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 slopes of 100 dB/octave [Warren et al., Percept. Psychophys. 57, 175-182 (1995)]. A subsequent study by Warren and Bashford [J. Acoust. Soc. Am. 106, L47-L52 (1999)] found that it was not the 1/3-octave passband, but the transition bands that were chiefly responsible for this high intelligibility: When the passband and transition bands were segregated using filter slopes of 1000 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 present study, experiment 1 examined the distribution of information along the transition bands' slopes by truncation at graded downpoints: Truncation at downpoints 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 passband (as defined conventionally by 3-dB downpoints) and an intelligibility score of 60%; truncation at downpoints of 50 dB or more produced no significant change in intelligibility. Experiment 3 determined the intelligibilities of rectangular bands (1000-dB/octave slopes) centered on 1500 Hz. Their bandwidths ranged from 3 to 12 semitones in 1-semitone steps, resulting in intelligibility scores increasing monotonically from 14% to 94%. Calculations based upon experiments 2 and 3 showed that the triangular band truncated at 30-dB downpoints had half the intelligibility of a rectangular band having the same frequency range.