**Statistical
Description of Community Noises**

Figure B1 shows the A-weighted
sound levels for two different time periods in a typical suburban environment. These
curves show the following characteristics of most community noises:

(a) The noises comprise two parts: a
fairly steady **'residual noise level'** which comes from distant,
unidentifiable sources, together with some **'discrete'** noise events of
identifiable origins.

(b) The residual noise level varies
slowly with time, usually displaying diurnal, or weekly cycle but with maximum deviations
rarely exceeding about 10 dB(A).

(c) The individual noise events vary
in magnitude and duration, rising as much as 40 dB(A) above the residual level for
seconds, minutes, or even longer.

Figure B1 Typical Community A-weighted
Sound Levels in

(a) Daytime (b) Night-time

Figure B2 Statistical Representation of
Community Noise

(a) __Equivalent Continuous Sound
Level__, LAeq

This is the steady-state A-weighted
sound level that has the same acoustic energy as that of the time-varying sound averaged
over the specified time interval. See Figure B3.

Figure B3 LAeq
and SEL

LAeq
can be estimated from a record of A-weighted sound level verse time by using the
definition :

(1)

where *L**A(t)*
= instantaneous A-level of sound

T = specified time period during which sound is
sampled

By breaking the sound-level record
into n nos. of equal increments of time , equation (1) can be
approximated by :

(2)

where *L**Ai*
= average A-level over the ith increment of time