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Extended Bandwidth and Speech Perception
Pittman1 noted that, upon high school graduation, the average graduate commands some 60,000 words. Unfortunately, children with hearing impairment develop vocabulary in a delayed fashion, apparently related to their degree of hearing loss. Pittman evaluated 50 children between 8 and 10 years of age. A total of 36 of the children had normal hearing and 14 had moderate-to-severe hearing loss. Her study compared word-learning rates when children were exposed to restricted (4 kHz) versus extended high-frequency (9 kHz) bandwidths. Regardless of hearing status, Pittman found all children learned words significantly faster while using extended high-frequency bandwidths. The research noted that restricted hearing aid bandwidths may provide an ambiguous signal; children may require more exposures to the primary signal to perceive the subtle acoustic elements required for word-learning.
Stelmachowicz et al4 in 2001 investigated stimulus bandwidth as it relates to the perception of the phoneme /s/ across 80 subjects, including normal-hearing and hearing-impaired subjects. The speech stimuli were created by male, female, and child talkers. Speech stimuli were low-pass filtered at five settings between 2 kHz and 9 kHz. Although subjects' perceptual performance for the male speaker was maximal at a bandwidth of 5 kHz, mean performance for the female speaker improved until the widest bandwidth (9 kHz) was reached. Likewise, for the child speaker, the subjects' performance increased steadily as the bandwidth was increased. The authors stated aided audibility for high-frequency sounds is a problem for children with mild-to-moderate hearing loss, as well as for children with severe-to-profound hearing loss.
In 2004, Stelmachowicz et al5 echoed concerns about limited bandwidth in behind-the-ear (BTE) hearing aids as inadequate for accurate representation of high-frequency speech sounds. The authors stated that, even in BTEs, gain typically dropped precipitously at about 5 kHz (at least in 2004). Further, they concluded adult studies of hearing aid users (presumably with speech and language skills developed and intact) cannot be used to predict performance in children (in whom speech and language skills are being developed), and they suggested the greatest delays in hearing-impaired children (with respect to phonological development) occur with fricatives, consistent with inadequate hearing aid bandwidth.
Last year, Stelmachowicz et al6 reported on 32 children with normal hearing and 24 children with hearing loss. The children ranged in age from 7 to 14 years. Four auditory tasks were used to assess the effects of bandwidth. The speech stimuli were from a female talker, low pass filtered at 5 kHz and 10 kHz in noise. Normal-hearing children demonstrated significant bandwidth effects for nonsense syllables and words. Children with hearing loss listening to the 10 kHz bandwidth demonstrated significant improvements for monosyllabic words, seemingly related to improved phoneme perception. The authors concluded restricted bandwidths can negatively impact speech sound perception, particularly with regard to /s/ and /z/ when spoken by females, and noted an inability to correctly perceive these sounds may impede phonological and morphological development.
http://www.hearingreview.com/issues/articles/2008-10_02.asp |
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发表于 2012-6-8 06:51
