PRESCRIPTIONS FOR MODERN HEARING AIDS - COMPARISON OF ACOUSTICAL CHARACTERISTICS
Smeds K, Larsson J, Hertzman S, Bergman N.
ORCA Europe, Widex A/S, Stockholm, Sweden.
A number of modern hearing aids from various manufacturers were programmed for a sloping and a flat audiogram, and coupler gain measurements were conducted. The study answers the following questions.
1. How does the measured gain correlate with the NAL-NL1 prescribed gain?
2. In particular, how much gain is prescribed for conductive hearing losses?
3. How is gain acclimatization handled in the devices?
Similar gain measurements conducted ten years ago indicated huge variations in prescribed gain. At that time, there was no consensus about which prescription to use, which may explain the results. Today, however, almost every hearing-aid manufacturer offer the NAL-NL1 prescription as one choice, and it was therefore hypothesized that the difference in measured gain should be smaller if measurements were performed today.
However, the results show that the spread in prescribed gain for a specific audiogram is just as large today as it was ten years ago. Despite the use of the NAL-NL1 prescription, the gain differences, between highest and lowest measured gain at a particular frequency, are around 20 dB for all measured input levels, and the shape of the gain-frequency response also varies considerably.
When looking at amplification for conductive losses, the data also show a large spread. Some manufactures seem to have implemented a compensation for conductive losses that mirrors that defined by the NAL-NL1, whereas others follow other principles.
Also, in devices where gain acclimatization is realized, large variations in the implementation are seen. The difference in gain between the ?full prescription? and the gain prescribed for a first-time hearing-aid user varies from 2 to 10 dB. Most manufactures make gain reductions that are independent of input levels, whereas others implement larger gain reductions for low-level inputs than for high-level inputs.
The implications of the results on clinical work will be discussed.
E-mail: karolina.smeds@orca-eu.info
