(2) An acoustical network couples the back chamber of the diaphragm through four holes covered with acoustic
resistance fabric to the handset cavity. The chamber above the diaphragm exhausts through the holes in the receiver cap.
The receiver response is virtually flat from 400 to 3,500 cps (cycles per second)--an improvement over earlier receivers.
(3) A varistor, or nonlinear resistance, protects the user from high acoustic levels caused by transient electrical
disturbances in the telephone circuit. This varistor also protects the receiver magnet from demagnetization hazards of
c. The receiver and transmitter units are mounted for convenience in an instrument called a handset. Figure 20a
is a disassembled view of a handset, showing the transmitter element, transmitter cap, receiver element, and receiver cap.
When the receiver cap (earpiece) is screwed on tightly, it exerts a pressure on the receiver element, forcing the two
contacts against two contact springs. These contact springs are connected to the external wiring of the receiver. Like the
transmitter, the receiver element may be removed for servicing or replacement by unscrewing the cap. In the typical
modern combined hand-telephone set shown in figure 20b, the handset rests on a cradle base when not in use.