Telephone speech quality and intelligibility in noisy environments

Abstract

Telephone listening often takes place in high ambient noise environments. Listening tests were performed on the relationship between intelligibility and quality, and received speech level and line noise level, over a wide range of ambient noise levels. Intelligibility is usually shown as a function of the signal‐to‐noise ratio. In listening over the telephone three sources of noise can be identified that affect the acoustic signal‐to‐noise ratio. These are line noise, room noise entering the ear acoustically via the leakage path between the receiver cap and the pinna, and noise picked up by the transmitter and mixed with the speech electrically via the sidetone network. Speech intelligibility and quality tests were performed with received line noise levels from 30 to 72 dB(A) and speech levels of 68 and 84 dB(A). Ambient noise levels ranged from 55 to 82 dB(A). Intelligibility was measured using the Modified Rhyme Test, and quality was measured on a five point scale. For all conditions sound levels were measured at the eardrum using KEMAR. The results of these tests show that the relationship between intelligibility, quality, and signal‐to‐noise ratio can best be predicted when all sources of noise are taken into account and measured at a single location.