A critical examination of distortion measurement in hearing aids.

This study was designed to examine relationships between measures of total distortion (TD) and individual 2nd and 3rd harmonic distortion in various types of hearing aids. Measures of 2nd harmonic, 3rd harmonic, and TD were determined for six instruments. Analysis of the results revealed that little additional information is obtained from a measure of TD as opposed to individual harmonic measures. Furthermore, in two instances TD measures resulted in erroneously high distortion readings because of ambient and system noise. It is recommended that in less than ideal acoustic environments, measures of 2nd and 3rd harmonic distortion be used in lieu of TD.