48 BELL SYSTEM TECHNICAL JOURNAL 



In the early microphones employing air damping the diaphragm was 

 composed of a thin sheet of steel which was stretched to give it a 

 relatively high stiffness. When assembled in the microphone the 

 stiffness was further increased by that of the air film between diaphragm 

 and the damping plate with the result that the resonant frequency 

 was well above the frequencies which it was desired to transmit and 

 the diaphragm vibrated in its normal mode over a wide frequency 

 range. In such a structure the mechanical impedance for frequencies 

 below resonance is due almost entirely to stiffness reactance. Hence a 

 constant sound pressure produces substantially the same displacement 

 of the diaphragm at all frequencies within this range and uniform 

 response results except at the very low frequencies where an appreciable 

 reduction in the stiffness of the air film occurs. The effective mass of 

 a steel diaphragm is, however, relatively large and necessitates a 

 comparatively high stiffness to secure the desired resonant frequency. 

 From the standpoint of securing maximum sensitivity of the micro- 

 phone, i.e. displacement of the diaphragm per unit force, it is of course 

 important to make the stiffness as low as possible and employ as small 

 a value of mechanical resistance as is consistent with the degree of 

 damping required. An improvement in both respects can be effected 

 by decreasing the mass of the diaphragm for with a reduced mass a 

 given resonant frequency can be obtained with lower values of stiffness 

 and the desired damping constant secured with less mechanical 

 resistance. 



The aluminum alloys have therefore replaced steel in the diaphragms 

 of most of the condenser microphones in use today. A typical example 

 of such a microphone is the Western Electric Company's instrument 

 (394-type) shown in the photograph. Fig. 1, and the cross-sectional 

 view, Fig. 2. The diaphragm of this instrument is made from alu- 

 minum alloy sheet .0011 inch in thickness. The edges are clamped 

 securely between threaded rings, gaskets of softer aluminum being 

 provided to prevent damage at the clamping surfaces. The requisite 

 stiffness is obtained by advancing the stretching ring until a resonant 

 frequency of 5,000 cycles is obtained. The method of determining 

 the resonant frequency of the diaphragm is as follows. The diaphragm 

 assembly to be tested is coupled to a condenser microphone which is 

 provided with a suitable circuit for measuring its output. A special 

 telephone receiver is placed in contact with the diaphragm on the 

 side opposite to the coupler. Current from a vacuum tube oscillator 

 is then passed through the winding of the receiver, setting up eddy 

 currents in the diaphragm under test. The forces which are developed 

 as a result of the reaction of the magnetic field produced by the eddy 



