Combined effects of early reflections and background noise on speech intelligibility

Abstract

An experiment, similar to the one devised by Lochner and Burger to find the integration characteristics of the ear, was performed in an anechoic chamber. The experiment was conducted in the absence of any disturbing background noise at a speech level approaching the threshold of hearing (22 dB(A) re 20 μPa). It is concluded that, under these conditions, the ear integrates reflections fully as described by Lochner and Burger. An experiment is presented which demonstrates the differences in speech intelligibility scores that occur when different speakers are employed. Further experiments were performed to investigate the combined effects of a single early reflection and the background noise level on speech intelligibility. The tests were conducted at speech levels of 35 dB(A), 45 dB(A) and 55 dB(A) for signal to noise ratios of −5, 0 and +5 dB. It was found that under these more realistic speech and noise levels a single reflection is no longer integrated fully. It appears that a reflection becomes more useful as its delay time is reduced to 0 ms. Also, the usefulness of a reflection is directly dependent on the background noise level. A further experiment revealed that, as for a single reflection, the ear does not fully integrate multiple reflections in the presence of background noise. Some of the implications of these findings are considered.