Page 66 - Master Handbook of Acoustics

acoustics of the environment. Still farther away from the source, the reverberant sound field
dominates. Its level remains constant, even at greater distances from the source. This level depends
on the amount of absorption in the room. For example, in a very absorptive room, the reverberant
level will be low.

Hemispherical Field and Propagation

True spherical divergence requires an absence of reflecting surfaces. Because this is difficult to
achieve, it is sometimes approximated by placing a sound source such as a loudspeaker face-up, on a
hard, reflective surface. This creates a hemispherical sound field, radiating upward. This can be
used, for example, to measure the response of the loudspeaker. In this case, the sound field radiates
2
2
over a surface that is 2πr in area. Therefore the intensity varies as I = W/2πr . As in a spherical
sound field, the sound-pressure level in a hemispherical field attenuates by 6 dB for every doubling
of distance. However, in a hemispherical sound field, the level begins 3 dB higher than in a spherical
sound field.
How do we characterize hemispherical sound propagation over the earth’s surface? Estimates

made by the “6 dB per doubling distance” rule are only rough approximations. Reflections from the
earth outdoors usually tend to make the sound level with distance something less than that indicated
by the 6 dB approximation. The reflective efficiency of the earth’s surface varies from place to place.
Consider the sound level of a sound at 10 ft and again at 20 ft from the source. In practice, the
difference between the two will probably be closer to 4 dB than 6 dB. For such outdoor
measurements, the distance law must be taken at “4 or 5 dB per doubling distance.” General
environmental noise can also influence the measurement of specific sound sources.