228 MISCELLANEOUS TRANSDUCERS 



response Is very high. Above the resonance frequency the amplitude de- 

 creases rapidly with frequency. The peak at 3000 cycles is the second 

 resonance frequency of the diaphragm. 



The response frequency characteristic labeled A^ Fig. 10.1, was obtained 

 with no leak between the ear and the ear cap. In all hard ear caps a leak 

 occurs between the ear and the telephone receiver and the acoustic im- 

 pedance presented to the telephone receivers is considerably more complex 

 than that of a capacitance of a small cavity. In the case of telephone 

 receivers worn in the customary manner the acoustic impedance has three 

 components, namely: the resistive and inertive components due to the leak 

 between the ear cap and the ear and the acoustic capacitance due to the 

 ear cavity. These factors will be considered in detail in the section on the 

 testing of telephone receivers. See Sec. 11.4. 



The response characteristic indicated as B in Fig. 10.1 was taken on an 

 artificial ear which simulates the conditions encountered in actual practice. 

 The artificial ear, see Sec. 11.45, introduces a leak which corresponds to 

 the leak between the ear and the ear cap. It will be seen that the effect 

 of this leak is to reduce the response at the lower frequencies. Those 

 familiar with telephone receivers have noticed that the low frequency re- 

 sponse is increased when the leak is reduced by pressing the telephone re- 

 ceivers tightly against the ears. 



Since the development of the bipolar telephone receiver by Alexander 

 Graham Bell the construction has remained essentially the same. Im- 

 provements have been made in sensitivity and response by the use of better 

 materials. However, the clamped plate diaphragm characterized by 

 prominent resonant peaks was retained. Referring to Fig. 10.1, it will be 

 seen that the peaks due to the first and second resonance fall within the 

 response range. These resonances not only introduce frequency distortion, 

 but increase the intensity of reproduction of cHcks due to the poor tran- 

 sient response. See Sec. 7.16. 



A new receiver^ has been designed in which all the prominent resonances 

 within the response range have been eliminated and the response frequency 

 characteristic improved both from the standpoint of uniformity as well as 

 from the frequency range. The new receiver is of the bipolar permanent 

 magnet type. The magnetic circuit consists of pole pieces of 45 per cent 

 Permalloy, two straight bar magnets of Remalloy and a Permandur dia- 

 phragm. The use of these materials increases the efficiency of the unit. 



The equivalent electrical circuit of the mechanical system is shown in 



1 Jones, W. C, Jour. A.I.E.E., Vol. 57, No. 10, p. 559, 1939. 



