Sound is measured on a logarithmic (nonlinear) scale in decibel (dB) units. There are different dB scales. Transportation noise is usually measured in dBAs, the so-called A-weighted scale that emphasizes sound frequencies that people hear best. On this scale, a 10 dBA increase in sound level is generally perceived by humans as a doubling of sound.
For airport noise exposure, the U.S. Department of Transportation, Federal Aviation Administration, has adopted a dB measurement called day-night noise level (DNL) established by a federal interagency group. The DNL is a yearly day-night average sound level, a measure of exposure to cumulative events over time. Aircraft engine noise standards, on the other hand, have been set based on a measurement of effective perceived noise level, or EPNdB, which accounts for the presence of different tones in sound.
Contour sound maps of airports are constructed from sound levels measured on the ground. People living or working within DNL 65 dB or higher contours are considered to be subject to significant sound. In general, sound levels are highest in the immediate area of flight pathways and increase in proximity to the airport. Sound levels also differ between takeoffs and landings and type and size of aircraft.
Stage 3 aircraft standards differ based on aircraft size and operations and range from 89 to 106 EPNdB. For landings, aircraft must meet the appropriate standard measured over a reference point 2,000 meters from the runway threshold. For takeoffs, the reference point is 6,500 meters from the start of an aircraft's takeoff roll.