Glossary of Noise Terminology



The method of reducing the degree of intensity of noise and the use of such a method.


The Airport Noise and Operations Management System (ANOMS) is a sophisticated, acoustical system which monitors aircraft flight tracks, fleet mix, and noise levels by time of day, season and on an annual basis.

Cumulative Noise Equivalent (CNEL)

A method for predicting, by a single number rating, cumulative aircraft noise that affects communities in airport environs. The CNEL value represents noise as measured by an A-weighted sound level (see also) meter. The metric includes a 4.8-decibel penalty for aircraft operations during evening hours (7 p.m. to 10 p.m.) and a 10-decibel penalty for aircraft operations during nighttime hours (10 p.m. to 7 a.m.). This is equivalent to multiplying the number of operations occurring during the evening hours by 3 and multiplying the number of operations occurring during the nighttime hours by 10. The A-weighted decibel scale was developed to closely represent the response of the human ear to sound. CNEL is only used in the State of California and is similar to Ldn, which does not include the evening penalty.


The Decibel (dB) is the unit used to measure the magnitude or intensity of sound. Decibel means 1/10 of Bel (named after Alexander Graham Bell). The decibel uses a logarithmic scale to cover the very large range of sound pressures that can be heard by the human ear. Under the decibel unit of measure, a 10 dB increase will be perceived by most people to be a doubling in loudness, i.e., 80 dB seems twice as loud as 70 dB.


The Day-night Average Sound Level (DNL) is the level of noise expressed (in decibels) as a 24-hour average. Nighttime noise, between the hours of 10:00 p.m. and 7:00 a.m. is weighted; that is, given an additional 10 decibels to compensate for sleep interference and other disruptions caused by nighttime noise. An annual average of DNLs is used by the Federal Aviation Administration to describe airport noise exposure. Areas with noise impacts less than 65 dB DNL are considered “compatible” with residential use; areas at or above 65 dB DNL are designated “incompatible” with residential use. DNL is used by all Federal agencies (EPA, HUD, DOE, DOD, etc.) and internationally in the assessment of potential noise impacts. It is used interchangeably with DNL.

General Aviation

Non-commercial airline aviation – primarily business aircraft and individuals traveling in private aircraft.


Instrument Flight Rules. When aircraft operate in the clouds or visibility is restricted, aircraft must use IFR. IFR procedures are published by the Federal Aviation Administration (FAA.)  When an aircraft flies IFR it must file a flight plan and get a clearance from Air Traffic Control. This clearance specifies the routes an aircraft must follow taking off, en route and landing.


An Instrument Landing System (ILS) is a precise landing aid consisting of several components giving the pilot vertical and horizontal electronic guidance. Elements usually include: 1.) an outer marker, a radio beam 4 to 6 miles from the touchdown point where the electronic signal begins; 2.) an approach lighting system at the runway end; 3.) a localizer radio beam which provides the horizontal guide; and 4.) a glide slope which provides vertical guidance on the angle of descent for landing.

Noise Abatement

A procedure or technique used by aircraft at an airport to minimize the impact of noise on the communities surrounding an airport.

Noise Level

For airborne sound, unless specified to the contrary, it is the A-weighted sound level.

Noise Contour

A Noise Contour is a line on a map that represents equal levels of noise exposure.

Noise Models

“Noise models” are computer models used to predict the impacts of aircraft noise over a geographic area. Such models are used to develop the noise exposure contours and noise exposure maps submitted to the Federal Aviation Administration and other government agencies (state and local). The Integrated Noise Model (INM) is the most commonly applied aviation noise model.

Noise Study

Investigation of existing noise conditions, flight patterns and land use surrounding an airport.

Peak Level (in DB)

The Peak Level is the highest level of sound pressure measured during a noise event.

Single Event Noise Exposure Level

SENEL  is a noise measure which quantifies the effect of duration and magnitude for a single event measured above a specified threshold. SENEL is defined by California Division of Aeronautics, “Noise Standards for California Airports,” California Administrative Code Chapter 9, Title 4 (Register 70, No. 48, November 28, 1970).  For more information, refer to the Noise Reports web pages.

Stage 2 and Stage 3 Aircraft

Commercial jet engines currently meet either Stage 2 or Stage 3 noise standards. Stage 2 engines are older and noisier than Stage 3 engines. Stage 3 aircraft incorporate the latest technology for suppressing jet-engine noise and, in general, are 10 dB quieter than Stage 2 aircraft. This represents a halving of perceived noise; however, actual noise reduction varies by aircraft. All aircraft greater than 75,000 lbs. had to meet Stage 3 noise standards as of January 1, 2000.

Time Above

The Time Above is a measure identifying the number of minutes in a day which exceed a certain noise level. For example, a location may experience 10 minutes a day when the noise level exceeds 65 dBA.


Visual Flight Rules. When aircraft can stay out of the clouds and the pilots can see a certain minimum distance, aircraft can fly using VFR.