Effect of Noise on Listening Levels in Conference Telephony

Abstract

The present study outlines the effects of background noise on requested loudspeaker levels of incoming speech signals at the conference table for four basic listening assessments. Two general classes of loudspeaker systems were employed proximity units in the “wings” of individual high-back chairs and a loudspeaker display above the long axis of the conference table. In addition, a brief outline of the effect of noise on vocal output has been included. The results show that for either loudspeaker system the preferred listening level under relatively quiet conditions (NC-23) was 63 dB (A+B)/2 as measured at the ears of the observers. This is approximately the level of so called conversational speech at a meter. For a 37-dB increase in the noise level (to NC-60), the participants requested a 15.5 dB increase in the preferred listening level. This compares with a 13.5-dB increase in the vocal output of the participants for the same 37-dB increase in noise level. The range between the strain level (threshold of message intelligibility) and the maximum acceptable listening level for conference purposes was about 50 dB under quiet conditions but only about 20 dB at NC-60. This rather rapid convergence stresses the importance of operating at a relatively low noise level in order to insure good intelligibility of all incoming voice signals.