Page 272 - The Master Handbook Of Acoustics

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DIFFRACTION OF SOUND

face of the wall, as expected. The upper edge of the wall acts as a new,
virtual source sending sound energy into the “shadow” zone behind the
wall by diffraction. The mechanism of this effect will be considered in
more detail later in this chapter.
In Fig. 11-1B the plane wavefronts of sound strike a solid barrier
with a small hole in it. Most of the sound energy is reflected from the
wall surface, but that tiny portion going through the hole acts as a vir-
tual point source, radiating a hemisphere of sound into the “shadow”
zone on the other side.

Diffraction of Sound
by Large and Small Apertures

Figure 11-2A illustrates the diffraction of sound by an aperture that is
many wavelengths wide. The wavefronts of sound strike the heavy
obstacle: some of it is reflected, some goes right on through the wide
aperture. The arrows indicate that some of the energy in the main
beam is diverted into the shadow zone. By what mechanism is this
diversion accomplished?

A B
FIGURE 11-2
(A) An aperature large in terms of wavelength of sound allows wavefronts to go
through with little disturbance. These wavefronts act as lines of new sources radiat-
ing sound energy into the shadow zone. (B) If the aperature is small compared to the
wavelength of the sound, the small wavefronts which do penetrate the hole act almost
as point sources, radiating a hemispherical field of sound into the shadow zone.