Intelligibility of bandpass speech: Effects of truncation or removal of transition bands

An intelligibility of over 90% was reported for keywords in "everyday" 1/3- octave sentences centered on 1500 Hz and having steep transition band slope s of 100 dB/octave [Warren et al., Percept. Psychophys. 57, 175-182 (1995)] . A subsequent study by Warren and Bashford [J. Acoust. Sec. Am. 106, L47-L 52 (1999)] found that it was not the 1/3-octave passband, but the transitio n bands that were chiefly responsible for this high intelligibility: When t he passband and transition bands were segregated using filter slopes of 100 0 dB/octave, the isolated passband had an intelligibility score of only 24% , while the pair of transition bands had a score of over 80%. In the presen t study, experiment 1 examined the distribution of information along the tr ansition bands' slopes by truncation at graded downpoints: Truncation at do wnpoints of 40 dB or more produced no significant change in intelligibility . Experiment 2 closed the gap separating the transition bands so that their slopes intersected at 1500 Hz. This triangular band had a negligible passb and (as defined conventionally by 3-dB downpoints) and an intelligibility s core of 60%; truncation at downpoints of 50 dB or more produced no signific ant change in intelligibility. Experiment 3 determined the intelligibilitie s of rectangular bands (1000-dB/octave slopes) centered on 1500 Hz. Their b andwidths ranged from 3 to 12 semitones In 1-semitone steps, resulting in i ntelligibility scores increasing monotonically from 14% to 94%. Calculation s based upon experiments 2 and 3 showed that the triangular band truncated at 30-dB downpoints had half the intelligibility of a rectangular band havi ng the same frequency range. (C) 2000 Acoustical Society of America. [S0001 -4966(00)01009-2].