Page 274 - The Master Handbook Of Acoustics

249
DIFFRACTION OF SOUND

A B
FIGURE 11-3
(A) An obstacle very much smaller than the wavelength of sound allows the wave-
fronts to pass essentially undisturbed. (B) An obstacle large compared to the wave-
length of sound casts a shadow that tends to be irradiated from sources on the
wavefronts of sound that go past the obstacle.

Figure 11-5 gives some idea of the effectiveness of highway barri-
ers and of the intensity of the sound in the shadow of a high, mas-
sive wall. The center of the highway is taken to be 30 ft from the wall
on one side, and the home or other sensitive area is considered to be
30 ft on the other side of the wall (the shadow side). A wall 20 ft
high yields something like 25 dB of protection from the highway
noise at 1,000 Hz. At 100 Hz, the attenuation of the highway noise
is only about 15 dB. At the higher audible frequencies, the wall is
more effective than at lower frequencies. The shadow zone behind
the wall tends to be shielded from the high-frequency components
of the highway noise. The low-frequency components penetrate
the shadow zone by diffraction.

Diffraction of Sound by a Slit

Figure 11-6 diagrams a classical experiment performed by Pohl in
acoustical antiquity and described by Wood in somewhat more recent
1
antiquity. One must admire the precise results obtained with crude
measuring instruments (high-pitched whistle, sound radiometer). The