Page 65 - Master Handbook of Acoustics

FIGURE 3-3 Even in an enclosed space, an approximate free field can exist close to the source,
where sound-pressure level is attenuated 6 dB for every doubling of distance. By definition, the
critical distance is that distance at which the direct sound-pressure level is equal to the reverberant
sound-pressure level. Farther from the source, sound-pressure level becomes constant and depends
on the amount of absorption in the room.

After moving several loudspeaker dimensions away from the loudspeaker, significant
measurements can be made in the far field. This far field consists of the free field and the reverberant
field, and a transition region between them. Approximate free-field conditions exist near the

loudspeaker where it operates as a point source. Direct sound is predominant, spherical divergence
prevails in this limited space, and reflections from the surfaces are of negligible comparative level.
In this region, sound-pressure level decreases 6 dB for every doubling of distance.
Moving away from the loudspeaker, sound reflected from the surfaces of the room affects the

result. We define the critical distance as the location in a room where the direct and the reflected
sound levels are equal. The critical distance is useful as a rough single-figure description of the