Page 160 - Master Handbook of Acoustics

FIGURE 7-10 The calculated effects of loudspeaker edge diffraction on the direct signal in the set-up
of Fig. 7-9. There is a significant change in overall frequency response of the system. (Vanderkooy,
Kessel.)

This cabinet diffusion effect can be controlled by setting the loudspeaker face flush in a much

larger baffling surface. Diffusion can also be reduced by rounding cabinet edges and using foam or
other absorbing material around the front of the cabinet.

Diffraction by Various Objects

Early sound-level meters were simply boxes with a microphone. Diffraction from the edges and

corners of the box seriously affected the accuracy of the readings at high frequencies. Modern sound-
level meters have cases with carefully rounded contours, and the microphone is mounted on a smooth,
slender, rounded stalk to remove it from the meter case.
Similarly, diffraction from the casing of a studio microphone can cause deviations from the

desired flat response. This must be taken into account when designing a microphone.
When measuring sound absorption in large reverberation chambers, the common practice is to
place the material to be measured in an 8- × 9-ft frame on the floor. Diffraction from the edges of this
frame may result in absorption coefficients greater than unity. In other words, diffraction of sound
makes the sample appear larger than it really is. Similarly, in practice, the absorption of materials is
increased because of diffraction at their edges. For this reason, and others, it is preferable to space

absorbing panels apart from each other, rather than join them into one section. This takes advantage of
increased absorption because of edge diffraction.