Influence of calibration method on distortion-product otoacoustic emission measurements: II. threshold prediction.

OBJECTIVES: Distortion-product otoacoustic emission (DPOAE) stimulus calibrations are typically performed in sound pressure level (SPL) before DPOAE measurements. These calibrations may yield unpredictable DPOAE response levels, presumably because of the presence of standing waves in the ear canal. Forward pressure level (FPL) has been proposed as an alternative method for stimulus calibration because it avoids complications due to standing waves. DPOAE thresholds after four FPL calibrations and one SPL calibration were compared with behavioral thresholds to determine which calibration results in data that yield the highest correlations between the two threshold estimates.
DESIGN: Fifty-two subjects with normal hearing and 103 subjects with hearing loss participated in this study, with ages ranging from 11 to 75 yr. These were the same individuals whose data were used to address the influence of calibration method on test performance in an accompanying article. DPOAE input/output (I/O) functions were obtained at f2 frequencies of 2, 3, 4, 6, and 8 kHz with the primary frequency ratio fixed at f2/f1 approximately 1.22. L(1) was set according to the equation L(1) = 0.4 L(2) + 39 with L(2) levels ranging from -20 to 70 dB SPL and FPL in 5-dB steps. I/O functions were obtained at each frequency for each of the five stimulus calibrations: SPL, daily FPL at room temperature, daily FPL at body temperature, reference FPL at room temperature, and reference FPL at body temperature. DPOAE thresholds were estimated using two methods. In the first method, DPOAE threshold was taken as the lowest L(2) for which DPOAE level is 3 dB or greater than the noise floor (signal- to-noise ratio > or =3 dB). In a second method, a linear regression method first described by Boege & Janssen (2002) and later adapted by Gorga et al. (2003), all DPOAE levels in each I/O function are converted to linear pressure and extrapolated to 0 microPa, at which the L(2) is taken as threshold. Correlations of DPOAE thresholds with behavioral thresholds were obtained for each frequency, calibration method, and threshold-prediction method.
RESULTS: Correlations were greatest for frequencies of 3 to 6 kHz and lowest for 8 kHz, consistent with previous frequency effects. Calibration method made little difference in correlations between DPOAE and behavioral thresholds at any frequency. A small difference was noted in correlations for the two threshold prediction methods, with the linear regression method yielding slightly higher correlations at all frequencies.
CONCLUSIONS: Little difference in threshold correlations was observed among the five calibration methods used to calibrate the stimuli before DPOAE measurements. These results were not anticipated, given the known effects of standing waves on ear-canal estimates of SPL at the plane of the probe. In addition, there was no effect of temperature (body versus room) or timing (daily versus reference) for FPL calibrations. It may be important to note that differences between SPL and FPL calibrations should not be seen if a standing wave does not occur at the plane of the probe at or near the frequency being tested. The frequencies (2 to 8 kHz) were chosen because it was expected that effects from standing waves would occur between these frequencies because of the typical lengths of ear canals for the age group tested. Because measurements were taken at only five discrete frequencies in the interval, it is possible that standing waves were present but did not affect the specific test frequencies. In total, these results suggest that SPL calibrations may be adequate when attempting to predict pure-tone thresholds from DPOAEs, despite the fact that they are known to be susceptible to errors associated with standing waves.