 and an artificial voice. These are used to detect humidity areas 20kV to 30kV is recommended. and measure the received signal and generate the tones to stimulate The telephone handset must be balanced SO that the user feels the transmitter. comfortable holding the handset for lengthy periods of time. Many manufacturers add weight to the handset. This is not always just for 58.6.1 Sending sensitivity 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 The sending sensitivity is defined as the ratio of the voltage mechanical design, the switch to seize the line (hookswitch or measured at the terminating impedance of the exchange feed bridge cradleswitch) is not returned to its normal position at the end of the and the sound pressure level injected into the microphone. This ratio call by the weight of the handset, and the exchange equipment is normally measured over the full frequency range. The sensitivity remains seized preventing incoming calls arriving. depends on the frequency response of the microphone, the acoustic Finally the telephone handset can take considerable abuse during path to the microphone, and the transmit gain of the telephone its lifespan, therefore it is required to withstand severe shock tests, circuit. The sending sensitivity and frequency response are designed dropping on the floor, being hit against a desk, and still continue to to have a rising characteristic within the speech band. The reason function. for this is that although most of the power in speech is at the lower This presents the designer with a considerable problem in meeting frequencies, the sharpness of speech is created by the higher fre- all the above diverse requirements to ensure reliable efficient tele- quencies. However, this is the region were losses in the cable are phone service. highest, therefore by increasing the sensitivity at the higher frequen- cies this tends to compensate for the effect of the cable and increases the clarity of