Page 157 - Master Handbook of Acoustics

FIGURE 7-6 A consideration of Pohl’s classic experiment in diffraction. (A) An approximation of

the equipment arrangement showing source and slit. (B) Diffraction causes the beam to broaden with
a characteristic pattern. The narrower the slit, the greater this broadening of the beam. (Wood.)

Diffraction by a Zone Plate

The zone plate shown in Fig. 7-7 can be considered an acoustic lens. It consists of a circular plate

with a set of concentric, annular slits of specific radii. If the focal point is at a distance of r from the
plate, the next longer path must be r + λ/2, where λ is the wavelength of the sound falling on the plate
from the source. Successive path lengths are r + λ, r + 3λ/2, and r + 2λ. These path lengths differ by
λ/2, which means that the sound through all the slits will arrive at the focal point in phase which, in
turn, means that they add constructively, intensifying the sound at the focal point.

FIGURE 7-7 The zone plate acts as an acoustic lens. The slits are arranged so 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 constructively. (Olson.)

Diffraction around the Human Head

Figure 7-8 illustrates the diffraction caused by a sphere roughly the size of the human head. This
diffraction by the head as well as reflection and diffraction from the shoulders and upper torso
influences human perception of sound. In general, for sound of frequency 1 to 6 kHz arriving from the
front, head diffraction tends to increase the sound pressure in front and decrease it behind the head.

As expected, for lower frequencies, the directional pattern tends to become circular.