evice Shielded case Winding on former Magnet Figure Piezo-electric transmitter Figure 58.6 Moving coil receiver Ear piece Iron diaphragm current on the line, inducing a change in the magnet field which causes the diaphragm to vibrate, reproducing sound from the re- Hole pattern ceived speech signal. Telephone handset design The telephone handset is designed to provide the optimum ear/mouth geometry for the average human head. Early designs of telephones used a fixed mouthpiece, mainly due to the early forms of carbon microphone, which were subject to the carbon granules generating unwanted noise when the microphone section of the telephone was moved. It was not until manufacture of the carbon microphone improved that it was possible to design a telephone handset that contained both a transmitter and receiver. The ergonomics of the handset design are equally important as the Coils acoustic design, to provide the user with a telephone of high perfor- Magnet mance and reliability of operation. The earpiece needs to be de- signed to sit comfortably on the user's ear. It must form an acoustic seal with the ear to ensure that received sound is directed into the Figure 58.5 Electromagnetic receiver ear and does not leak away into the surrounding air. If the handset has a poor seal to the ear the listener will pick up external noise and will probably complain that the telephone is not loud enough. greater the current detected in the coils the closer the diaphragm is Furthermore, others in close proximity to the user will hear the pulled to the poles of the magnet. The diaphragm therefore vibrates telephone conversation. in unison with the received speech signal. The positioning of the microphone is also critical. For reliable performance it must be possible to comfortably position the handset 58.4.1 Rocking armature receiver such that with the receiver in place on the ear the microphone sits directly in front of the mouth, to ensure that the microphone is in a The construction of this type of receiver is the same as the 