Page 278 - The Master Handbook Of Acoustics

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DIFFRACTION OF SOUND

r /z
r
3
r /z
r 2
r
Focal
Source point

FIGURE 11-7
The zone plate or acoustic lens. The slits are so arranged that the several path lengths differ by multiples of
a half wavelength of the sound so that all diffracted rays arrive at the focal point in phase, combining con-
2
structively. (After Olson. )

Diffraction by Loudspeaker Cabinet Edges

Loudspeaker cabinets are notorious for diffraction effects. If a loud-
speaker is mounted near a wall and aimed away from the wall, the
wall is still illuminated with sound diffracted from the corners of
the box. Reflections of this sound can affect the quality of the sound
at the listener’s position. Measurements of this effect have been
6
5
scarce, but Vanderkooy and Kessel have recently computed the
magnitude of loudspeaker cabinet edge diffraction. The computa-
tions were made on a box loudspeaker with front baffle having the
dimensions 15.7 × 25.2 in and depth of 12.6 in (Fig. 11-9). A point
source of sound was located symmetrically at the top of the baffle.
The sound from this point source was computed at a distance from
the box. The sound arriving at the observation point is the combi-
nation of the direct sound plus the edge diffraction. This combina-
tion is shown in Fig. 11-10. Fluctuations due to edge diffraction for
this particular typical situation approached plus or minus 5 dB.
This is a significant change in overall frequency response of a
reproduction system.