Previous research regarding the utility of non-linear frequency compression in hearing aids has revealed conflicting results for speech recognition, marked by high individual variability. The aims of the study were to determine the effect of non-linear frequency compression on aided speech recognition in noise and listening effort using a dual-task test paradigm and to determine if listener variables of hearing loss slope, working memory capacity, and age predicted performance with non-linear frequency compression. 17 older adults with symmetrical sensorineural hearing loss were tested in the sound field using hearing aids. Speech recognition in noise and listening effort were measured by adapting the Revised Speech in Noise Test into recognition/recall dual-task paradigm. On average, speech recognition in noise performance significantly improved with the use of non-linear frequency compression. Individuals with steeply sloping hearing loss received more recognition benefit. Recall performance also significantly improved at the group level with non-linear frequency compression revealing reduced listening effort. Older participants within the study cohort received less recall benefit than the younger participants. Evidence supports individualized selection of non-linear frequency compression, with results suggesting benefits in speech recognition for individuals with steeply sloping hearing losses and in listening effort for younger individuals.

Frequency-lowering signal processing in hearing aids has re-emerged as an option to improve audibility of the high frequencies by expanding the input bandwidth. However, few studies have investigated the usefulness of the scheme as a bimodal option for cochlear implant users. In this study, that question was posed. It was hypothesized that, following fitting and a period of adjustment to a frequency-compression hearing aid, sound localization and speech perception would be improved compared to conventional amplification. More specifically, more high-frequency cues would be perceived in the hearing aid ear using frequency compression, thereby providing better sensitivity to interaural level differences when a cochlear implant is used contralaterally. There were two experiments in this study. In the first experiment, the goal was to determine if frequency compression was a better bimodal option than conventional amplification. Performance was assessed on tests of sound localization, speech perception in a background of noise, and using questionnaires. Ten subjects with a cochlear implant plus hearing aid participated in experiment one. In the second experiment, the goal was to determine the impact of frequency compression on speech perception in quiet. Consonant and vowel perception in quiet was assessed using the frequency-compression and conventional hearing aid. Seventeen adult subjects participated in the second experiment. In both experiments, subjects alternated daily between a frequency-compression and conventional hearing aid for two months. The parameters of frequency compression were set individually for each subject and audibility was measured for the frequency compression and conventional hearing aid programs by comparing estimations of the Speech Intelligibility Index (SII) using a modified algorithm (Bentler, R., Cole, B., Wu, Y-H. (2011, March). Deriving an audibility index for frequency-lowered hearing aids. Poster session presented at the meeting of the American Auditory Society, Scottsdale, AZ). In both experiments, the outcome measures were administered following the hearing aid fitting to establish baseline performance and after two months of use. Results revealed no significant difference between the frequency-compression and conventional hearing aid on tests of localization and consonant recognition. Spondee-in-noise and vowel perception scores were significantly higher with the conventional hearing aid compared to the frequency-compression hearing aid after two months of use. These results suggest that, for the subjects in this study, frequency compression is not a better bimodal option than conventional amplification. In addition, speech perception may be negatively influenced by frequency compression because formant frequencies are too severely compressed and can no longer be distinguished.

Listeners with severe-sloping losses often don’t perceive high-frequency sound cues. Conventional amplification fails to provide these cues due to loudness discomfort experienced by the listener, and/or acoustic feedback. Alternative signal-processing solutions include shifting higher frequencies down to lower frequencies, or providing electrical stimulation via a speech processor. Three experiments were carried out on adult hearing-impaired listeners to determine the best way of providing high-frequency information: conventional amplification, frequency compression or cochlear implantation.

Nonlinear frequency compression is a signal processing technique used to increase the audibility of high frequency speech sounds for hearing aid users with sloping, high frequency hearing loss. However, excessive compression ratios may reduce spectral contrast between sounds and negatively impact speech perception. This is of particular concern in infants and young children, who may not be able to provide feedback about frequency compression settings. This study explores use of an objective cortical auditory evoked potential that is sensitive to changes in spectral contrast, the auditory change complex (ACC), in the verification of frequency compression parameters. We recorded ACC responses in adult listeners to a spectral ripple contrast stimulus processed with a range of frequency compression ratios (1:1 to 4:1). Vowel identification, consonant identification, speech recognition in noise (QuickSIN), and behavioral ripple discrimination thresholds were also measured under identical frequency compression conditions. In Experiment 1, these measures were completed in ten adult normal hearing individuals to determine the effects of this type of signal processing in individuals with optimal hearing. In Experiment 2, these same measures were repeated in ten adults with sloping, high frequency hearing loss, which is the clinical population for whom this signal processing technique was intended. No significant relationship of compression ratio and vowel identification was found in the normal hearing listeners, though a significant negative effect of increasing frequency compression ratio was observed in the hearing impaired group. Significant effects of compression ratio on ACC amplitude, consonant identification, ripple discrimination threshold, and speech perception in noise were found for both the normal hearing and hearing impaired groups. These results indicate that the ACC response, like speech and non-speech perceptual measures, is sensitive to frequency compression ratio. Further investigation of this technique with hearing impaired individuals is needed to determine optimal stimulus and recording parameters for the ACC in both adults and children.

Purpose: To evaluate sound quality preferences of participants wearing hearing aids with different strengths of nonlinear frequency compression (NFC) processing versus no NFC processing. Two analysis methods, one without and one with a qualifier as to the magnitude of preferences, were compared for their percent agreement to differentiate a small difference in perceived sound quality as a result of applied NFC processing. Research Design: A single-blind design was used with participants unaware of the presence or strength of NFC processing (independent variable). The National Acoustic Laboratories-Nonlinear 2 (NAL-NL2) prescription of amplification was chosen because audibility is intentionally not prescribed in the presence of larger sensorineural hearing loss thresholds. A lack of prescribed audibility, when present, was deemed an objective qualifier for NFC. NFC is known to improve the input bandwidth available to listeners when high-frequency audibility is not otherwise available and increasing strengths of NFC were examined. Experimental condition 3 (EC3) was stronger than the manufacturer default (EC2). More aggressive strengths (e.g., EC4 and EC5), however, were expected to include excessive distortion and even reduce the output bandwidth that had been prescribed as audible by NAL-NL2 (EC1). Study Sample: A total of 14 male Veterans with severe high-frequency sensorineural hearing loss. Data Collection and Analysis: Participant sound quality preference ratings (dependent variable) without a qualifier as to the magnitude of preference were analyzed based on binomial probability theory, as is traditional with paired comparison data. The ratings with a qualifier as to the magnitude of preference were analyzed based on the nonparametric statistic of the Wilcoxon signed rank test. Results: The binomial probability analysis method identified a sound quality preference as well as the nonparametric probability test method. As the strength of NFC increased, more participants preferred the EC with less NFC. Fourteen of 14 participants showed equal preference between EC1 and EC2 perhaps, in part, because EC2 showed no objective improvement in audibility for six of the 14 participants (42%). Thirteen of the 14 participants showed no preference between NAL-NL2 and EC3, but all participants had an objective improvement in audibility. With more NFC than EC3, more and more participants preferred the other EC with less NFC in the paired comparison. Conclusions: By referencing the recommended sensation levels of amplitude compression (e.g., NAL-NL2) in the ear canal of hearing aid wearers, the targeting of NFC parameters can likely be optimized with respect to improvements in effective audibility that may contribute to speech recognition without adversely impacting sound quality. After targeting of NFC parameters, providers can facilitate decisions about the use of NFC parameters (strengths of processing) via sound quality preference judgments using paired comparisons.

Objective: To date, the main focus in frequency lowering hearing aid studies has been in relation to speech perception abilities. With improvements in hearing aid technology, there is a growing interest in musical perception as a dimension that could improve hearing aid users’ quality of life. The purpose of this study was two-fold: Firstly, to develop a test of music perception for adult hearing aid users and secondly, to evaluate the influence of non-linear frequency compression (NFC) on music perception with the use of the Music Perception Test (MPT) compiled by the researcher. Research design and research sample: Phase 1 entailed the compilation of the MPT and can be described as design-based. A quasi-experimental research design was selected to establish the structure of the method employed in Phase 2, which involved the fitting of participants (n=40) with NFC hearing aids. Objective data was obtained with the hearing aids with NFC active and inactive. Phase 3 was characterized by a survey design which elicited subjective impressions of the participants’ musical experiences with NFC active and inactive. Results: Results proved that normal hearing adults as well as adults using hearing aids were able to complete all the sub-tests of the MPT. Furthermore, the use of NFC resulted in a statistically significant improvement in hearing aid users’ perception of timbre and melody, but not of pitch. Overall, no statistically significant improvement in their perception of rhythm was observed, although their performance on some rhythm sub-tests improved significantly. The use of NFC also brought about a statistically significant improvement in hearing aid users’ perception of the music qualities of overall fidelity, tinniness and reverberance. Although participants experienced the loudness, fullness, crispness, naturalness and pleasantness of music more positively with NFC, these benefits were not statistically significant. Conclusion: The MPT can be used successfully for assessing music perception in hearing aid users within the South African context and may therefore result in more accountable hearing aid fittings. The use of NFC may increase hearing aid users’ appreciation of music whilst not influencing music perception negatively. Given that a large percentage of hearing aid users express a loss in enjoyment of music, audiologists should not ignore the possible benefits of NFC, especially if one takes into account that previous research indicated speech perception benefits with this technology. / Thesis (DPhil)--University of Pretoria, 2012. / Speech-Language Pathology and Audiology / Unrestricted