Page 275 - The Master Handbook Of Acoustics

250 CHAPTER ELEVEN

P

Source Image of source

FIGURE 11-4

The classic sound barrier case. The sound striking the wall is reflected as though the
sound is radiated from a virtual image of the source. That sound passing the top edge
of the wall acts as though the wavefronts are lines of sources radiating sound energy
into the shadow zone.

equipment layout of Fig. 11-6A is very approximate. Actually the
source/slit arrangement rotated about the center of the slit and
the measuring radiometer was at a distance of 8 meters. The slit
width was 11.5 cm wide, the wavelength of the measuring sound
was 1.45 cm (23.7 kHz). The graph of Fig. 11-6B shows the intensity
of the sound versus the angle of deviation. The dimension B indi-
cates the geometrical boundaries of the ray. Anything wider that B 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 is the striking feature of
this experiment.

Diffraction by the Zone Plate

The zone plate can be considered an acoustic lens. It consists of a
circular plate with a set of concentric, annular slits of cunningly
devised radii. If the focal point is at a distance of r from the plate,