Distortion-product otoacoustic emission (DPOAE) suppression tuning curves (STCs) were measured in 65 hearing-impaired (HI) subjects at f2 frequencies of 2.0, 2.8, 4.0, and 5.6 kHz and L2 levels relative to sensation level (SL) from 10 dB to as much as 50 dB. Best frequency, cochlear-amplifier gain (tip-to-tail difference, T-T), and tuning (QERB) were estimated from STCs. As with normal-hearing (NH) subjects, T-T differences and QERB decreased as L2 increased. T-T differences and QERB were reduced in HI ears (compared to normal) for conditions in which L2 was fixed relative to behavioral threshold (dB SL). When STCs were compared with L2 at constant sound pressure levels (dB SPL), differences between NH and HI subjects were reduced. The large effect of level and small effect of hearing loss were both confirmed by statistical analyses. Therefore, the magnitude of the differences in DPOAE STCs between NH and HI subjects is mainly dependent on the manner in which level (L2) is specified. Although this conclusion may appear to be at odds with previous, invasive measures of cochlear-response gain and tuning, the apparent inconsistency may be resolved when the manner of specifying stimulus level is taken into account.
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