CONDENSER AND CARBON MICROPHONES 



53 



The difference between the pressure and held calibrations is due to 

 several factors. In the first place the sound is diffracted around the 

 microphone differently at different frequencies. At frequencies where 

 the wave-length is large as compared with its external dimensions the 

 pressure is the same as that of the undisturbed wave. At the higher 

 frequencies where the microphone is large in comparison with the wave- 

 length of the sound, the pressure is twice that developed at the lower 

 frequencies. In the 394-type microphone the effect of diffraction 



-46 



O db = I VOLT (OPEN CIRCUIT) PER BAR 

 POLARIZING VOLTAGE = 200 VOLTS 



Fig. -i- 



lOO 1000 



FREQUENCY IN CYCLES PER SECOND 



Pressure calibration of the 394 type condenser microphone. 



first becomes noticeable in the region of 1200 cycles and reaches a 

 maximum of 6 db at approximately 2200 cycles. The second factor 

 which causes a difference between the pressure and field calibrations is 

 acoustic resonance in the shallow^ cavity in front of the microphone. 

 This causes the pressure actuating the diaphragm to be higher than 

 that of the incident sound wave in the frequency region of 1500 to 

 5500 cycles. The maximum increase in pressure occurs at approx- 

 imately 3500 cycles. If the sound source is so located relative to the 



I -55 



100 1000 10000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 5— Field calibration of the 394-type condenser niicrophone for a direction of 

 approach of sound normal to the diaphragm. 



microphone that the waves approach from a direction normal to the 



diaphragm and reflection from surrounding walls and objects is 



negligible, the combined effect of diffraction and resonance is to 



produce a maximum departure from flatness of approximately 12 db 



as is shown by the field calibration Fig. 5.^ If the sound wave travels 



9 These curves are taken from unpublished work of P. B. Flanders of the Bell 

 Telephone Laboratories, Inc. 



