Sound Measurement Terms

A Quick Reference To Sound Measurement Terminology:

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.

DNL - The day-night level (DNL) is the average annual daily sound level, in decibels, computed on the basis of a 24-hour period with a 10-dB penalty for sound occurring between 10 p.m. and 7 a.m. to account for the greater intrusiveness of noise during late night or early morning hours.

DNL was developed as a single-number measurement of community noise exposure by EPA. DNL was adopted by the FAA for use by airports.

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. The composite noise contours combine past and projected noise exposure including use of a planned third parallel runway.

The least severe exposure occurs outside the DNL 55 contour, the level at which the Federal Aviation Administration describes noise exposure as minimal. The FAA considers residential development and other noise-sensitive land uses as acceptable in these areas and recommends no special noise-control measures.

Noise exposure is regarded as moderate between the DNL 55 and DNL 65 contours, and the FAA recommends that land-use controls be considered. Noise exposure is regarded as significant between the DNL 65 and DNL 75 contours, and the FAA recommends that land-use controls be implemented. The Airport Authority has worked with local governments to develop airport-compatible zoning measures for this area.