Appendix I to Subpart B of Part 51 - Definition of Acoustical Quantities
24:1.1.1.1.30.2.59.8.3 : Appendix I
Appendix I to Subpart B of Part 51 - Definition of Acoustical
Quantities
1. Sound Level. The quantity in decibels measured with an
instrument satisfying requirements of American National Standard
Specification for Type 1 Sound Level Meters S1.4-1971. Fast
time-averaging and A-frequency weighting are to be used, unless
others are specified. The sound level meter with the A-weighting is
progressively less sensitive to sounds of frequency below 1,000
hertz (cycles per second), somewhat as is the ear. With fast time
averaging the sound level meter responds particularly to recent
sounds almost as quickly as does the ear in judging the loudness of
a sound.
2. Average Sound Level. Average sound level, in decibels,
is the level of the mean-square A-weighted sound pressure during
the stated time period, with reference to the square of the
standard reference sound pressure of 20 micropascals.
Day-night average sound level, abbreviated as DNL, and
symbolized mathematically as Ldn is defined as:
Time t is in seconds, so the limits shown in hours and minutes
are actually interpreted in seconds. LA(t) is the time varying
value of A-weighted sound level, the quantity in decibels measured
by an instrument satisfying requirements of American National
Standard Specification for Type 1 Sound Level Meters S1.4-1971.
3. Loud Impulsive Sounds. When loud impulsive sounds such
as sonic booms or explosions are anticipated contributors to the
noise environment at a site, the contribution to day-night average
sound level produced by the loud impulsive sounds shall have 8
decibels added to it in assessing the acceptability of a site.
A loud impulsive sound is defined for the purpose of this
regulation as one for which:
(i) The sound is definable as a discrete event wherein the sound
level increases to a maximum and then decreases in a total time
interval of approximately one second or less to the ambient
background level that exists without the sound; and
(ii) The maximum sound level (obtained with slow averaging time
and A-weighting of a Type 1 sound level meter whose characteristics
comply with ANSI S1.4-1971) exceeds the sound level prior to the
onset of the event by at least 6 decibels; and
(iii) The maximum sound level obtained with fast averaging time
of a sound level meter exceeds the maximum value obtained with slow
averaging time by at least 4 decibels.
[44 FR 40861, July 12, 1979; 49 FR 10253, Mar. 20, 1984; 49 FR
12214, Mar. 29, 1984]