HIGHLY DIRECTIONAL MICROPHONES 



217 



where d is the path length of the delay introduced for the point furthest 

 removed from the common junction. 



R. = 



sin — {I — I cos Q -\- d) 



A 



- (/ - / cos + ^) 



9.70 



The directional characteristic of the microphone of Fig. 9.34 for various 

 ratios of the length of the Hne to the wavelength, and for a delay path of 



MAGNET 



3 



2 



Fig. 9.34. Line Microphone. Useful directivity on the line axis. This microphone differs 

 from Fig. 9.32 in that a delay is inserted in each small pipe. The amount of delay is pro- 

 portional to the distance from the pipe opening to the pickup point nearest the common 

 junction. 



LENGTH = A 



LENGTH = A 



LENGTH =2A LENGTH = 4A 



Fig. 9.35. The directional characteristics of the microphone shown in Fig. 9.34 for a time 

 delay equivalent to one-quarter of the length of the line as a function of the ratio of the 

 length of the hne to the wavelength. The polar graph depicts the output, in volts, as a 

 function of the angle, in degrees. 



one fourth times the length of the Hne is shown in Fig. 9.35. Comparing 

 Fig. 9.35 with Fig. 9.33, it will be seen that the same directional charac- 

 teristic can be obtained with a shorter line by introducing appropriate 



