Page 271 - The Master Handbook Of Acoustics

246 CHAPTER ELEVEN

that diffraction can cause light to change its direction of travel. In fact,
all types of wave motion, including sound, are subject to diffraction.
The shorter the wavelength (the higher the frequency), the less
dominant is the phenomenon of diffraction. Diffraction is less notice-
able for light than it is for sound because of the extremely short wave-
lengths of light. Obstacles capable of diffracting (bending) sound must
be large compared to the wavelength of the sound involved. The well-
worn example of ocean waves is still one
of the best. Ocean waves sweep past a pil-
ing of a dock with scarcely a disturbance.
New Ocean waves, however, are bent around an
Source end of an island.

Diffraction and Wavelength

The effectiveness of an obstacle in dif-
fracting sound is determined by the
acoustical size of the obstacle. Acoustical
size is measured in terms of the wave-
A length of the sound. One way of looking
at the illustration shown later, Fig. 11-3,
is that the obstacle in B is the same phys-
ical size as that of A, but the frequency of
New Source the sound of A is one tenth that of B. If

the obstacle in B is 1 ft long and that of A
0.1 ft long, the frequency of the sound in
A could well be 1,000 Hz (wavelength
1.13 ft), and that of B could be 100 Hz
(wavelength 11.3 ft). The same drawing
could be used if the obstacle of A were
0.01 ft long with a frequency of 10,000 Hz
B (wavelength 0.113 ft) and the obstacle of
FIGURE 11-1 B were 0.1 ft long with a frequency of
(A) If the brick wall is large in terms of the wave- 1,000 Hz (wavelength 1.13 ft).
length of the sound, the edge acts as a new source, In Fig. 11-1, two types of obstructions to
radiating sound into the shadow zone. (B) Plane plane wavefronts of sound are depicted. In
waves of sound impinging on the heavy plate with a
small hole in it sets up spherical wavefronts on the Fig. 11-1A a heavy brick wall is the obsta-
other side due to diffraction of sound. cle. The sound waves are reflected from the