We carried out a study into the use of insertion gain measurement for hearing aid fitting. The subjects for our study were 80 adults with mild/moderate hearing losses who had been referred for hearing aid fitting. We were using basic, low powered analogue aids for most of our first fittings. These aids were allocated to the subjects alternately, and insertion gain measurements were carried out on each subject with the allocated aid, using the NAL prescription formula to determine the target gain. If the insertion gain differed from the target by more than 10 dB at any frequency up to 3kHz, tone control adjustments and earmould modifications (vents, filters, Libby horns, high/low pass tubing) were used as appropriate to try to modify the response. If the target was still not met, the other aid was tried.
After careful consideration of the figures, we have decided that when fitting hearing aids like the ones used in our study without the use of insertion gain, Libby horns should be used, if possible, whenever the difference between 1 kHz and 4kHz thresholds is 20dB or more. The same should apply with other NHS hearing aids which provide relatively little high frequency gain.
Norman M, James PN. The case for using Libby horns. BAAT News, Spring 2001.
Dr. Meg Norman, Sunderland Unit, Sunderland. Tel: +44 (0)191-569-9001.
Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne, NE7 7DN, UK.
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