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Noise Impact
Assessment
 

Noise Effects
On Animals

 Sound results from small and rapid changes in atmospheric pressure.  These cyclical changes in pressure propagate through the atmosphere and are often referred to as sound waves.  For air, the waves travel at the speed of sound, about 1128 feet per second at 70º Fahrenheit. The audible range of human hearing is about 20 to 20,000 cycles per second.  Cycles per second, or sound frequency, are commonly referred to as hertz (Hz).  The greater the amount of variation in atmospheric pressure (amplitude); the greater the loudness (sound level).

Sound levels are most often measured on a logarithmic scale of decibels (dB).  The decibel scale compresses the audible acoustic pressure levels which can vary from 20 micropascals (µPa), the threshold of hearing and reference pressure (0 dB), to 20 million µPa, the threshold of pain (120 dB).  Because sound levels are measured in dB, the addition of two sound levels is not linear.  To add sound levels in dB, the levels (dB) must be converted into  “energy” terms (10 dB/10 ), which are added and then converted back to dB (10 Log “energy”).  Adding two equal sound levels creates a 3 dB increase in overall level.  An example is 50 dB (100,000 in “energy”) added to 50 dB (100,000 in “energy”) equals 53 dB (200,000 in “energy”).  If more than a 10 dB difference exists between sound levels, there is no significant additive effect.  An example is 50 dB (100,000 in “energy”) added to 60 dB (1,000,000 in “energy”)  equals 60.4 dB (1,100,000 in “energy”).  A figure is provided to illustrate the range of acoustic pressures (µPa) and their relationship to the acoustic decibel scale. 
  

To see a figure that illustrates the range of acoustic pressures (µPa) and their relationship to the acoustic decibel scale:
Common SPLs


Because the human ear does not hear sound energy linearly (on a one-to-one basis), humans do not perceive changes in sound level as equally loud.   Research indicates the following general relationships between sound level and human perception:
  • A 3 dB increase is a doubling of acoustic energy and is the threshold of perceptibility.  The average person will not be able to distinguish a 3 dB difference in sound level in a laboratory condition.
  • A 10 dB increase is a tenfold increase in acoustic energy but is perceived as a doubling in loudness to the average person.  The average person will judge a 10 dB change in sound level to be twice or half as loud.
The human ear does not perceive sound levels from every frequency as equally loud.  As part of the hearing process, the human ear will attenuate low and high-frequency sounds.  To compensate for this phenomenon in perception, the A-weighted decibel scale, referred to as dBA, is used to measure and evaluate environmental noise levels.  The A-weighted scale adjusts sound pressure levels by frequency, reducing low and high-frequency sound, similar to the way people hear sound. 

A variety of sound level indicators can be used for environmental noise analysis.  These indicators describe the variations in intensity and temporal pattern of the sound levels.  Some indicators commonly used in environmental noise assessments are:

  • L(eq) is the energy averaged sound level.  It represents the average sound (level) energy that was recorded during the time period and accounts for both loud events and quiet background sound levels.
  • L(10) is the A-weighted sound level which is exceeded for 10 percent of the time period.
  • L(90) is the A-weighted sound level which is exceeded for 90 percent of the time period. The L(90) is sometimes considered to be the background sound level.
  • L(max) is the maximum A-weighted sound level measured during the time period.
  • L(min) is the minimum A-weighted sound level measured during the time period.
The L(dn) is a 24 hour, weighted average sound level that the Department of Housing and Urban Development (HUD), Federal Aviation Administration, Federal Transit Administration, and Department of Defense commonly use for noise impact assessment.  The L(dn) is derived from hourly L(eq) values.  Studies have shown that additional annoyance occurs during the nighttime since background sound levels are typically at their minimum and many people are noise sensitive while trying to sleep. A 10 dBA nighttime (10:00 PM to 7:00 AM) penalty is therefore added to nighttime L(eq) values to account for increased noise annoyance during these hours.