Encyclopedia of Environmental Science and Engineering, Volume I and II

NOISE 773

where I 
 sound intensity (W/m^2 ), where sound pressure

and particle velocity are in-phase, W 
 sound power of the

source (W) and r 
 distance from the source (m). The above

equation is called the inverse square law.

Scalar sound intensity level is given by

LIIr
10 lg[]⁄Ief (7.2)

where L I 
 scalar sound intensity level and I ref 
 10 ^12 W/m^2.

For airborne sound under typical conditions, sound pres-

sure level and scalar sound intensity level are approximately

equal, from which

LLpI
 
^10 lgW^20 lgr^109 (7.3)

for the spherical wave where L p and L I are expressed in dB. If

sound power has been A-weighted, L p and L I are in dBA.

When the inverse-square law applies, then sound levels

decrease with distance at the rate: 20 lg r. Thus, if sound

level is known at one location, it may be estimated at another

location. Table 4 and Figure 3 show the distance adjustment

to be added to sound level at distance r 1 from the source to

obtain the sound level at distance r 2.

MEASUREMENT AND INSTRUMENTATION

Sound level meters. The sound level meter is the basic tool

for making noise surveys. A typical sound level meter is a

hand-held battery-powered instrument consisting of a micro-

phone, amplifiers, weighting networks, a rootmean-square

rectifier, and a digital or analog sound level display. The A-

weighting network is most commonly used. This network

electronically adjusts the signal in accordance with Table 1,

so that sound level is displayed in dBA. When measuring out-

of-doors, a windscreen is used to reduce measurement error

due to wind impinging on the microphone. Integrating sound

level meters automatically calculate equivalent sound level. If

a standard sound level meter is used, equivalent sound level

may be calculated from representative measurements, using

the procedure described later.

Frequency analysis. The cause of a noise problem may

sometimes be detected by analyzing noise in frequency

bands. An octave band is a frequency range for which the

upper frequency limit is (approximately) twice the lower

TABLE 4

Spherical wave attenuation

r2/r1 ADJ

0.5 6.0

0.6 4.4

0.7 3.1

0.8 1.9

0.9 0.9

1.0 0.0

1.1 0.8

1.2 1.6

1.3 2.3

1.4 2.9

1.5 3.5

1.6 4.1

1.7 4.6

1.8 5.1

1.9 5.6

2.0 6.0

2.1 6.4

2.2 6.8

2.3 7.2

2.4 7.6

2.5 8.0

2.6 8.3

2.7 8.6

2.8 8.9

2.9 9.2

3.0 9.5

Distance adjustment based

on the inverse-square law.

L(r2) 
 L(r1)  ADJ.

Distance ratio r2/r1

Add to sound level at r1

10

5

0

–5

–10

0.5 1.0 1.5 2.0 2.5 3.0

FIGURE 3 Distance adjustment based on the inverse-square law.

Number of equal contributions

Add to level due to one source

10

8

6

4

2

0

1 2 345678910

FIGURE 2 Combining n equal contributions.

C014_003_r03.indd 773C014_003_r03.indd 773 11/18/2005 10:46:09 AM11/18/2005 10:46:09 AM