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.

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:

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 (PDF-Page 24) and Comprehensive Plan policy also apply the DNL metric.

Lmax:

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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