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Lars Bramsløw

Research Engineer, PhD, Project Leader

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mail@eriksholm.com

Since its inception, Eriksholm has been involved in the derivation of prescription rules for hearing-aid fitting. At first, it was just linear gain and output limiting, then came nonlinear input/output functions, and more recently, compression time constants were integrated into Oticon’s prescription formulae.

POGO

In 1983, based on work at Eriksholm, the Prescription Of Gain/Output (POGO) prescription rule was formulated by McCandless and Lyregaard1. The basic idea was to prescribe insertion gain such that “sound levels which are most important in daily life be audible without being excessively loud”; thus commonly-occurring sound levels should be presented to the listener at his/her Most Comfortable Level (MCL), and the output from the hearing-aid should not exceed his/her Uncomfortable Loudness Level (UCL).

Measurements of hearing threshold (HTL), MCL and UCL were made on a large sample of listeners. This enabled the development of a very simple formula prescribing the gain required for a given HTL, and the UCL likely to occur for a given HTL. Thus, both gain and maximum output could be prescribed based on knowledge of HTL alone2.

Early multi-channel, non-linear prescription: MultiFocus

After Villchur’s seminal publication on recruitment in sensorineural hearing loss and its compensation via multi-channel compression3, many laboratories (Eriksholm included) began to research the idea. A long series of in-house studies progressed from rack equipment to configurable, wearable prototypes.

Comparing performance and preference for various combinations of compression action in a two-channel device, it was concluded that the best arrangement involved fast-acting compression with a low compression threshold in the low-frequency (below 1500 Hz) channel, and linear gain in the high frequencies. This was an unexpected result. Even more surprising was the conclusion that with this hearing aid, most users did not need to adjust the volume control. The final outcome of this work was MultiFocus, the world’s first hearing-aid without a volume control4. MultiFocus was fitted using proprietary prescription rules derived from the research studies.

Going digital: DigiFocus and the beginning of multi-prescription fitting software

In 1995, Oticon announced its first fully digital hearing aid, DigiFocus. Some years before that, research had begun to investigate the best ways to exploit the flexibility of the upcoming device. A series of studies at Linköping University by Lunner et al. began to reveal that the optimal configuration of gain and compression in a multi-channel aid depends on the audiometric configuration of the user5.

Based on these studies, the original general-purpose prescription formulae for fitting DigiFocus (‘Adaptive Speech Alignment’ (ASA)) was supplemented with the ‘SKI’ prescription for precipitous high-frequency losses. By now, fitting was done via software, and algorithms could be incorporated for recommending which prescription to choose, based on the client’s audiogram.

Beyond the audiogram: further personalisation

The issue of candidature had been apparent from early on in all the above work, as there were sometimes distinct groups of people liking or disliking the experimental prescriptions being tested. This was the background for the large study of candidature, which compared various linear and non-linear prescriptions, including contrasts in compression time constants. Strong patterns of differential benefit for individual hearing-aid users appeared, indicating that the choice of prescription makes a difference.


Further reading

1 McCandless GA, Lyregaard PE. Prescription of gain/output (POGO) for hearing aids. Hearing Instruments 34:1 p.16-21, 1983.

2 Lyregaard PE. POGO and the theory behind. Proc. 13th Danavox Symposium, p. 81-96, 1988.

3 Villchur E. Signal processing to improve speech intelligibility in perceptive deafness J. Acoust. Soc. Am. Volume 53, Issue 6, pp. 1646-1657, 1973.

4 Brunved PB. How studying loudness growth led to the development of MultiFocus. Hearing instruments 45:Supplement 1 p. 8-10, 1994.

5 Lunner T, Hellgren J, Arlinger S, Elberling C. A digital filter-bank hearing aid. Predicting user preference and performance for two signal processing algorithms. Ear and Hearing 18:1 p. 12-25, 1997.