Department of Planning and Development

CONTACT INFORMATION: Our offices are open 8:00 a.m. to 4:00 p.m., Monday through Friday.
703-324-1380 TTY 711
12055 Government Center Parkway
Fairfax, VA 22035
Tracy Strunk

Noise Basics

In its simplest form, noise can be defined as undesired sound.

Measuring sound

Sound is measured by using a unit known as the “decibel" (abbreviated “dB").

  • The decibel scale is a logarithmic scale rather than a linear scale. Relative differences in sound energy and perceived loudness do not vary linearly along this scale.
  • Sound energy doubles for every 3 dB increase in level.
  • “Loudness" of sound (the subjective perception of sound by humans) is generally considered to double for every 10 dB increase in sound level. For example, a sound level of 70 dB is generally considered to be twice as loud as a sound level of 60 dB.

Human response to sound

The human response to sound is dependent on the frequency of the sound wave.

  • In general, the range of human hearing is between 20 and 20,000 Hertz (Hz, or cycles per second).
  • Within this range, the human ear is not equally sensitive to all frequencies of sound.
  • In order to approximate human response to sounds, sound levels are typically weighted to emphasize frequencies that are most audible by humans.
  • The “A-weighted" sound level (or dBA) approximates the sound level perceived by humans. A-weighted levels are used extensively to describe transportation related noise impacts.


Sound levels can be characterized by a number of descriptors (or “metrics"). Sound can be described in terms of average levels, maximum levels, thresholds and single-event sound exposure. There are a large number of sound metrics. The key metrics relating to Fairfax County noise policy and regulation include:

Leq or Leq(x):

Equivalent Sound Level—An average sound level over a specific period of time x (typically a 24-hour period). This metric is often applied in transportation noise analyses; the Federal Highway Administration, for example, applies peak hour Leq guidelines in its noise regulations.

DNL (or Ldn):

Day-Night Average Sound Level—An average sound level over a 24-hour period adjusted to account for the more intrusive nature of noise during nighttime hours. This metric is a refinement of the 24-hour Leq metric, with a 10 dB penalty applied to sound levels that occur between 10:00 P.M. and 7:00 A.M. Federal interagency noise guidelines apply the DNL metric; the Airport Noise Impact Overlay District of the county's Zoning Ordinance and Comprehensive Plan policy also apply the DNL metric.


Maximum Sound Level—The maximum sound level occurring within a specific period of time. The county’s Noise Ordinance (Chapter 108.1 of the County Code) establishes noise limits for stationary noise sources. In addition to frequency-specific limits, A-weighted maximum sound level requirements have been established.

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