Audio Engineering

84 Chapter 3

3.3 Atmospheric Absorption .........................................................................................

These other processes represent actual dissipation of sound energy. Energy is lost due
to the combined action of the viscosity and heat conduction of the air and relaxation
of behavior in the rotational energy states of the molecules of the air. These losses
are independent of the humidity of the air. Additional losses are due to a relaxation of
behavior in the vibrational states of the oxygen molecules in the air, as this behavior is
strongly dependent on the presence of water molecules in the air (absolute humidity).
Both of these energy loss effects cause increased attenuation with increased frequency
( Figure 3.2 ).

This frequency-discriminative attenuation is referred to as excess attenuation and must
be added to the level change due to divergence of the sound wave. Total level change is

100K50K 20K10K 5K 2K 1K 500 200100

0.001

0.002

0.005

0.001

0.002

0.05

0.1

0.2

0.5

1.0

0.02

0.01

0.005

Frequency (Hz)

Attenuat

ion constant (dB/m)

Temperature (20 C)

20% RH

40% RH

60% RH

80% RH

Figure 3.2 : Absorption of sound for different frequencies and values of relative humidity.