increased sidetone. sound pressure level, and line length. High levels of sidetone are also undesirable in environments with A loudness rating is a standardised method of measuring the high ambient noise since room noise is picked up by the micro- transmission loss of a speech path. It is a single value related to the phone, which is also transmitted to the ear, and this may mask the loudness with which the listener perceives, speech that has been incoming speech signal, making the telephone unusable. emitted by the talker. The performance of a telephone is assessed by Acoustic shock is another significant factor in the design of measuring between the telephone and an impedance representing telephone handsets. As the user will have the handset close to the the telephone termination, at the telephone exchange, over varying ear during a conversation, it is essential that the telephone circuit line lengths and over the full frequency range in which the telephone has a mechanism to prevent large transients or power surges from operates. Known sound pressure levels are applied to the micro- the telephone line being transmitted to the receiver, which could phone of the telephone and the resulting voltage changes appearing produce a sudden surge of sound pressure into the listener's ear- at the simulated exchange terminations are measured. drum. In the other direction signals are applied to the simulated ex- With modern electronic designs of telephone circuits it is necess- change termination and the resulting sound pressure levels are ary to protect the electronics from static discharge from the user into measured at the telephone receiver. These measurements are nor- either the transmitter or receiver, or through the body of the handset mally carried out with the telephone in a sound absorbing box or in where the two sections join. No regulatory specifications exist for an anechoic chamber, with the telephone clamped into a jig which electrostatic discharge but 10kV is the minimum and for dry low has an artificial ear