566 



BELL SYSTEM TECHNICAL JOURNAL 



data obtained by L. J. Sivian on a large number of ears, that constant 

 amplitude of motion of the diaphragm per unit current throughout 

 the frequency range is on the average the best condition to strive for 

 in the design of a high quality receiver. We shall therefore assume 

 that at any frequency the amplitude of motion of the diaphragm per 

 unit current is a correct measure of the response of the receiver. It 

 will be assumed also that the impedance of the cavity is without effect 

 on the displacement of the diaphragm. For the receivers to be con- 

 sidered this assumption introduces but little error, although the effect 

 is not negligible in general. 



The voltage generated by a moving coil in a magnetic field is pro- 

 portional to the velocity; therefore, the diaphragm of a uniformly 

 sensitive microphone with a rigidly attached coil should have, at all 

 frequencies, the same velocity per unit of pressure in the actuating 

 sound wave. Expressed in another way, if the diaphragm has a 

 constant effective area, the mechanical impedance (force per unit 

 velocity) of a microphone diaphragm should be the same at all fre- 

 quencies, whereas that of the receiver should be inversely proportional 

 to the frequency. The receiver and the microphone to be described 

 are quite similar in design and construction, but their dynamical con- 

 stants differ so as to approach these conditions of impedance. 



If a receiver or microphone is constructed with a diaphragm having 

 a single degree of freedom, the operating conditions of the diaphragm 

 can be represented by the circuit diagram shown in F'ig. 1, where nio 



30 500 1,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 1 — Response of a simple resonant system. 



5,000 10,000 



