Glossary of Noise Terms

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.

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

ANOMS:  The ANOMS is an acronym for Airport Noise and Operations Monitoring System which is designed to provide DIA officials with accurate runway use counts specific to aircraft type, aircraft flight path information, and 24-hour noise monitoring data at selected sites within residential communities. In addition to providing reliable airport operations data, ANOMS archives data for future airport decisions, validates complaint information, tracks effects of air traffic routing procedures, and validates computer-generated noise models.

dB:  The Decibel (dB) is the unit used to measure the magnitude or intensity of sound. 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.

dBA:  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 to the normal response of the human ear at conversational levels.

Commuter Aircraft:  Scheduled passenger aircraft with fewer than 50 seats.

Commercial Aviation:  The sum total of air carrier and air taxi flights.

General Aviation:  Non-commercial airline aviation - primarily business aircraft and individuals traveling in private aircraft, including those making connections to commercial flights.

Husk kitted Aircraft:  Hush kitted Stage III aircraft are previously Stage II aircraft that have been adapted to meet Stage III requirements, typically by means of engine modification.

IFR:  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.

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

INM:  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.

Ldn:  The Day-night Average Sound Level (Ldn) is the level of noise expressed (in decibels) as a 24-hour average. Nighttime noise, between the hours of 10 pm and 7 am is weighted; that is, given an additional 10 decibels to compensate for sleep interference and other disruptions caused by nighttime noise.

Ldn 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.

Lmax:  The Maximum Instantaneous Noise Level (Lmax) is the maximum level of noise measured during a given measurement period.

Noise:  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 fluctuation.

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

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

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.

Preferential Runway Use:  Taking off or landing on specified runways during certain hours to avoid residential areas.

RMT:  Remote Monitoring Terminals, DIA has 28 RMTs places strategically throughout the Denver metro area.

SEL:  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 vs. Stage 3 Aircraft:  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 reduction in noise levels; 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.

VFR:  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.

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