Harmonic distortion measurements in clinical hearing and fitting. Preliminary report

Measurement of the distortion from hearing aids and its influence on their fitting has become practicable with the implementation of real ear measurement in the audiological clinic. Thus the harmonic distortion measured in the ear fitted with the hearing aid can be measured by means of a probe in the ear canal. The objective of the present investigation was to examine the value of harmonic distortion measurements in the fitting of hearing aids within a clinical setting. The material comprised 101 consecutive patients who had been prescribed different types of hearing aids based on the POGO formula, using insertion gain measurements. 160 hearing aids were fitted and with the fitting strategy used, a distortion less than 10% was indicated in 122 fittings (76%), a distortion between 10 and 20% in 30 fittings (19%) and finally a distortion greater than 20% in 8 fittings (5%). In the cases with the distortion greater than 10%, the hearing aid setting was maintained and the magnitude of the harmonic distortion was controlled with a 2-cc coupler. At the hearing aid fitting, the audiology technician can listen to the signal at the patient's drum and thus may hear distortion products. The relationship between the 'distortion' heard by the audiology technician and the 'distortion' heard by the patient was fairly good. Based on these preliminary findings it is concluded that harmonic distortion measurements may prove useful in the hearing aid fitting procedure.