Evaluation of the National Acoustic Laboratories' new hearing aid selection procedure

This study evaluated the National Acoustic Laboratories' (NAL) new formula for prescribing the gain and frequency response of a hearing aid. The frequency response prescribed for 44 clients (67 fitted ears) was compared with a series of variations having increased or decreased low-frequency and/or high-frequency emphasis. The evaluations consisted of paired-comparison judgments of the intelligibility of speech in quiet and the pleasantness of speech in noise. There were only 4 ears (6%) where a comparison response was more intelligible than the NAL response, but there were 16 ears (24%) where one of the comparison responses was more pleasant. On the average, hearing aid gain that was used by each subject agreed closely with prescribed gain. These trends were not affected by audiogram configuration, experience in aid usage, or type of aid limiting. The formula was found to be highly effective, but there were some cases where a change in aid prescription was indicated. A simple evaluation procedure using paired-comparison judgments is proposed for detecting such cases.