Audio Engineering

86 Chapter 3

the frequency interval between comb fi lters from two sources and then converting from
frequency to time and fi nally to distance.

The velocity of sound under conditions likely to be encountered in connection with
architectural acoustic considerations is dependent on three fundamental factors. These are:

1. γ is the ratio of specifi c heats and is 1.402 for diatomic molecules (air molecules).

2. PS is the equilibrium gas pressure in Newtons per square meter (1.013 1 0^5 N/m^2 ).

3. ρ is the density of air in kilograms per cubic meter (kg/m^3 ).

c

ps



γ

ρ

(3.4)

where c is the velocity of sound in m/s.

The density of air varies with temperature, and an examination of the basic equations
reveals that, indeed, temperature variations are the predominant infl uence on the velocity
of sound in air.

The equation for calculating the density of air is

Density of air

H

C





1 293

1 0 00367 76

.

[. (° )]( )

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

(3.5)

where density of air is in kg/m^3 ; H is the barometric pressure in centimeters of mercury,
Hg; °C is the temperature in degrees Celsius; 9/5 (°C)  3 2  °F; and 5/9 (°F) –
32  °C. Hg in inches times 2.54 equals Hg in centimeters.

3.4.1 Example

If we were to measure a temperature of 72°F and a barometric pressure of 29.92 in cm
Hg, we would fi rst calculate the density of the air according to data gathered:

5

9

72 32 22 22

29 92 2 54 76

()

in Hg cm Hg

–.

..





°C

Density

()

[ ( )]( )

kg/m







1 293 76

1 0 00367 22 22 76

1 1955^3

.

..

..