Input Impedance of Telephone Receivers

If a receiver is connected to an impedance bridge as in Fig. 25, if the test current indicated by the milliammeter is held constant by rheostat r, and if the resistance R and the inductance L required to balance the receiver impedance at different frequencies are measured, it will be found that these measured values vary greatly with test conditions. If the receiver is radiating sound energy into a small closed box the impedance will be greatly different from that if it is radiating into a large room. Or if the receiver is held to the ear the impedance will be different from that measured if it is radiating into a room. The impedance of a receiver or loudspeaker varies with the acoustic load on the diaphragm.

Figure 25. Impedance bridge for measuring receiver constants.

The vector difference between the normal and the blocked impedance of an electroacoustic transducer such as a telephone receiver or a loudspeaker is defined as the motional impedance¹ of the device. This motional impedance is proportional to the back electromotive force induced in the receiver windings by the motion of the receiver diaphragm. The reluctance of the magnetic circuit of a receiver changes with the motion of the receiver diaphragm, and thus the flux linking the receiver coils varies, inducing a back electromotive force in these coils. If the diaphragm is damped, the diaphragm motion is influenced, and thus the induced back electromotive force and therefore the impedance are changed.