July 18, 2007
OMAHA, Neb — Researchers at Boys Town National Research Hospital have concluded that a restricted listening bandwidth (below 6-7 kHz) can negatively affect children’s perception of /s/ and /z/ sounds when spoken by female talkers like mothers and other female caregivers. The paper by Patricia Stelmachowicz, Dawna Lewis, Sangsook Choi, and Brenda Hoover appears in the August edition of Ear and Hearing.
Stelmachowicz and Boys Town researchers, including Mary Pat Moeller, have been at the forefront of a sometimes-contentious debate regarding the possible need for children to require increased high and/or low frequency gain compared to adults. In pediatric audiology, adult data are often extrapolated to apply to children. However, adults can miss much of the high frequencies in meaningful speech, and still fill in the gaps; with children who have not fully acquired speech and language understanding, that reduction of high-frequency signals may have a larger negative impact. For example, the /s/ sound of an average female speaker can be at 9 kHz, and until recently few hearing instruments have come close to amplifying that frequency region. If it can be shown that it makes sense to provide audibility in this high frequency range, then manufacturers may have something more to work toward.
In the present study, the researchers conducted experiments to determine the effects of bandwidth on a range of auditory skills in children with normal hearing and children with hearing loss. The subjects included 32 children with normal hearing (NL) and 24 children with mild to moderately severe hearing loss (HL). The experiments included measures of phonology, language and short-term memory, fricative/affricate perception, novel-word learning, and listening effort. Although the results from both groups of children suggest that the effects of bandwidth do not significantly influence either the learning of new words or listening effort, bandwidth does have a significant effect on their perception of words.
In their summary, the authors state: “For children with NL, a significant bandwidth effect was observed for both nonsense syllables and words. For the children with HL, no overall bandwidth effect was observed in the nonsense syllable task, but a significant bandwidth effect was found for PBK words [phonetically balanced Kindergarten test of word recognition used in younger children]. ”
The researchers believe that they did not see a significant effect on novel-word learning and listening effort due to the insensitivity of the tests employed. “Despite the lack of a significant overall bandwidth effect for nonsense syllables in the group with HL, large mean improvements were observed for both /s/ (56 percentage points) and /z/ (42 percentage points) in the 10 kHz bandwidth condition,” write the authors. “Given the importance of these phonemes in the English language, an inability to perceive these sounds correctly may have a negative impact on phonological and morphological development as well as speech production.”
Source: American Auditory Society