rocking direct line of any sound being generated. The choice of material for armature transmitter. When this device is used as a receiver it is the handset and the size and positioning of any holes, is also critical similar in operation to the electromagnetic type. An a.c. current is to the acoustic performance of the handset. Mounting of the trans- detected in the telephone line and this causes the armature to move mitter and receiver should also be done with care, with sufficient back and forth in sympathy with the a.c. current on the line. The vibration absorbing material placed between the body of the handset armature is linked to a diaphragm which vibrates causing sound to and the transmitter or receiver to ensure that sound vibrations do not be reproduced. travel through the handset body from transmitter to receiver or in the other direction and impair performance. 58.4.2 Moving coil receiver Sidetone is a unique feature of telephone handset design and is essential for good performance of the telephone. When using a This type of receiver is similar in construction to a loudspeaker, as telephone handset the normal acoustic feedback from mouth to ear shown in Figure 58.6. A flexible diaphragm, normally of a plastic is partially blocked, therefore to compensate a portion of the talker's material, is formed into a shallow conical shape and a former, own voice is fed back into the receiver. Subjective tests have shown wound with very thin wire, is attached to the diaphragm. The that, at high levels of sidetone, the speech power levels transmitted assembly is then suspended in the field of a permanent magnet. The to the telephone network are significantly reduced, resulting in the coil is connected to the telephone line and detects the a.c. speech distant end experiencing a low volume of received sound. This 58/6 Telephones and headsets effect is due to the talker's voice being lowered and moving the into a loudness rating i.e. a single number for a given frequency, handset away as he tries to compensate for the 