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 LA(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 LAi = average A-level over the ith increment of time