Page 155 - Master Handbook of Acoustics

diverted into the shadow zone. Each wavefront passing through the aperture becomes a row of point
sources radiating diffracted sound into the shadow zone. The same principle holds for Fig. 7-5B,
except that the aperture is small compared to the wavelength of sound. Most of the sound energy is
reflected from the wall surface and only a small amount of energy passes through it. The points on the

limited wavefront going through the aperture are so close together that their radiations take the form
of a hemisphere. Following Huygens’ principle, the sound emerges from the aperture as a
hemispherical wave that diverges rapidly. Most of the energy passing through a small aperture is
through diffraction. Because of diffraction, even a small opening in a barrier can transmit a relatively
large amount of sound energy, particularly at low frequencies.

FIGURE 7-5 When plane waves of sound strike a barrier with an aperture, diffraction depends on the
wavelength of sound and the relative size of the opening. (A) An aperture large in terms of
wavelength allows wavefronts to go through with little directional disturbance. (B) If the aperture is
small compared to wavelength, a hemispherical field of sound radiates on the other side of the

barrier.

Diffraction by a Slit

Figure 7-6 diagrams a classic experiment first performed by Pohl, and described more recently by

Wood. The equipment layout of Fig. 7-6A is very approximate. The source/slit arrangement rotated
about the center of the slit and the measuring radiometer was at a distance of 8 m. The slit width was
11.5 cm wide, and the wavelength of the measuring sound was 1.45 cm (23.7 kHz). Figure 7-6B
shows the intensity of the sound versus the angle of deviation. The dimension X indicates the
geometrical boundaries of the ray. Any response wider than X is caused by diffraction of the beam by
the slit. A narrower slit would yield correspondingly more diffraction and a greater width of the

beam. The increase in width of the beam, that is, the extent of diffraction, is the striking feature of this
experiment.