TELEPHONE RECEIVERS 



227 



/here A = effective area of one pole, in square centimeters, 

 A^ = number of turns per coil, 



R\ = reluctance of the permanent field circuit, in gilberts per max- 

 well, 

 R<i = reluctance of the alternating magnetic circuit, in gilberts per 



maxwell, 

 M = magnetomotive force of the magnet, in gilberts, 

 Vax = maximum current in the coil, in abamperes, 

 CO = lirf, 



/ = frequency, in cycles per second, and 

 / = time, in seconds. 



1000 

 FREQUENCY 



CROSS -SECTIONAL VIEW 



Fig. 10.1. Cross-sectional view ot a bipolar telephone receiver. The graph shows the pressure 

 response frequency characteristics. A. Receiver feeding a closed cavity. B. Receiver feed- 

 ing an artificial ear. 



The first and third term on the right-hand side of equation 10.1 represent 

 a steady force, the second term represents a force of the same frequency 

 and the last term represents a force of twice the frequency of the current 

 in the coil. Equation 6.9 shows the necessity for the polarizing field ^i 

 in order to obtain high sensitivity. Further, <^i must be large compared 

 to <^2 in order to reduce second harmonic distortion. 



The diaphragm in the bipolar receiver is a circular plate clamped at the 

 edge. See Sec. 3.5. The first resonance is usually placed at 1000 cycles. 

 In the range below 1000 cycles the system is stiffness controlled. 



If a telephone receiver is very carefully sealed to the ear so that no leak- 

 age occurs between the ear cap and the ear, the acoustic impedance pre- 

 sented to the telephone receiver by the ear is an acoustic capacitance. In 

 order that the sound pressure be independent of the frequency under these 

 conditions the ratio of the current to the amplitude must be independent 

 of the frequency. The sound pressure delivered by a bipolar telephone 

 receiver to a cavity as a function of the frequency is shown in Fig. 10.1. 

 In the range below the resonance frequency the response is independent of 

 the frequency. At the first resonance frequency of the diaphragm the 



