EXPLORING WAYS TO IMPROVE STI'S RECOGNITION OF THE EFFECTS OF POOR SPECTRAL BALANCE ON SUBJECTIVE INTELLIGIBILITY

摘要

Over their many years of designing and commissioning sound systems, the authors have acquired considerable field evidence showing that an unbalanced frequency response can greatly affect subjective speech intelligibility. Relatively small changes to the response, sometimes as small as 1 dB, can noticeably affect the intelligibility of conversational speech and the degree of listening concentration that is required. The speech transmission index (STI) (1) has gained international acceptance as a useful measure of the ability of a transmission path to faithfully transmit speech intelligibility. However, recent work (2), (3) indicates that changes in subjective intelligibility due to poor frequency response do not appear to be reflected in STI measurements of those systems. The improvements to intelligibility that equalisation can provide suggest that either the speech spectrum or the model of psychoacoustic upward-masking used by STI might not correctly reflect the subjective process of listening to speech. Other mechanisms are also likely to be contributing to this situation. The work presented in this paper draws upon prior work (2), (4). In (2), Leembruggen and Stacey compared measured STI and subjective word scores and noted a considerable mismatch between those scores, especially when various filters were applied to the speech. This paper examines the effects on STI of a range of spectra that occur during regular speech and explores the use of alternative masking models with the STI method.