How Is Airplane Noise Measured?

The noise an airplane makes depends on a number of factors, including the type of engines and type of aircraft, the weather and wind conditions, and whether the plane is landing or taking off. All these factors are taken into consideration when describing aircraft noise.

The standard way of measuring noise levels around an airport is by calculating DNL, or “Day-Night Sound Level,” over a 24-hour period. DNL is a cumulative measure of all of the noise events during a typical day, taking into account aircraft sound levels, number of flights, time of day and flight paths.

In the DNL calculation, nighttime flights (10 p.m. to 7 a.m.) are “weighted” ten decibels higher – about twice as loud as they actually are – than the otherwise equivalent daytime flights. This takes into account the greater sensitivity to noise during these hours.

Another calculation, a “Sound Exposure Level” (SEL) measures the sound made by a single aircraft. SEL data are also used in the Part 150 study to gain a full understanding of noise impact around the airport.

More about DNL

The standard way of measuring noise levels around an airport is by calculating DNL, or “Day-Night Sound Level,” over a 24-hour period. DNL is a cumulative measure of all of the noise events during a typical day, taking into account aircraft sound levels, number of flights, time of day and flight paths.

After selection of DNL by the Environmental Protection Agency (EPA) in 1973, the Federal Aviation Administration (FAA) adopted DNL as the accepted measure of aircraft noise exposure. The FAA and the U.S. Department of Housing and Urban Development (HUD) recognize a DNL of less than 65 decibels as being compatible with residential land use.

DNL is the most widely accepted descriptor for aviation noise because it is:

• The product of extensive research and long experience.
• A measurable quantity.
• Simple to understand and use.
• Useful for comparison of alternative scenarios.
• The best predictor of how a community reacts to noise.
• The best descriptor of all noise sources for land use planning.

More About SEL

A “Sound Exposure Level,” or SEL, measures the sound made by a single aircraft of a particular type taking off or landing. The SEL is another metric that is considered to gain a fuller understanding of noise impact around an airport.

• The noisier the plane is, the higher the SEL. Landings are quieter than takeoffs.
• Because of the way it is calculated, an SEL is about 10 decibels higher than the maximum noise level that is actually produced by the aircraft.
• An outdoor SEL is about 20-30 decibels higher than the SEL that is heard inside a typical home.
• SEL data help assess the effects of aircraft noise on sleep.

How Loud Is Loud?

The actual experience of noise is variable and also subjective, affected by personal preferences, habits and disposition. Not everyone will react to airplane noise in the same way. Even so, the EPA, the FAA and acoustical engineers around the world have developed effective ways of measuring and comparing the sounds. Here are some examples:

Will nighttime flights affect my sleep?

The effect of aviation noise on sleep is a long recognized concern of those interested in addressing the effects of noise on people. The latest sleep studies in the U.S. and in England have shown that aircraft noise has less of an effect on sleep than previously thought, according to the Federal Interagency Committee on Aviation Noise (FICAN), a committee of the EPA and other Federal agencies. Earlier studies tested the effects of noise on subjects sleeping in laboratories. Recent studies, using SEL measurements, were conducted in actual residences near airports. FICAN reports that only a very small percent of the population are awakened by aircraft noise.

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