Audio Engineering

82 Chapter 3

Research in recent years has advanced the knowledge of atmospheric absorption
signifi cantly from the original base laid by Kneser, Knudsen, followed later by Harris,
and, more recently, by the work of Sutherland, Piercy, Bass, and Evans (see Figure 3.1 ).
This prediction graph is felt to be reliable within 5% for the temperature indicated
(20°C) and 10% over a range of 0 to 40°C.

The June 1977 Journal of the Acoustical Society of America had an exceptional tutorial
paper entitled “ Review of Noise Propagation in the Atmosphere, ” pages 1403–1418, and
included a 96 reference bibliography.

3.2 Inverse Square Law .................................................................................................

The geometrical spreading of sound from a coherent source (inverse square law rate
of level change), which is a change in level of 6 dB for each doubling of distance for a
spherical expansion from a point source, is well known to most sound technicians.

LL

D

PPD

r

m

at measurement pointRef distance 20 log (3.1)

where Dr is the reference distance and Dm is the measured distance.

Not as well recognized is the change in level of 3 dB per doubling of distance for
cylindrical expansion from an infi nite line source. The ambient noise from a motor race
track with the fi eld of cars evenly spread during the early stages of a race can come very
close to being effectively an infi nite line source.

LL

D

PPD

r

m

at measurement pointRef distance 10 log (3.2)

Finally, there is the case of the parallel “ loss free ” propagation from an infi nite area
source—the crowd noise viewed from the center of the audience.

Descriptions of the spreading out of sound for coherent sources remain true for
incoherent sources as well. The size of the near fi eld may be more restricted and the
propagation less directional but the general rate of level change remains the same. Note
that this “ spreading out ” of sound does not constitute absorption or other loss but merely
the reduction of power per unit of area as the distance is increased. Unfortunately, other
processes also are going on.