Effects of low pass filtering on the intelligibility of speech in noise for people with and without dead regions at high frequencies

People with high-frequency sensorineural hearing loss differ in the benefit they gain from amplification of high frequencies when listening to speech. Using vowel–consonant–vowel (VCV) stimuli in quiet that were amplified and then low pass filtered with various cutoff frequencies, Vickers et al. [J. Acoust. Soc. Am. 110, 1164–1175 (2001)] found that the benefit from amplification of high-frequency components was related to the presence or absence of a cochlear dead region at high frequencies. For hearing-impaired subjects without dead regions, performance improved with increasing cutoff frequency up to 7.5 kHz (the highest value tested). Subjects with high-frequency dead regions showed no improvement when the cutoff frequency was increased above about 1.7 times the edge frequency of the dead region. The present study was similar to that of Vickers et al. but used VCV stimuli presented in background noise. Ten subjects with high-frequency hearing loss, including eight from the study of Vickers et al., were tested. Five had dead regions starting below 2 kHz, and five had no dead regions. Speech stimuli at a nominal level of 65 dB were mixed with spectrally matched noise, amplified according to the “Cambridge” prescriptive formula for each subject and then low pass filtered. The noise level was chosen separately for each subject to give a moderate reduction in intelligibility relative to listening in quiet. For subjects without dead regions, performance generally improved with increasing cutoff frequency up to 7.5 kHz, on average more so in noise than in quiet. For most subjects with dead regions, performance improved with cutoff frequency up to 1.5–2 times the edge frequency of the dead region, but hardly changed with further increases. Calculations of speech audibility using a modified version of the articulation index showed that application of the Cambridge formula was at least partially successful in making high-frequency components of the speech audible for subjects with dead regions, and that such subjects often failed to benefit from increased audibility of the speech at high frequencies.