Stimulus Ratio and Level Dependence of Low- and Mid-Frequency Distortion-Product Otoacoustic Emissions

Active amplifiers within the cochlea generate, as a by-product of their function, distortion-product otoacoustic emissions (DPOAEs) in response to carefully chosen two-tone stimuli. Focus has been on invoking emissions in a mid-frequency range from 500 to 4000 Hz. Below 500 Hz, physiological noise from heartbeating, breathing and swallowing contaminates the measurement. Above 4000 Hz, calibration of the equipment involved is challenging, if not impossible, to do reliably. Here, we investigate alternative measurement parameters to see if the DPOAE is limited to frequencies above 500 Hz. Few examples of low-frequency DPOAEs exist in the literature. Overcoming the decreasing response level and increasing noise level with decreasing frequency may provide a non-invasive window into the inner-ear mechanics of low-frequency hearing.Eighteen out of 21 young human adults screened (19-30 years) had audiometrically normal hearing for inclusion in our experiment. DPOAEs were measured with pure-tone stimuli in four configurations: f2 fixed around a mid-frequency (2050-2180 Hz), f2 fixed around a low frequency (512-545 Hz), fdp fixed at a mid-frequency (1231 Hz) and fdp low frequency (246 Hz). Eight stimulus ratios f2/f1 (1.05-1.50) and three stimulus sound pressure levels L1/L2 (65/45, 65/55, 70/60) were measured in each configuration. The DPOAE response was isolated with the discrete Fourier transformation (DFT). The DFT measures the DPOAE response accurately only when the DPOAE frequency and both stimulus frequencies are exactly at a DFT bin. This makes for slightly odd parameter choices.Trends in ratio-magnitude responses for the mid-frequency DPOAE agree well with those reported in previous literature. Not reported previously is the change when going from mid- to low-frequency DPOAEs. In the mid-frequency range, ratio-magnitude responses typically have maxima at ratios <1.30. In the low-frequency range, maxima are typically at ratios >1.30.DPOAEs are not limited to frequencies above 500 Hz, but the stimulus ratio invoking the largest DPOAE in the mid-frequency range does not do so in the low-frequency range. The stimulus level has similar effects in both frequency ranges, that is, the ratio-magnitude response increases and broadens with increasing level. The combined observations could indicate a difference between apical and basal cochlear physiology