Noise 101

Glossary of Terms and Acronyms

Click here to see a glossary of terms and acronyms related to aviation and aircraft noise.

Introduction to Sound

Sound is created by a vibrating source that induces vibrations in the air. The vibration produces alternating bands of relatively dense and sparse particles of air, spreading outward from the source like ripples on a pond. Sound waves dissipate with increasing distance from the source. Sound waves can also be reflected, diffracted, refracted or scattered. When the source stops vibrating, the sound waves disappear almost instantly and the sound ceases.

Sound can be defined in terms of three components:

  1. Level (amplitude)

    The level of sound is measured by the difference between atmospheric pressure (without the sound) and the total pressure (with the sound). Amplitude of sound is like the relative height of the ripples caused by the stone thrown into the water. Although physicists typically measure pressure using the linear Pascal scale, sound is measured using the logarithmic decibel (dB) scale. By definition, a 10 dB increase in sound is equal to a tenfold (10X) increase in the mean square sound pressure of the reference sound.

  2. Pitch (frequency)

    The pitch (or frequency) of sound can vary greatly from a low-pitched rumble to a shrill whistle. Consider the analogy of ripples in a pond; high frequency sounds are vibrations with tightly spaced ripples, while low rumbles are vibrations with widely spaced ripples. The rate at which a source vibrates determines the frequency. The rate of vibration is measured in units called hertz (Hz) – the number of cycles, or waves, per second. The ability to hear a sound depends greatly on the frequency composition. Humans hear sounds best at frequencies between 1,000 and 6,000 Hz.

  3. Duration (time pattern)

    The duration of sounds – the patterns of loudness and pitch over time – can vary greatly. Sounds can be classified as continuous like a waterfall, impulsive like a firecracker, or intermittent like aircraft overflights. Intermittent sounds are produced for relatively short periods, with the instantaneous sound level during the event roughly appearing as a bell-shaped curve. An aircraft noise event is characterized by the period during which it rises above the background sound level, reaches its peak, and then recedes below the background level.

Click here to see a ‘Comparison of Sound’ exhibit for both common indoor and common outdoor sound levels. The smallest detectable change by a human ear is +/- 1 dB (laboratory setting). A change of +/- 3 dB is noticeable to most people. Adding two equally like sounds adds a 3 dB increase. Generally, in a community setting, it takes a noise level difference of more than 3 dB for nearly every listener to tell which noise event was louder, and this applies to two events that follow closely in time. A +/- 5 dB change is readily noticeable while +/- 10 dB change is judged by most people as a doubling (when the noise level is increased) or a halving (when the noise level is decreased) of the loudness of the sound.

Introduction to Noise

Noise is a sound that is unpleasant, unexpected, or unwanted. Noise can be produced by many sources – a running engine, an aircraft overhead, an operating machine tool, etc.

Click here to review a background paper regarding the principles of noise, noise analysis and modeling, as well as the preparation of airport noise exposure maps and how the estimates of noise impacts inside a 65 DNL noise contour are determined.

The DNL Metric and its History

The day-night average sound level (DNL) metric describes the total noise exposure during a given period. DNL can only be applied to a 24-hour period and is defined by federal regulation at 14 CFR 150.7. In computing DNL, an extra weighting of 10 decibels (dB) is assigned to any sound levels occurring between the hours of 10:00:00 p.m. and 6:59:59 a.m. This penalty is intended to account for the greater annoyance that nighttime noise is presumed to cause for most people. Recalling the logarithmic nature of the dB scale, this extra weight treats one nighttime noise event as the equivalent to ten daytime events of the same magnitude. For this reason DNL values are strongly influenced by the loud events. For example, 30 seconds of sound of 100 dB, followed by 23 hours, 59 minutes, and 30 seconds of silence would compute to a DNL value of 65 dB. If the 30 seconds occurred at night, the same example would yield a DNL value of 75 dB.

DNL is the standard metric used for environmental noise analysis in the U.S. This practice originated with the U.S. Environmental Protection Agency’s (USEPA’s) effort to comply with the Noise Control Act of 1972. The USEPA designated a task group to “consider the characterization of the impact of airport community noise and develop a community noise exposure measure.” The task group recommended using the DNL metric. The USEPA accepted the recommendation in 1974, based on the considerations listed below. Click here to review the March 1974 USEPA report.

  1. The measure is applicable to the evaluation of pervasive, long-term noise in various defined areas and under various conditions over long periods of time.
  2. The measure correlates well with known effects of the noise environment on individuals and the public.
  3. The measure is simple, practical, and accurate. 5. The metric at a given location is predictable, within an acceptable tolerance, from knowledge of the physical events producing the noise.

Soon thereafter, other federal agencies adopted the use of DNL. At about the same time, the Acoustical Society of America developed a standard which established DNL as the preferred metric for outdoor environments (ANSI S3.23-1980). This standard was reevaluated in 1990, and the same conclusions were reached regarding the use of DNL (ANSI S12.40-1990).

In 1980, the Federal Interagency Committee on Urban Noise (FICUN) met to consolidate federal guidance on incorporating noise considerations in local land use planning; participating federal agencies included the Federal Aviation Administration (FAA) and the USEPA. The committee selected DNL as the best noise metric for this purpose, thus endorsing the earlier work of the USEPA and making it applicable to all Federal agencies. Click here to review the June 1980 FICUN report.

In 1981, the FAA adopted the DNL 65 dB noise metric (often written as 65 DNL) and related land use compatibility guidelines in Part 150 of Title 14 of the Code of Federal Regulations (14 CFR Part 150) in response to the Aviation Safety and Noise Abatement Act of 1979 (ASNA) and the recommendations of the USEPA and FICUN. The Part 150 regulations were finalized in 1984 and became effective in 1985.

In the early 1990s, Congress authorized the creation of a new interagency committee to study airport noise issues. The Federal Interagency Committee on Noise (FICON) was formed with membership from the FAA, the USEPA, and other federal agencies. FICON concluded in its 1992 report that Federal agencies should “continue the use of the DNL metric as the principal means for describing long term noise exposure of civil and military aircraft operations.” FICON further concluded that there were no new descriptors or metrics of sufficient scientific standing to substitute for the DNL cumulative noise exposure metric. Click here to review the August 1992 FICON report.

In 1993, the FAA issued its Report to Congress on Effects of Airport Noise. Regarding DNL, the FAA stated, “Overall, the best measure of the social, economic, and health effects of airport noise on communities is the Day-Night Average Sound Level (DNL).” In that report, the FAA also committed to the establishment of an interagency committee and subsequently convened the Federal Interagency Committee on Aviation Noise (FICAN) in November 1993; participating federal agencies include the FAA and the USEPA. Since that time, FICAN has served as the federal government’s forum for aviation noise research and development.

Airport Noise Contours

Currently, noise contours for Midway are computed using the Integrated Noise Model (INM). The INM was developed under the guidance of the FAA and since 1978 was the FAA's standard tool for determining the predicted noise impact in the vicinity of airports. The noise pattern calculated by the INM for an airport is a function of several factors, including: the number of aircraft operations during the period evaluated, the types of aircraft flown, the time of day when they are flown, the way they are flown, how frequently each runway is used for landing and takeoff, and the routes of flight used to and from the runways. Substantial variations in any one of these factors, when extended over a long period of time, may cause marked changes to the noise pattern.

Click here to see a map depicting historic changes in noise contours for Midway.

The FAA will allow the use of INM for existing airport developments, and will require the use of AEDT version 2b or higher for new airport development projects initiated after May 29, 2015.

Aircraft Noise Footprints

An aircraft noise footprint illustrates the noise levels produced by a specific aircraft type during landing and takeoff. The noise footprint represents the maximum sound level experienced on the ground as the aircraft flies over.

Click here to see an exhibit depicting noise footprints for specific aircraft at Midway.
Click here​ to see a video depicting historic changes in noise contours for Midway.

Quantifying Noise Exposure

Noise exposure can be quantified using measurements and modeling. Measuring sound levels will accurately tell us:

  • The sound levels at a specific location for the time period measurements were made
  • A historical record of the sound levels at a specific location
  • Historical trends

Measurements, however, do not predict future noise levels. Modeling sound exposure will accurately tell us:

  • The projected sound levels over broad geographical areas.

The FAA will allow the use of INM for existing airport developments, and will require the use of AEDT version 2b or higher for new airport development projects initiated after May 29, 2015.

Abating and Mitigating Noise Exposure

Given the large amount of local and transient flight activity in the Chicago metropolitan area, some exposure to aircraft noise is inevitable. However, the governmental and business stakeholders at Chicago Midway International Airport are working hard to minimize aircraft noise exposure as much as possible while still serving the needs of the region.

There are existing policies in place that reduce noise exposure. For example:

  • The Fly Quiet Program is a voluntary noise abatement program that encourages pilots and air traffic controllers to use designated nighttime preferential runways and flight tracks at Midway.
  • The insulation of 8,040 homes through the Midway Residential Sound Insulation Program
  • The insulation of 41 schools at a cost of more than $105 million through the Midway School Sound Insulation Program

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