A hearing aid fitting test and the clinical results.

Field hearing threshold of the subjects aided with a hearing aid was measured by automatically interrupted test tones at a distance of one meter from a speaker in a sound proof room. The dB dial of a sound generator was calibrated at a distance of one meter from the speaker in 5dB step in reference to 0.0002dynes/cm in a range of 0dB to 100dB. The frequency range of the test tones was limited to a range of 250 to 3000Hz containing the important frequencies for speech intelligibility. The aided thresholds reached to the fitting levels by controlling the volume and the frequency characteristic of the hearing aid. The fitting levels were 50dB at 500Hz, 45dB at 1000Hz, 30dB at 2000Hz and 25dB at 3000Hz, respectively. These were 20dB lower than the average speech spectrum of Piece and David. The highest score of intelligibility was obtained when the aided thresholds reached to the fitting levels. The clinical results were classified into 3 types and discussed. A theory of fitting hearing aid was presented as to what characteristic of hearing aid could fit the field hearing levels of the subjects without a hearing aid.