Fachbereich Physik, Univ. Oldenburg, D-26111 Oldenburg, Germany
Transiently evoked otoacoustic emission spectra typically show several maxima and minima with a frequency spacing, which might represent a periodic structure in the cochlea. A method studying the spectral properties of transiently evoked otoacoustic emissions is presented. A modified cepstral analysis is performed by calculating a backward FFT on samples of the power spectrum, which were taken equivally spaced on the bark scale, representing the frequency mapping on the basilar membrane. This results in a type of cepstrum, known from speech analysis, which can tell something about any periodicity in the spectrum. Analyzing EOAE results from 60 normally hearing subjects, half of the ears showed a periodicity with a spacing ranging from 0.4 to 0.8 bark. The synchronization of a strong spontaneous emissions by the transient stimulus results in an emission spectrum dominated by this single frequency and with no detectable periodicity. The finding of a periodic structure in case of broader emission spectra supports the model of ``Bragg reflection'' due to a periodic fluctuation of BM impedance [H. W. Strube, Hear. Res. 38, 35--45 (1989)] for the generation of TEOAE.