PRESSURE MICROPHONES 



181 



motive force by the deformation of a crystal having piezoelectric properties. 

 Section 6.5 considered the piezoelectric crystal as a driver. In the case of 

 the microphone the reverse effect is used. The voltage generated due to 

 a deformation of the crystal is proportional to the displacement. There- 

 fore, to obtain a uniformly sensitive microphone with respect to frequency 

 the displacement for a constant applied force must be independent of the 

 frequency. Rochelle salt exhibits the greatest piezoelectric activity of all 

 of the known crystals. For this reason it is used in audio frequency micro- 

 phones. There are two general classifications of crystal microphones, 

 namely: the direct actuated and the diaphragm actuated. In the direct 



TERMINALS 

 TWISTER BIMORPH 



DIAPHRAGM 



TERMINALS 

 BASE 



BENDER BIMORPH 

 CRYSTAL ELEMENTS 



CRYSTAL 

 MICROPHONE 

 UNIT OR CELL 



DIRECT 



ACTUATED 



CRYSTAL 



MICROPHONE 



DIAPHRAGM 

 ACTUATED CRYSTAL 

 MICROPHONE 



Fig. 9.5. Crystal elements and sound cells. A direct actuated crystal microphone. A cross- 

 sectional view of a diaphragm actuated crystal microphone. 



actuated the sound pressure acts directly upon the crystal. In the dia- 

 phragm actuated the sound pressure acts upon a diaphragm which is 

 coupled to a crystal. The crystal element, Fig. 9.5, is made up of two crys- 

 tals cut so that a voltage is generated when forces are applied as shown. 

 The two types of bimorph elements, namely, " twisters " and " benders," are 

 shown in Fig. 9.5. The advantages of a bimorph construction over the 

 single crystal are as follows: it lends itself to a more efiicient size and 

 shape; it becomes more sensitive (a gain of 15 times for practical shapes); 

 it reduces the variations of the mechanical and electrical constants of the 

 crystal for changes in temperature. The temperature limits of bimorph 

 crystals are from — 40° F. to 130° F. If exposed to temperatures in excess 

 of 130° F. the crystal loses its piezoelectric activity permanently. The 

 sensitivity or voltage output of the crystal varies with temperature due 

 primarily to a change in the capacitance and in a lesser degree to a change 

 in the developed voltage. 



