The role of modulation spectrum amplitude and phase in consonant intelligibility

This paper considers an acoustic basis for speech intelligibility and evaluates various acoustically based speech intelligibility prediction algorithms. Earlier research indicates that speech intelligibility does not require preservation of spectral and temporal fine‐structure, but is highly dependent on the preservation of the amplitude component of the modulation spectrum [R. Drullman, J. Acoust. Soc. Am. 97, 585–592 (1995)]. This study assessed the importance of the phase component of the modulation spectrum using a 21‐alternative forced‐choice consonant perception test. Temporal and spectral fine‐structure were removed by modulating a white noise carrier with 50 Hz low‐pass filtered speech amplitude envelopes in 4, 8, or 24 discrete bands. Modulation spectrum phase was distorted by imposing a random delay in each discrete band. Behavioral results are discussed in light of intelligibility predictions generated by the articulation index [N. R. French and J. C. Steinberg, J. Acoust. Soc. Am. 19, 90–119 (1947)] and the speech transmission index [H. J. M. Steeneken and T. Houtgast, J. Acoust. Soc. Am 67, 318–326 (1980)].