Page 279 - The Master Handbook Of Acoustics

254 CHAPTER ELEVEN

Direction of
6 kHz plane wave
1 kHz

5
0
5

FIGURE 11-8

Diffraction around a solid sphere about the size of a human head. For sound in the
1-6 kHz range, sound pressure is generally increased in the front hemisphere and
2
generally reduced in the rear. (After Muller, Black and Davis, as reported by Olson. )
This effect can be controlled (eliminated?) by setting the loud-
speaker box face flush in a much larger baffling surface. There is also
the possibility of rounding edges and the use of foam or “fuzz”. 7

Diffraction by Various Objects

Sound level meters were, in early days, boxes with a microphone pro-
truding. Diffraction from the edges and corners of the box seriously
affected the calibration of the microphones. Modern sound level
meters have carefully rounded contours with the microphone
mounted on a smooth, slender, rounded neck.
Diffraction from the casing of a microphone can cause deviations
from the desired flat sensitivity.
In the measurement of sound absorption in large reverberation
chambers, the common practice is to place the material to be measured