Influence of side tone upon the intelligibility of telephone communication

Interference from room noise is controllable in two ways: (1) by the design of a good anti-side tone subscriber's set, and (2) by suitably designing the transmitter capsule and the handset, the latter being arranged so that the mouth-piece is close to the mouth. Measurements are made on the relative intelligibility under noisy conditions of subscriber's sets having short and long handsets and effective and ineffective anti-side tone circuits. The results show that under difficult noisy conditions the absence of side tone may compare in importance with improvements in speech quality. Poor commercial speech may be reasonably intelligible in the absence of side tone, while better quality speech of the same volume may be unintelligible in the presence of loud side tone.