578 THE BELL SYSTEM TECHNICAL jOtJfeNAL, JULY 1951 



to Fig, 17 except that the connections to one of the sound sources were 

 reversed. With the two sources out of phase, cancellation now occurs straight 

 ahead. 



The diffraction pattern of one wide slit would be similar to that of the 

 4 wavelength radiator of Fig. 4. 



To show the diffraction rings produced at the focal point of a lens, the 

 sound field of a strip lens was scanned in a plane perpendicular to that of the 

 previous photos. Figure 20 shows such a scan made in a plane passing through 

 the focal point and perpendicular to the lens axis. The usual optical formulae 



Fig. 20— By scanning a plane perpendicular to the axis of radiation, the diffraction rings 

 around the focal spot of the lens of Fig. 3 are portrayed. / = 9 KC. 



determine the size of the focal spot and the position of the surrounding 

 diffraction rings. They are functions of the focal distance, the aperture, and 

 the wavelength. In the previous lens patterns of Figs. 3 and 7, these diffrac- 

 tion rings show up as minor lobes. For a perfectly symmetrical lens construc- 

 tion, the rings would be more perfect, and the minor ''lobes" would in reality 

 be cones of energy surrounding the major lobe. 



Diffusion of Sound 



The previous lenses have been of the convergent type for focusing or 

 beaming energy. In the next pair of pictures is shown the effect of a diver- 

 gent lens diffusing energy. The phase pattern of a 6" square aperture horn 



