182 MICROPHONES 



1. Direct Actuated Crystal Microphone. — In the direct actuated crystal 

 microphone the sound pressure acts directly upon the crystal. A common 

 form of sound cell for a direct actuated crystal microphone consists of two 

 bimorph elements assembled as shown in Fig. 9.5. The cavity formed by 

 the two crystal elements is completely enclosed so that the application of 

 an external pressure causes a deformation of the crystal. 



The internal voltage e developed by the crystal is 



e = Kx 9.13 



where K = constant of the crystal, and 



X = effective amplitude of the deformation of the crystal by an 

 applied force. 

 The effective periodic force, Jm, required to produce a periodic dis- 

 placement, x^ in the crystal is 



X 



Im = — 9.14 



where Cm = the effective compliance of the crystal. 



The force acting in the case of the directly actuated crystal is 



Im = pA 9.15 



where p = sound pressure, and 



A = effective area of the crystal. 

 From equations 9.13, 9.14 and 9.15 the generated voltage is 



e = KCuAp 9.16 



Equation 9.16 shows that the internal voltage generated by the micro- 

 phone is in phase with the sound pressure in the sound wave. A typical 

 direct actuated microphone shown in Fig. 9.5 consists of four sound cells. 

 The use of several cells in parallel reduces the high internal impedance as 

 compared to that of the single cell. The response characteristic of sound 

 cells can be varied in design so that uniform response can be obtained up 

 to 17,000 cycles. In the case of a small crystal element and an open struc- 

 ture the system is nondirectional. 



2. Diaphragm Actuated Crystal Microphone. — In the diaphragm ac- 

 tuated crystal microphone the sound pressure acts upon a diaphragm which 

 in turn drives a crystal. The output of the diaphragm actuated type is 

 considerably higher than the direct actuated type because the diaphragm 

 acts as a coupling unit between the relatively low impedance of the air and 



