Comparison of total harmonic distortion measures among hearing aid test systems

Different types of hearing aid test equipment use different methods to calculate total harmonic distortion (THD). Comparisons of the THD measured by four commercially available systems were made by evaluating eight hearing aids, three times each. The results obtained were significantly different (p less than 0.001) among three of the test instruments. Furthermore approximately 10% of the hearing aids, which met the ANSI S3.22-1976 specifications for THD when tested with an apparatus that measures true THD, failed this measurement when evaluated on the other three systems.