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Glossary of Terms

There are many acronyms and terms for aircraft noise.

The method of reducing the degree of intensity of noise and the use of such a method.
Air Carriers
Airlines holding a certificate of public convenience and necessity that operate aircraft designed to have a maximum seating capacity of more than 60 seats or a maximum payload capacity of more than 18,000 pounds or conduct international operations.
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.
Commercial Aviation
The sum total of air carrier and air taxi flights.
Community 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-dcibel 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.
Commuter Aircraft
Scheduled passenger aircraft with fewer than 50 seats.
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. 

The Effective Perceived Noise Level (EPNdB) is another unit of measure for aircraft noise. It is based on how people judge the annoyance of sounds they hear with corrections for the duration of the event and for pure tones. The Federal Aviation Administration (FAA) uses EPNdB in the certification of large transport planes for Federal Noise Regulations (FAR Part 36). 

General Aviation
Non-commercial airline aviation – primarily business aircraft and individuals traveling in private aircraft, including those making connections to commercial flights.
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.
Preferential Runway Use
Taking off or landing on specified runways during certain hours to avoid residential areas.
The Sound Exposure Level (in dB) is computed by converting the total noise energy measured during a noise event to an equivalent dBA level for a single event that would only be one second in duration. The SEL accounts for both the magnitude and the duration of the noise event; noise analysts use SEL to calculate the day-night average noise level.
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.

Air Taxi
Non-scheduled passenger aircraft with 50 or fewer seats.

The A-weighted Decibel (dBA) is the most common unit used for measuring environmental sound levels. It adjusts, or weights, the frequency components of sound to conform with the normal response of the human ear at conversational levels. dBA is an international metric that is used for assessing environmental noise exposure of all noise sources.

Hertz (Hz)
The Hertz is a unit of measurement of frequency, numerically equal to cycles per second of the measure of the rate of the vibration of the sound. High frequencies can be thought of as having a high pitch; like a whistle; low frequency sounds are more like a rumble of a truck or airplane.

Huskkitted Aircraft
Hushkitted Stage III aircraft are previously Stage II aircraft that have been adapted to meet Stage III requirements.

Instrument Flight Rules govern flight procedures during limited visibility or other operational constraints. Under IFR, pilots must file a flight plan and fly under the guidance of radar.

The Federal Aviation Administration’s (FAA), Office of Environment and Energy (AEE-100) has developed the Integrated Noise Model (INM) for evaluating aircraft noise impacts in the vicinity of airports. The INM has been the FAA’s standard tool since 1978 for determining the predicted noise impact in the vicinity of airports. The FAA requires airports use the INM in assessing environmental impacts for soundproofing, evaluating physical improvements to the airfield, analyzing changes to existing or new procedures and in assessing land use compatibility.

The INM Model utilizes flight track information, aircraft fleet mix, standard and user defined aircraft profiles and terrain as inputs. The INM model produces noise exposure contours that are used for land use compatibility maps. The INM program includes built in tools for comparing contours and utilities that facilitate easy export to commercial Geographic Information Systems. The model also calculates predicted noise at specific sites such as hospitals, schools or other sensitive locations.

The sound energy flow through a unit area in a unit time.

1. Unwanted sound. 2. Any sound not occurring in the natural environment, such as sounds emanating from aircraft, highways, industrial, commercial and residential sources. 3. An erratic, intermittent, or statistically random oscillation.

Noise Event
A Noise Event is the measured sound produced by a single source of noise over a particular period of time. An aircraft noise event begins when the sound level of an overflight exceeds a noise threshold and ends when the level drops down below that threshold.

An aircraft maintenance procedure; a “revving” of the engine.

Visual Flight Rules (VFR) are air traffic rules allowing pilots to land by sight without relying solely on instruments. VFR conditions require good weather and visibility.