PRESSURE MICROPHONES 177 



sinusoidal motion of the diaphragm the current, in amperes, in one of the 

 buttons may be written as 



9.3 



rso + ^^ sin ut 



when e = voltage of the battery, in volts, 



rEo — resistance of the circuit, when ;c = 0, in ohms, 



X = amplitude of the diaphragm, in centimeters, 



h = constant of the carbon element, in ohms per centimeter, 



CO = Itt/", and 



/ = frequency, in cycles per second. 

 The current in the other button is 



i, = f— - ~ 9.4 



^£0 — ^^ sm oit 



The difference of equations 9.3 and 9.4 after expanding is, 



1e ( hx . AV . 



ii — ii = — ( — sm ut + — - sm^ co/ 



rEo V'Eo rE(i 



■) 



2e/hxs\nwt 3 h^x^ . h^x^ . \ r, r 



= — I • + sm CO/ — sm 3co/ ... 1 9.5 



rsoX rEo 4 rso^ 4rEo^ / 



Comparing equation 9.5 with equation 9.2 shows that the large second 

 harmonic term has been eliminated by the use of a push-pull two button 

 microphone. 



One common cause of faulty operation of the carbon microphone is due 

 to the cohering of the carbon granules caused by the breaking of the circuit 

 when the current is flowing. The use of electric filters as shown in the cir- 

 cuit diagram will protect the microphone against cohering. 



The frequency range and response of the double button carbon micro- 

 phone compares favorably with the condenser microphone. The carbon 

 microphone is several times more sensitive than the condenser microphone. 

 However, the limitation is carbon noise. 



B. Condenser Microphone. — A condenser microphone is a microphone 

 which depends for its operation on variations in capacitance. The typical 

 condenser microphone ^' ^^ consists of a thin stretched plate separated from a 



3 Wente, E. C, Phys. Rev., Vol. 10, No. 1, p. 39, 1917. ^ 



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

 phia. 



