Encyclopedia of Environmental Science and Engineering, Volume I and II

770 NOISE

c is a wave a propagation speed; it does not represent particle

velocity within the medium.

Wavelength. If a pure-tone pressure wave could be

observed at a given instant, the length of one cycle of the

wave in the propagation directly could be identified as the

wavelength. Thus,

λ 
 c / f (1.2)

where λ 
 wavelength (m), c 
 propagation speed (m/s) and

f 
 frequency (Hz). The effectiveness of noise barriers and

sound-absorbing materials is dependent on the sound wave-

length (thus, effectiveness is frequency-dependent).

SOUND PRESSURE AND SOUND PRESSURE LEVEL

One standard atmosphere is defined as a pressure of 1.01325 

10 5 Pa (about 14.7 psi). Typical sound pressure waves rep-

resent very small disturbances in ambient pressure. Sound

pressure level is defined by

Lpp pppgrms

2

ref

2



 10 lg⎣⎡ ⁄ ⎤⎦ 20 1[]rmsref (2.1)

where L p 
 sound pressure level in decibels (dB), lg 


common (base-ten) logarithm, p rms 
 root-mean-square sound

pressure (Pa) and p ref 
 reference pressure 
 20  10 −6 Pa.

Sound pressure represents the difference between instan-

taneous absolute pressure and ambient pressure. For a pure-

tone sound wave of amplitude P,

p rms 
 P /2 1/2.

The reference pressure is the nominal threshold of hearing,

corresponding to zero dB. Sound pressure may be determined

from sound pressure level by the following relationship:

pp

L L

rms ref


 10 p⁄^20 
2 10[]P^100 ⁄^20.

(2.2)

A-WEIGHTING

Human hearing is frequency-dependent. At low sound levels,

sounds with frequencies in the range from about 1 kHz to

5 kHz are perceived as louder than sounds of the same sound

pressure, but with frequencies outside of that range. A-, B-

and C-weighting schemes were developed to compensate for

the frequency-dependence of human hearing at low, moder-

ate and high sound levels. Other weightings are also used,

including SI-weighting which relates to speech interference.

A-weighting has gained the greatest acceptance; many stan-

dards and codes are based on sound levels in A-weighted

decibels (dBA). When noise is measured in frequency bands,

the weighting adjustment may be added to each measured

value. Sound level meters incorporate weighting networks

so that weighted sound level is displayed directly. A-weight-

ing adjustments are shown in Table 1.

Some representative sound levels are given in Table 2.

Most values are approximate; actual noise sources produce a

wide range of sound levels.

EQUIVALENT SOUND LEVEL

Sound energy is proportional to mean-square sound pressure.

Equivalent sound level is the energy-average A-weighted

sound level over a specified time period. Thus,

(^)
LTtL
T
eq
^10 lg^110 d
10
( ⁄ ) 0
⎡
⎣⎢
⎤
⎦⎥
⁄
∫ (4.1)
where L eq
equivalent sound level (dBA), L
instantaneous
sound level (dBA) and T
averaging time, often 1 hour, 8
TABLE 1
A-weighting
Frequency
Hz
Adjustment
dB
20 50.5
25 44.7
31.5 39.4
40 34.6
50 30.2
63 26.2
80 22.5
100 19.1
125 16.1
160 13.4
200 10.9
250 8.6
315 6.6
400 4.8
500 3.2
630 1.9
800 0.8
1,000 0
1,250 0.6
1,600 1.0
2,000 1.2
2,500 1.3
3,150 1.2
4,000 1.0
5,000 0.5
6,300 0.1
8,000 1.1
10,000 2.5
12,500 4.3
16,000 6.6
20,000 9.3
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