Audio Engineering

Acoustic Environment 85

the sum of inverse-square-law level change and excess attenuation. Figure 3.3 shows the
excess attenuation difference between 1000 and 10,000 Hz at various distances.

3.4 Velocity of Sound ....................................................................................................

For a given frequency, the relation of the wavelength to the velocity of sound in the
medium is

λ

λ

λ







c

f

cf

f

c

(3.3)

where λ is the wavelength in feet or meters, c is the velocity of sound in ft/s or m/s, and f
is the frequency in Hz.

In dealing with many acoustic interactions, the wavelength involved is signifi cant and
the ability to calculate it is important. Therefore we need to be able to both calculate and
measure the velocity of sound quickly and accurately.

The velocity of sound varies with temperature to a degree suffi cient to require our
alertness to it. A knowledge of the exact velocity of sound when using signal-delayed
signal analysis allows very precise distance measurements to be made by observing

Distance from source (feet)

10 20 50 100 200 500 1K 2K 5K 10K

140

120

100

80

60

40

20

0

Excess attenuat

ion (dB)

10,000 Hz

3000 Hz

9.14 dB 1000 Hz

45.7 dB

68

 F and 20% RH

2000 Hz

Figure 3.3 : Excess attenuation for different frequencies and distances from the source.