SOUND WAVES 



L/X 



.04 .06 0.1 



0.4 0.6 



Fig. 1.5. The diffraction of a sound wave by a cylinder, cube and sphere. (After Muller, 



Black and Dunn.) 



Another example of difFraction of sound is illustrated by the zone plate 

 shown in Fig. 1.6. The path lengths of the sound from the source to the 

 focus varies by an integral wavelength. As a consequence, all the pencils 



FRONT VIEW CROSS-SECTIONAL VIEW 



Fig. 1.6. Zone plate. The source S and the focus F are equidistant from the zone plate. 



of sound are in phase at the focus with the result that the sound pressure 

 is considerably greater at this point than any other position behind the 

 zone plate. 



