210 



MICROPHONES 



D. Distortion of the Directional Pattern in the Unidirectional Micro- 

 phone. — Deviations from the cardioid characteristic in the unidirectional 

 microphone are due to: 



1. Phase shift in the velocity microphone due to deviation from a pure 

 mass reactance. 



2. Phase shift in the velocity microphone due to diffraction. 



3. Phase shift in the oressure microphone due to deviation from resistance 

 control. 



4. Phase shift in the pressure microphone due to diffraction. 



5. Deviation in the output from a cosine directional characteristic in the 

 velocity microphone. 



6. Deviation in output with angle in the pressure microphone. 

 7 Unequal sensitivity of the two elements. 



The phase angle between the output of a velocity microphone and the 

 particle velocity in a plane wave has been considered in Sec. 9.35. The phase 

 shift in a pressure ribbon microphone has been considered in Sec, 9.2D3. 

 It is possible to adjust these phase shifts and those due to diffraction so 

 that the cancellation for 180° will be of the order of —20 db up to 10,000 

 cycles. In the case of the dynamic pressure unit the problem of maintain- 

 ing appropriate phase shifts is more difficult. 



RIBBON DAMPED PIPES 



Fig. 9.29. A directional microphone employing a phase shifting system. The polar graphs 

 show the directional characteristics for various ratios o( d/D. The polar graph depicts the 

 output, in volts, as a function of the angle, in degrees. The maximum response is arbi- 

 trarily chosen as unity. 



E. Phase Shifting Unidirectional Microphones. — A unidirectional micro- 

 phone consisting of a nondirectional and bidirectional microphone has been 

 described in the preceding section. It is the purpose of this section to 

 describe other means for obtaining directional response. 



The elements of a phase shifting microphone are shown in Fig. 9.29. 



