Modifications of a single saturating non-linearity account for post-onset changes in 2 f₁-f₂ distortion product otoacoustic emission

2 f1-f2 distortion product otoacoustic emissions (DPOAEs) were recorded from guinea pigs. DPOAEs showed complex time dependence at the onset of stimulation. The DPOAE, measured during the first 500 ms, can either decrease or increase at the onset depending on both the frequencies and levels of the primary tones. These changes are closely associated with amplitude minima (notches) of the DPOAE I/O functions. These notches are characteristic of DPOAE growth functions measured from guinea pigs for primary tones of 50-60-dB sound-pressure level (SPL). Apparent changes in the DPOAE amplitude occur because the notch shifts to higher levels of the primaries during the onset of stimulation. This shift of the notch to higher levels increases for lower f2/f1 ratios but does not exceed about 2 dB. DPOAE amplitude increases for a constant level of the primaries if the onset emission is situated at the low-level, falling slope of the notch. If the onset DPOAE is located on the high-level, rising slope of the notch, then the upward shift of the notch causes the emission either to decrease monotonically, or to decrease initially and then increase. By establishing that the 2 f1-f2 onset changes reflect a shift in the growth-function notch, it is possible to predict the temporal behavior of DPOAEs in the two-dimensional space of the amplitude of the primaries and for their different frequency ratios.