Distortion product emissions in humans II. Relations to acoustic immitance and stimulus frequency and spontaneous otoacoustic emissions in normally hearing subjects

Multifrequency and multicomponent evaluations of aural acoustic immitance, including tympanometry and acoustic reflex testing were performed on 44 normal ears to examine the influence of middle ear functioning on the generation and detection of disortion product otoacoustic emissions (DPEs). In the same ears, the prevalence and parametric features of spontaneous and stimulus frequency emissions were also assessed so that their relationship to the detection 'thresholds' and amplitudes of corresponding DPEs could be determined. The general outcome was that none of the examined features of acoustic immittance provided an explantation for the discrete, low-amplitude DPE regions observed in about one third of normal ears. Moreover, the presence of typical spontaneous and stimulus frequency emissions in these same 'irregular' ears indicated that emission generation and reverse cochlear transmission were also operating normally within these regions of reduced DPEs. Consequently, other, as yet undetermined influences appear to contribute to the DPE variability noted in some ears. Finally, the stimultaneous presence of stimulus frequency emissions, but not spontaneous emissions, appeared to reduce the detection 'thresholds' and increase the amplitudes of low-frequency DPEs.