216 



MICROPHONES 



The absolute value of the term on the right is given by 



* I *J -1/2 



2-n-j{x—xcose)/\ 



dx 



Re = 



/ 7 7 \ 



Sin - (/ — /cos B) 

 X ^ 



- (/ - I cos 6) 



9.67 



9.68 



The directional characteristics of the microphone of Fig. 9.32 for various 

 ratios of length of the line to the wavelength are shown in Fig. 9.33. 



LENGTH = ^ 



LENGTH = — 



LENGTH =2X 



LENGTH - a\ 



Fig. 9.33. The directional characteristics of the microphone shown in Fig. 9.32 as a function 

 of the ratio of the length of the line to the wavelength. The polar graph depicts the output, 

 in volts, as a function of the angle, in degrees. 



These characteristics are surfaces of revolution about the line as an axis. 

 This microphone is useful for collecting sounds arriving from directions 

 making small angles with the microphone axis. 



2. Line Microphone: Useful Directivity on the Line Axis. Line with 

 Progressive Delay. — As in the case of Fig. 9.32 this microphone consists of 

 a number of small pipes with the open ends, as pickup points, equally 

 spaced on a line and the other ends joined at a common junction. In 

 addition, there is inserted a delay which is proportional to the distance 

 from the end of the line or the pickup point nearest the common junction 

 (Fig. 9.34). 



1 I /^'/2 



R^ = —\ / Bj'''^^'' - ^ ^^^ "/'^ + '^/^' dx 



Bnl\*J -1/2 



9.69 



