Karen L. Payton
ECE Dept., Univ. of Massachusetts, North Dartmouth, MA 02747
Louis D. Braida
MIT, Cambridge, MA 02139
The speech transmission index (STI) is highly correlated with speech intelligibility scores when the environment is degraded by noise and/or reverberation and/or the listener's hearing is impaired [e.g., Payton et al., J. Acoust. Soc. Am. 95, 1581--1592 (1994)]. The STI is typically computed from modulation transfer functions (MTFs) that are determined theoretically, based on effective SNR, or on measurements using the RASTI procedure. In principle, however, MTFs can be computed directly from speech envelope spectra. For the current study, envelope spectra were computed for both conversational and clearly articulated speech. Three environments were considered: quiet/anechoic, reverberant (0.6 s RT), and additive noise (0 dB SNR). Results indicate that reliable MTFs can be computed from speech envelope spectra if the coherence function is used to limit the range of modulation frequencies (to reduce the effects of computational artifacts). Also, while MTFs for the two speaking styles are very similar in additive noise, in reverberation MTFs differ as one would expect on the basis of the higher intelligibility of clear speech in reverberant environments. [Work supported by NIH.]