Effect of speaking styles on the relevance of the Speech Transmission Index in the presence of reverberation
Sander J. van Wijngaarden, Tammo Houtgast · The Journal of the Acoustical Society of America · 2003
The Speech Transmission Index (STI) predicts speech intelligibility under various speech degrading conditions, including noise and reverberation. Measurements of sentence intelligibility in noise and reverberation, for three different talkers adopting different speaking styles, indicate that the STI is sometimes inaccurate in the presence of reverberation. For a trained talker speaking clearly, the STI predictions are accurate, but for conversational speech by untrained talkers, the effect of reverberation is underestimated. By using a wider range of modulation frequencies than prescribed for the standardized STI calculation (up to 31.5 Hz instead of 12.5 Hz), more accurate predictions are obtained for conversational speech. This can be understood by comparing the envelope spectrum between talkers and speaking styles: conversational speech tends to spread more of the energy in its envelope spectrum to higher modulation frequencies. Since higher modulation frequencies are more susceptible to reverberation, conversational speech tends to be affected more by reverberation than clear speech. To investigate the range of between-talker variations, envelope spectra were calculated for a population of 134 talkers. Upper boundaries of the STI modulation frequency range of 12.5 and 31.5 Hz seem approximately appropriate for the 5th and 95th percentile of this population.