Index of content:
Volume 108, Issue 3, September 2000
- SPEECH PERCEPTION 
108(2000); http://dx.doi.org/10.1121/1.1287710View Description Hide Description
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.
108(2000); http://dx.doi.org/10.1121/1.1287711View Description Hide Description
Speech Intelligibility Index (SII) procedures were used to measure the amount of speechinformation perceived in five frequency bands (170–570, 570–1170, 1170–1768, 1768–2680, and 2680–5744 Hz) by 15 users of the Cochlear Ltd. CI-22M implant and Spectra-22/SPEAK processor. The speechinformation perceived was compared to that perceived by normal-hearing listeners. The ability of these subjects to discriminate between stimulation on adjacent electrodes corresponding to each frequency band was also investigated, using a 4IFC procedure with random current level variations of between 0% and 60% of the dynamic range. Relative to normal-hearing listeners, speechinformation was, on average, significantly more reduced in the four frequency regions between 170 and 2680 Hz than in the region 2680–5744 Hz. There was a significant correlation between electrodediscrimination ability (when the random level variation encompassed 20% or more of the dynamic range) and the amount of speechinformation perceived in the four frequency regions between 170 and 2680 Hz. There was no such correlation in the region 2680–5744 Hz, regardless of the extent of random level variation. These results indicate that speechinformation in the low to medium frequencies is more difficult for implantees to perceive, that this difficulty is correlated with the difficulty in discriminating electrode place in the presence of random loudness variations, and that fine spectral discrimination may be relatively more important in the vowel-formant regions than in higher frequency regions.