and number of single noise events (along with nighttime weighting), it also is not a good estimator of the single-event sound levels which are experienced. For example, a DNL of 65 dB may be generated by any of the following combinations of average sound exposure level and the effective number of those events, where “effective” number is the sum of the number of daytime events plus 10 times the number of nighttime events:
Consider two communities: one near a large airport, the other near a small one. Both are exposed to a DNI, of 65 dB. Although people near the small airport experience only 50 aircraft operations in a day, the average SEL of each of these is about 97 dB. On the other hand, the community near the large airport is impacted by 500 daily operations, but each of these has an average SEL of about 87 dB. This does not invalidate the usefulness of the DNL measure, but should be considered, for example, in determining needs for structural sound insulation.
Some criticism of DNL stems from beliefs that the levels identified with land-use compatibility are too high. Any compatibility guideline, such as a DNL of 65 dB, must represent a balance between that level which is most desirable to protect communities and that which can be achieved with cost-effective mitigation measures and available technology. There is no single criterion which can fit all airports and all communities. Local communities may choose to mitigate impacts below a DNL of 65 dB.
A time-average measure of noise impact, such as day-night average sound level, is also criticized because people feel that they are annoyed by individual sound events, rather than some “fictitious” average level. Clearly, people are bothered by individual noise events, but their sense of annoyance increases with the number of those noise events, and those which occur late at night.
DNL provides a combined “measure” of these factors which can be used to evaluate existing and predicted future conditions on an unambiguous, single-number basis. Other measures, such as maximum sound level, or sound exposure level, give valuable supplemental information in analyzing airport noise. For example, as noted above, in designing sound insulation for dwellings and schools, single-event measures are necessary. Nevertheless, day-night average sound level remains the best single measure for assessing the effects of airport noise on communities, and allows a standardized and effective means for measuring transportation noise.