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Lars Bramsløw

Research Engineer, PhD, Project Leader

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mail@eriksholm.com

The audiogram does not provide complete information about which hearing-aid fitting will be best for the individual client. In this study a range of other factors (in psychoacoustical, cognitive, and auditory ecology domains) were examined in a very large-scale design. All three domains demonstrated significant influence on the optimal choice of hearing-aid fitting.

Background

Linear amplification has largely been superseded by nonlinear amplification, but the superiority of nonlinear is not universally demonstrated. Furthermore, different forms of compression compete, without any clear ‘winner’ appearing. Much evidence suggests that individual differences in user needs and abilities - beyond differences on audiometric loss - play a role in determining the optimal form of amplification for the individual. However, robust demonstrations of this are few. We aimed to rectify this situation by executing a study designed to reveal such effects.

Methods

We evaluated the benefits of fast-acting compression, slow-acting compression, and linear reference fittings for speech intelligibility and reported disability, in a within-subject, within-device masked crossover design on 50 hearing-impaired listeners. Five hearing-aid fittings were implemented on the same hearing-aid platform. Each listener sequentially experienced each fitting for a 10-week period. Outcome measures included speech intelligibility under diverse conditions, and self-reported disability. Candidature dimensions included hearing thresholds, discomfort thresholds, spectro-temporal and masking abnormalities, cognitive capacity, self-reports, and acoustic measures of auditory ecology.

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Results

At a group level, all the nonlinear fittings were superior to the linear reference for benefits in listening comfort, listener satisfaction, reported intelligibility and speech intelligibility. Slow-acting compression outperformed the fast-acting compression fittings for listening comfort, while for reported and measured speech intelligibility the opposite was true. For listener satisfaction there were no group differences between the nonlinear fittings.

Analysis for individual listeners revealed definite divergences from the group data and hence a need for candidature criteria. A linear fitting was optimal for a substantial subset of the listeners. There were systematic differences between the benefits of nonlinear and linear fittings, and also within nonlinear fittings with fast-acting versus slow-acting compression. The patterns of benefit and individual optima depended on the domain of outcome being assessed. All the candidature variables were associated with patterns of individual performance differences across the different fittings.

A further striking discovery was that the distribution of preference rankings for some fittings was bimodal, whereas for others it was a more usual, unimodal shape. Figure 1 shows an example in which the distributions of ranking on ‘Satisfaction’ by the 50 listeners are displayed for two of the fittings.

Conclusions

Better fittings for the individual client may be achieved by gathering measures beyond audiogram data. Patterns of candidature found here include measures beyond auditory function in the domains of cognitive capacity and auditory ecology. Linear amplification may be superior to nonlinear for a sizeable minority of users. Some types of nonlinear fitting appear to meet with a ‘consensus of indifference’ among listeners, whereas others seem to provoke a strong liking or disliking.


Further reading

Gatehouse S, Naylor GM and Elberling C "Benefits from hearing-aids in relation to the interaction between the user and the environment", Int. J. Audiol. 42 Suppl. 1, pp S77-85, 2003.

Gatehouse S, Naylor G and Elberling C “Linear and nonlinear hearing-aid fittings – 1. Patterns of benefit”, Int. J. Audiol., 45, pp 130-152, 2006.

Gatehouse S, Naylor G and Elberling C “Linear and nonlinear hearing-aid fittings – 2. Patterns of candidature”, Int. J. Audiol., 45, pp 153-171, 2006.

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    Study Partners

    This study was carried out in collaboration with the late Stuart Gatehouse at the Medical Research Council Institute of Hearing Research, Glasgow, UK. The findings of this study have since provided the conceptual basis for individualisation tools in many of Oticon’s premium hearing solutions.