EFFECTS OF EAR-CANAL STANDING WAVES ON MEASUREMENTS OF DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS

At frequencies above kHz standing waves in the ear canal complicate ca libration of stimulus sound-pressure levels (SPLs) for measurements of distortion-product otoacoustic emissions (DPOAEs). In the literature, two stimulus-presentation strategies have been used for DPOAE measure ments. In the ''in-the-ear adjustment'' strategy, the voltage command to the speakers is adjusted to maintain a constant stimulus SPL across frequency at the DPOAE-measurement microphone. In the ''iso-voltage'' strategy, the voltage presented to the speakers is held constant acro ss frequency, on the basis of the assumption that the frequency respon se of the speakers is approximately flat at the eardrum in the average human ear canal. Because of standing-wave effects, there are large, s ystematic but idiosyncratic differences of stimulus SPL between the tw o strategies. DPOAE-versus-frequency functions (''DPOAE audiograms'') obtained using both stimulus-presentation strategies in the same ears are presented. The differences of stimulus SPL between the two strateg ies, and the associated differences of DPOAE amplitude, are described and quantified. Around frequencies of standing-wave minima at the DPOA E probe, the in-the-ear adjustment strategy resulted in smaller DPOAEs at high L(1)=L(2), but much larger DPOAEs at low L(1) =L(2), than did the iso-voltage strategy. For any L(1), the DPOAE-amplitude differenc es between the two strategies varied systematically with L(1)-L(2) At the stimulus levels used to construct previously published population norms for clinical applications (i.e., L(1) greater than or equal to 6 5 dB SPL), there are only small differences of mean DPOAE amplitudes, and of the standard deviations of these means, between the two strateg ies. (C) 1995 Acoustical Society of America.