To assess the extent to which hearing aids improve spatial benefit by restoring the availability of interaural difference cues, the benefit attributable to spatial separation of speech and babble with and without bilateral hearing aids was measured as a function of low-pass cutoff frequency.
Twenty-one older adults with sloping high-frequency hearing loss were provided commercially available bilateral hearing aids. After a 3 to 6 month acclimatization period, speech levels corresponding to 50% correct recognition of sentences from the Hearing in Noise Test (HINT) were measured in a 65-dB SPL babble, with speech and babble low-pass filtered at 1.8, 3.6, and 5.6 kHz. Sentences were always at 0° azimuth, and babble was at either 0° or 90°. Speech and babble spectra for all conditions were digitally recorded using a probe microphone placed in each ear canal of each subject. Spectra and levels of speech and babble and unaided thresholds for narrowband noises were used to calculate the aided audibility index and provide predictions of unaided and aided thresholds for HINT sentences, hearing aid benefit, and spatial benefit for each cutoff frequency. In addition, subjects' willingness to tolerate background noise with and without amplification was measured in the spatially coincident and spatially separated conditions using the Acceptable Noise Level (ANL) procedure.
Thresholds for HINT sentences in babble and ANL improved significantly when aided and when speech and babble were spatially separated. Specifically, hearing aid benefit improved significantly as cutoff frequency increased from 1.8 to 3.6 kHz but only when speech and babble were spatially separated; likewise, spatial benefit improved significantly from 1.8 to 3.6 kHz but only in the aided condition. No further improvement in hearing aid or spatial benefit was observed when cutoff frequency was increased from 3.6 to 5.6 kHz, although improvement in hearing aid benefit was predicted.
Hearing aid benefit, although significant, was poorer than predicted, suggesting that these older adults with high-frequency hearing loss did not take full advantage of the increase in audible speech information provided by amplification. Hearing aid benefit was also limited because hearing aids for some subjects did not restore speech audibility across the full bandwidth of speech. Unaided and aided spatial benefit was significantly greater than predicted, and aided spatial benefit was greater than unaided. This suggests that these older adults were able to take advantage of interaural level and time difference cues to improve speech recognition in babble and that they benefited from these cues to a greater extent with than without bilateral hearing aids. Finally, in contrast to results of previous studies, ANL may vary for an individual depending on the listening condition.
Observed and predicted sentence recognition in babble and benefit of spatial separation were measured as a function of low-pass cutoff frequency with and without bilateral hearing aids. Predictions were determined with an importance-weighted speech-audibility metric (aided audibility index). Hearing aid benefit improved significantly as cutoff frequency increased, but only with spatial separation. Likewise, spatial benefit improved significantly as cutoff frequency increased, but only when aided. Hearing aid benefit was significantly less than predicted, whereas spatial benefit was significantly greater than predicted. Questionnaires administered to assess listeners' perspectives on hearing aid success supported an association between subjective and objective measures of speech recognition.
Department of Otolaryngology-Head and Neck Surgery, Medical University of South Carolina, Charleston, South Carolina.
This work was supported, in part, by research grants P50 DC00422 and R01 DC00184 from NIH/NIDCD, the MUSC General Clinical Research Center (M01 RR01070), and the James E. and Pamela Knowles Foundation. This investigation was conducted in a facility constructed with support from Research Facilities Improvement Program Grant Number C06 RR14516 from the National Center for Research Resources, National Institutes of Health.
Address for correspondence: Jayne B. Ahlstrom, M.S., Department of Otolaryngology-Head and Neck Surgery, Medical University of South Carolina, 135 Rutledge Avenue, MSC 550, Charleston, SC 29425-5500. E-mail: firstname.lastname@example.org
Received November 2, 2007; accepted September 12, 2008.