A clinical comparison between a laboratory and a commercial impedance audiometer

Impedance audiometry is a useful tool in the diagnosis of retrocochlear lesions, provided that adequate normal values regarding the stapedius reflex thresholds and decay are used. The normal values in Sweden (and elsewhere) have been obtained with a laboratory impedance audiometer that is not in common use. We have compared a commercial impedance audiometer, the Grason-Stadler 1723, with this laboratory model to determine whether the normal values are valid. We found that the Grason-Stadler 1723 audiometer gives reliable stapedius reflex thresholds, but at 2-4 dB higher levels. The reflex decay curves, however, are unsatisfactory. Possible explanations for the results are discussed. We have also proposed a modification of the recorder of the commercial audiometer. The study demonstrates that normal values cannot be transferred freely between different equipments.