180 MICROPHONES 



The amplitude of the diaphragm is given by 



Jm 





9.12 



coCjtfi ajCjif2 



where /at = appHed force, in dynes, 



Vm = damping mechanical resistance of air film, in mechanical ohms, 

 m = effective mass of the diaphragm, in grams, 

 Cmi = compliance due to stiffness of the diaphragm, in centimeters 



per dyne, 

 Cm2 = compHance due to stiffness of the air film, in centimeters per 

 dyne, 

 CO = Itt/, and 



/ = frequency, in cycles per second. 

 Equation 9.12 shows that the sensitivity below the resonance frequency 

 is inversely proportional to the stiffness and the mechanical resistance. 

 For the same fundamental resonance frequency the stiffness can be re- 

 duced by decreasing the mass. This procedure also reduces the amount 

 of mechanical resistance required to damp the fundamental resonance and 

 thereby obtain uniform response. Aluminum alloys, due to the low den- 

 sity and high tensile strength, are the logical materials for use in dia- 

 phragms. The minimum diaphragm thickness suitable for the manufacture 

 of condenser microphones is about .001 inch. The capacity of a micro- 

 phone with a diaphragm diameter of If inches and a spacing of from .001 

 to .002 inch is from 400 to 200 mmfds. Due to the high impedance of 

 this capacitance it is necessary to locate the microphone near the vacuum 

 tube amplifier. The capacitance of a long connecting cable reduces the 

 sensitivity without frequency discrimination because the internal imped- 

 ance of the microphone is also a capacitance. The response frequency 

 characteristics of a condenser microphone for constant sound pressure on 

 the diaphragm and for constant free wave sound pressure are shown in 

 Fig. 9.4. 



C. Piezoelectric {Crystal) Microphones '^'^'^'^^. — A piezoelectric micro- 

 phone is a microphone which depends upon the generation of an electro- 



7 Sawyer, C. B., Proc. Inst. Rad. Eng., Vol. 19, No. 11, p. 2020, 1931. 



8 Williams, A. L., Jour. Soc. Mot. Pic. Eng., Vol. 18, No. 4, p. 196, 1934. 

 ^Nicolson, U. S. Patent 1,495,429. 



*^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



