Page 113 - The Master Handbook Of Acoustics

88 CHAPTER FOUR

conditions exist close to the loudspeaker. This means that spherical
divergence prevails in this limited space, and reflections from the sur-
faces are of negligible comparative level. Moving away from the loud-
speaker, the effects of sound reflected from the surfaces of the room
begin to be effective. At the critical distance the direct and the reflected
sound are equal. The critical distance may be taken as a rough single-
figure description of the acoustics of the environment.
In the region very close to the loudspeaker, the sound field is in
considerable disarray. The loudspeaker, at such close distances, can
in no way be considered a point source. This region is called the near
field. Only after moving several loudspeaker dimensions away from it
can significant measurements be made in the far field.

Hemispherical Propagation
True spherical divergence implies no reflecting surfaces at all. Tied to
this earth’s surface as we are, how about hemispherical sound propaga-
tion over the surface of this planet? Estimates made by the very conve-
nient “6 dB per distance double” rule are only rough approximations.
Reflections from the surface of the earth outdoors usually tend to
make the sound level with distance something less than that indicated
by the 6 dB per distance double. The reflective efficiency of the earth’s
surface varies from place to place. Note the sound level of a sound at
10 ft and again at 20 ft from the source. The difference between the two
will probably be closer to 4 dB than 6 dB. For such outdoor measure-
ments the distance law must be taken at “X dB (4?, 5?) per distance
double.” There is also the effect of general environmental noise that
can influence the measurement of specific sound sources.