Non-linear frequency compression (NLFC) hearing aids aim to deliver high-frequency sound to a better functioning, lower frequency region of the cochlea. Despite the widespread use of frequency lowering technology, it continues to be difficult to predict candidacy. Since the aim of NLFC is to improve high-frequency audibility, possible predictors of benefit may include high-frequency hearing thresholds. NLFC alters the harmonic relationship of speech, and it has frequently been proposed that auditory training may help users adapt to the distorted acoustic signal. The aim of this thesis was twofold: firstly, to identify audiometric predictors of benefit from NLFC, and secondly to explore the role of auditory training in adaptation to NLFC. The first study aimed to determine if outcome with NLFC was related to hearing thresholds. Twenty-one experienced adult hearing aid users, with mild-to-profound high-frequency hearing loss, completed speech recognition and self-report measures with NLFC enabled and disabled over 16 weeks. The additional benefit gained on phoneme detection and word recognition tasks with NLFC enabled was positively correlated with average high-frequency hearing loss. Additional benefit began to emerge as average high-frequency hearing thresholds exceeded 70 dB HL. There was some evidence of a ‘roll-off’ in additional benefit when hearing thresholds exceeded 100 dB HL. The second study aimed to identify which training method, if any, may help adaptation to NLFC. Forty young normally hearing adults were randomly allocated to one of four groups: a training group (sentence or consonant) or a control group (passive exposure or test-only). Training led to significant improvements on novel measures of speech recognition. Generalisation was limited to near transfer i.e. sentence training led to improved sentence recognition whilst consonant training led to improved consonant recognition. Improvements following passive exposure to frequency compressed sentences were equivalent to those achieved by active sentence training. Results from this second study suggest that auditory training may improve outcome for users of NLFC hearing aids, but passive exposure may be an important confounding variable. In the final study high-frequency focused sentence and consonant training was compared to sham training (passive exposure to a talking book). Twenty-six hearing-impaired adults who were new users of NLFC were allocated to either an auditory training or a sham training group. Measures of speech recognition and self-report benefit were administered before and after four weeks of home-based training. Auditory training significantly improved speech reception thresholds. In conclusion, benefit from NLFC was dependent on hearing thresholds; as hearing thresholds increased so did benefit. Participants with severe-to-profound high-frequency hearing loss gained most benefit from NLFC. Auditory training provided alongside NLFC significantly improved speech recognition in noise but changes to all other outcome measures were not significant.

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.

The hearing and understanding problems of the people with high frequency hearing loss are covered within the scope of this thesis. For overcoming these problems, two main studies, developing hearing loss simulation (HLS) and applying new frequency lowering methods (FLMs) were carried out. HLS was developed with the suprathreshold effects and new FLMs were applied with different combinations of the FLMs. For evaluating the studies, modified rhyme test (MRT) and speech intelligibility index (SII) were used as subjective and objective measures, respectively. Before both of the studies, offline studies were carried out for specifying the significant parameters and values for using in MRT. For the HLS study, twelve hearing impaired subjects listened to unprocessed sounds and thirty six normal hearing subjects listened to simulated sounds. In the evaluation of the HLS, both measures gave similar and consistent results for both unprocessed and simulated sounds. In FLMs study, hearing impaired subjects were simulated and normal hearing subjects listened to frequency lowered sounds with the specified methods, parameters and values. All FLMs were compared with the standard method of hearing aids (amplification) for five different noisy environments. FLMs satisfied 83% success of higher speech intelligibility improvement than amplification in all cases. As a conclusion, the necessity of using subject-specific FLMs was shown to achieve higher intelligibility than with amplification only. Accordingly, a methodology for selection of the values of parameters for different noisy environments and for different audiograms was developed.

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