SOUND WAVE AND MICROWAVE SPACE PATTERNS 567 



ment time of 1 minute allows the photographic record to be inspected almost 

 immediately after the scanning run has been completed. 



Amplitude Patterns 



The first pictures were taken to determine the amplitude distribution in 

 the focal region of a 10'' diam. acoustic lens. This acoustic lens is made of 

 rigid metal strips arranged in an open construction.^ Figure 2 shows an 

 early photograph in which the horizontal motion of the scanning device 

 was too rapid. The line structure of the scanner is therefore very coarse but 



Fig. 4 — Radiation pattern of a (d" square aperture horn. / = 9 KC. 



the lobe structure and focusing effect of the lens are evident. Figure 3 shows 

 a later photograph in which a finer gradation scan with a longer stroke 

 provides a smooth appearing pattern. 



The radiation pattern of a long horn with a 6" square aperture is shown 

 in Fig. 4, taken at a frequency of 9 KC. As in the lens pictures, minor lobes 

 can be seen forming at the sides of the major lobe, while several minima 

 appear faintly in the central region near the aperture (the close-in or Fresnel 

 field). 



The refracting property of a prism made of rigid strips is illustrated by 

 the amplitude pattern in Fig. 5. In combination with the strip lens of Fig. 

 3, it bends the sound beam downward away from the axis. The lens itself is 

 an example of a refractor, of course, but the prism is usually chosen to demon- 



