Noise Glossary and Acronyms

dB - The decibel (dB) is the unit used to measure the magnitude or intensity of sound. It uses a mathematical scale to cover the large range of sound pressures that can be heard by the human ear. A 10-dB increase will be perceived by most people to be a doubling of loudness. For example, 80 dB typically 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 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. 

DNL - In simple terms, Ldn or Day Night Average Sound Level (DNL) is the average noise level over a 24-hour period except that noise occurring at night (between the hours of 10:00 p.m. and 7:00 a.m.) are artificially increased by 10 dB. This weighting reflects the added intrusiveness of night noise events attributable to the fact that community background noise typically decreases by 10 dB at night. Under Federal Aviation Regulation (FAR) Part 150, the FAA has established Ldn/DNL as the cumulative noise exposure metric for use in airport noise analyses. 

FAR Part 150 Study - A program funded by the FAA under Federal Aviation Regulation Part 150; Airport Noise Compatibility Planning and by Local Government entities allowing airport operators to voluntarily develop and submit noise exposure maps and noise compatibility programs to the FAA. 

IFR – Instrument Flight Rules (IFR) govern flight procedures during limited visibility or other operational constraints. Under IFR, pilots fly under the guidance of radar. 

ILS – An Instrument Landing System (ILS) is a precise landing aid consisting of several components giving pilot vertical and horizontal electronic guidance. Elements usually include:

• An outer marker, a radio beam 4 to 6 miles from the touchdown point where the electronic signal begins;
• An approach lighting system at the runway end;
• A localizer radio beam which provides the horizontal guide; and
• A glide slope which provides vertical guidance on the angle of descent for landing.

Land Use Compatibility - Land uses identified in FAR Part 150 as normally compatible with the outdoor noise environment adjacent to an airport within a specific noise contour level (or an adequately attenuated noise level reduction for any indoor activities involved). 

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 Contour - A continuous line on a map that represents equal levels of noise exposure. Noise lessens from the innermost area within a contour line outward. Noise exposure is regarded as significant above the 65 DNL noise contour line, and the Federal Aviation Administration recommends that land-use controls be implemented. 

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 over-flight exceeds a noise threshold and ends when the level drops down below that threshold. 

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

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

Preferential Runway Use – Taking off or landing on specified runways during certain hours to avoid residential and other noise-sensitive areas, to the greatest extent possible. 

Stage 2 and Stage 3 Aircraft - As of January 1, 2016, all commercial jet engines currently meet Stage 3 noise standards. 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. 

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.