Vowel Identification by Listeners With Hearing Impairment in Response to Variation in Formant Frequencies

Abstract

Purpose This study examined the influence of presentation level and mild-to-moderate hearing loss on the identification of a set of vowel tokens systematically varying in the frequency locations of their second and third formants. Method Five listeners with normal hearing (NH listeners) and five listeners with hearing impairment (HI listeners) identified synthesized vowels that represented both highly identifiable and ambiguous examples of / i /, /ʊ/, and /ɝ/. Results Response patterns of NH listeners showed significant changes, with an increase in presentation level from 75 dB SPL to 95 dB SPL, including increased category overlap. HI listeners, listening only at the higher level, showed greater category overlap than normal and overall identification patterns that differed significantly from those of NH listeners. Excitation patterns based on estimates of auditory filters suggested smoothing of the internal representations, resulting in impaired formant resolution. Conclusions Both increased presentation level for NH listeners and the presence of hearing loss produced a significant change in vowel identification for this stimulus set. Major differences were observed between NH listeners and HI listeners in vowel category overlap and in the sharpness of boundaries between vowel tokens. It is likely that these findings reflect imprecise internal spectral representations due to reduced frequency selectivity.