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 acoustic design, to provide the user with a telephone of high perfor­mance and reliability of operation. The earpiece needs to be de­signed 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 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. Furthermore, others in close proximity to the user will hear the telephone conversation.

The positioning of the microphone is also critical. For reliable performance it must be possible to comfortably position the handset 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 direct line of any sound being generated. The choice of material for the handset and the size and positioning of any holes, is also critical to the acoustic performance of the handset. Mounting of the trans­mitter and receiver should also be done with care, with sufficient vibration absorbing material placed between the body of the handset and the transmitter or receiver to ensure that sound vibrations do not travel through the handset body from transmitter to receiver or in the other direction and impair performance.

Sidetone is a unique feature of telephone handset design and is essential for good performance of the telephone. When using a telephone handset the normal acoustic feedback from mouth to ear is partially blocked, therefore to compensate a portion of the talker's own voice is fed back into the receiver. Subjective tests have shown that, at high levels of sidetone, the speech power levels transmitted to the telephone network are significantly reduced, resulting in the distant end experiencing a low volume of received sound. This is due to the talker's voice being lowered and moving the handset away as he tries to compensate for the increased sidetone.

High levels of sidetone are also undesirable in environments with high ambient noise since room noise is picked up by the micro­phone, which is also transmitted to the ear, and this may mask the incoming speech signal, making the telephone unusable.

Acoustic shock is another significant factor in the design of telephone handsets. As the user will have the handset close to the ear during a conversation, it is essential that the telephone circuit has a mechanism to prevent large transients or power surges from the telephone line being transmitted to the receiver, which could produce a sudden surge of sound pressure into the listener's ear­drum.

With modern electronic designs of telephone circuits it is necess­ary to protect the electronics from static discharge from the user into either the transmitter or receiver, or through the body of the handset where the two sections join. No regulatory specifications exist for electrostatic discharge but lOkV is the minimum and for dry low humidity areas 20kV to 30kV is recommended.

The telephone handset must be balanced so that the user feels comfortable holding the handset for lengthy periods of time. Many manufacturers add weight to the handset. This is not always just for balance since it is sometimes necessary to counteract the 'walk on' problem. If a handset is too light, or the telephone is of a poor mechanical design, the switch to seize the line (hookswitch or cradleswitch) is not returned to its normal position at the end of the call by the weight of the handset, and the exchange equipment remains seized preventing incoming calls arriving.

Finally the telephone handset can take considerable abuse during its lifespan, therefore it is required to withstand severe shock tests, dropping on the floor, being hit against a desk, and still continue to function.

This presents the designer with a considerable problem in meeting all the above diverse requirements to ensure reliable efficient tele­phone service.

Дата добавления: 2015-03-31