Real-ear calibration of ipsilateral acoustic reflex stimuli from five types of impedance meter

The Hearing Level scale has been established in national and international standards for tones presented via supra-aural earphones. However, the standards do not encompass stimuli from the probes of ipsilateral impedance meters. This study transfers the calibration of standardised earphones to the probes of five types of impedance meter and therefore enables the intensities of their ipsilateral stimuli to be defined in terms of Hearing Levels. The method used for transfer of calibration is based on equality of acoustic reflex responses elicitied by the impedance meter probe and by a standardised earphone. Ten subjects were tested with all five instruments to ensure comparability between instruments. The resultant calibration data are expressed in terms of Reference Equivalent Threshold Sound Pressure Levels measured in both 2 cc and Zwislocki couplers. The results indicate a considerable variation amongst instruments emphasising the need for a set of calibration data for each design of probe. This variation is less marked for the Zwislock coupler than for the 2 cc coupler.