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.
Metrics
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:
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.
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.