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DEFINITIONS OF ACOUSTICAL TERMS TERM DEFINMON Decibel, dB A unit describing the amplitude of sound, equal to 20 times the logarithm to the base 10 of the ratio of the pressure of the sound measured to the reference pressure, which is 20 micropascals (20 micronewtons per square meter). Frequency, The number of complete pressure fluctuations per second above and Hz below atmospheric pressure. A-Weighted The sound pressure level in decibels as measured on a sound level Sound Level, meter using the A-weighting filter network. The A-weighting filter de- dBA emphasizes the very low and very high frequency components of the sound in a manner similar to the frequency response of the human ear and correlates well with subjective reactions to noise. All sound levels in this report are A-weighted. Imo„ L,o, L307 The A-weighted noise levels that are exceeded 01%, 10%, 50%, and Ll 90% of the time during the measurement period. Equivalent The average A-weighted noise level during the measurement period. Noise Level, Community The average A-weighted noise level during a 24-hour day, obtained Noise after addition of 5 decibels to levels in the evening from 7:00 PM to Equivalent 10:00 PM and after addition of 10 decibels to sound levels in the night Level, CNEL between 10:00 PM and 7:00 AM. Day/Night The average A-weighted noise level during a 24-hour day, obtained Noise Level, after addition of 10 decibels to levels measured in the night between L, 10:00 PM and 7:00 AM. Ambient The composite of noise from all sources near and far. The normal or Noise Level existing level of environmental noise at a given location. Intrusive That noise which intrudes over and above the existing ambient noise at a given location. The relative intrusiveness of a sound depends upon its amplitude, duration, frequency, and time of occurrence and tonal or informational content as well as the prevailing ambient noise level.