Narrowband stimulation and synchronization of otoacoustic emissions

Tone-burst-evoked otoacoustic emissions were obtained for several normal listeners with and without spontaneous otoacoustic emission using an optimized tone burst. The dependence of the response amplitude on stimulation level shows a linear increase below a certain value close to the threshold in quiet and levels off at higher levels exhibiting only small differences between non-linear and linear averaging modus. In addition, the latency of the response tends to decrease and the sharpness of the resonance decreases with increasing stimulation level. Synchronization tuning-curves were obtained using a fixed tone burst to elicit an evoked otoacoustic emission and a probe tone at different frequencies for synchronizing the SOAE. These synchronization tuning curves exhibit relatively sharp resonance characteristics (Q3 varying between 3 and 8) for subjects with spontaneous otoacoustic emissions and less sharp tuning (Q3 varying between 1 and 3) for subjects without spontaneous otoacoustic emissions. These experimental results could be reproduced very well with a model of a single non-linear Van-der-Pol-oscillator with the appropriate parameters. The results indicate that spontaneous otoacoustic emissions and narrow-band-evoked otoacoustic emissions are generated by the same mechanism which can be modelled as a self-sustained oscillator.