Static input-output non-linearity as the source of non-linear effects in maximum length sequence click-evoked OAEs

The application of the maximum length sequence (MLS) technique to the recor ding of click-evoked otoacoustic emissions (CEOAEs) allows for a reduction in test time by one to two orders of magnitude. This is because the techniq ue permits the use of extremely high click rates, as inter-click intervals are not constrained to be greater than the duration of the response. Howeve r, increasing the click rate also causes a progressive reduction in amplitu de, or 'suppression', of the CEOAE. The origin of this suppression is uncle ar, with diverse suggestions in the literature as to its nature and mechani sm. This paper presents a simple model of the well-known compressive non-li nearity of the CEOAE level function, based on a static amplitude non-linear ity within each of a number of narrowband frequency channels. The response of the model to MLS stimulation demonstrates suppression broadly of the for m and magnitude previously reported in experimental studies. Furthermore, t he model exhibits the generation of additional non-linear components that h ave been speculated on in connection with CEOAE recordings using the MLS te chnique. It is concluded that the MLS suppression phenomenon is derived lar gely, if not entirely, from the static non-linearity of the CEOAE level fun ction. The approach to modelling the phenomenon as described here also bear s promise for understanding various aspects of non-linearity in MLS -based CEOAE recordings.