UNIDIRECTIONAL MICROPHONES 



211 



The open ends of the pipes are separated by a distance D. A bend of 

 length d is placed in the shorter pipe. The ribbon element measures the 

 difference in pressure between the two pipes. The difference in pressure 

 between the two pipes is given by 



Ap = 2po sin 



'd-D Dt "^ 



IT + cos d 



. X X > 



9.63 



where po ^ sound pressure, in dynes per square centimeter, 



D = separation between the receiving ends of the pipes, in centi- 

 meters, 

 d = acoustic path introduced by the bend, in centimeters, 

 X = wavelength, in centimeters, and 



6 = angle the incident pencils of sound make with the axis of 

 the system. 

 If the distances D and d are small compared to the wavelength, Ap will 

 be proportional to the frequency. If a mass controlled electrodynamic 

 element is used, the output will be 

 independent of the frequency. 



A series of directional charac- 

 teristics for various ratios of D to 

 d is shown in Fig. 9.29. 



A diaphragm actuated crystal 



DIAPHRAGM 



ACOUSTIC 

 NETWORK 



CROSS -SECTIONAL VIEW 



SOUND INCIDENT AT 180 



v'ECTOR DIAGRAMS 



unidirectional microphone ^"^ em- 

 ploying a phase shifting network is 

 shown in Fig. 9.30. The principle 

 is essentially the same as that of 

 Fig. 9.29 described above. The 

 vector diagrams show the action Pic 9.30. Crystal unidirectional microphone 

 for sound incident at 0° and 180°. employing a phase shifting network. The 

 Considerable deviation from the T^"-- diagram depicts the magnitude and 

 . . phase available lor dnvmg the diaphragm tor 



cardioid characteristic occurs at o° and 180°. 



the higher frequencies due to the 



relatively large physical size of the microphone compared to the wave- 

 length of the sound. Since the actuating force upon the crystal, and 

 hence the voltage output, is proportional to the frequency, compensation 

 must be employed to obtain a microphone uniformly sensitive with respect 

 to frequency. 



30 Baumzweiger, Benj., Electronics, Vol. 12, No. 2, p. 62, 1939. 



