Wavelet analysis of chirp-evoked otoacoustic emissions

Click evoked otoacoustic emissions (CEOAEs) are very low level sounds produced by the inner ear in response to an acoustic click. An acoustic sinusoidal signal, whose instantaneous frequency changes continuously over a finite period of time, is called a chirp. While clicks and chirps differ in duration, they may contain spectral energy throughout a specific bandwidth. The instantaneous rate of frequency change of an increasing frequency chirp signal can be determined by the traveling wave velocity along the basilar membrane and the functional relationship between stimulus frequency and place of maximum displacement. This physiologically derived chirp stimulus should elicit larger amplitude (higher SNR) evoked otoacoustic emissions with a more complete frequency content (more high frequencies) than click evoked OAEs. In this study, two such chirps with different low frequency content and duration were used to elicit transient otoacoustic emissions (Chirp Evoked OAEs or ChEOAE) and auditory brainstem responses (ABRs). The results were compared to responses evoked by conventional clicks. Simultaneously recorded OAEs and ABRs were obtained from 16 normal adult hearing subjects and analyzed in terms of amplitude, latency, and frequency content. Chirp offset time was used to determine latencies in chirp evoked OAEs and ABRs. The responses from both types of chirps produced similar OAE and ABR amplitudes and latencies. Click ABR amplitudes, however, were significantly lower than chirp ABR amplitudes, while latencies were similar. The chirp acoustic stimulus evoked an almost identical OAE to that of click evoked OAE, without any stimulus artifact. In addition, at low stimulus levels ChEOAEs showed higher SNR and RMS amplitudes than CEOAEs. Wavelet analysis was used to compare latencies of several frequency components of both ChEOAEs and CEOAEs. Chirp and click evoked OAEs yielded similar frequency dispersion characteristics in time-frequency maps, yet, chirp OAEs showed shorter relative latencies between dominant frequency components, showing compression in time. A peak energy detection scheme of the time-frequency distribution was used to determine significant frequencies of CEOAE and ChEOAE. Wavelets corresponding to these selected frequencies were used to model the individual\u27s evoked OAE. ChEOAEs were modeled with fewer wavelets, while preserving a higher correlation value to the original evoked OAE, than the CEOAE. In addition, a composite time-frequency map of peak energies for all subjects showed that the ChEOAE had a tighter grouping to a power curve fit than the CEOAE. Determination of a normative range around the frequency dispersion curve fit using PBL wavelets may aid in diagnosing hearing loss in newborns and other difficult to test populations, where the behavioral test cannot be done.The overall objective of this dissertation was to investigate the time-frequency characteristics of the OAEs evoked by acoustical chirp stimuli and determine whether the use of acoustical chirp signals elicit larger amplitude (higher SNR) evoked otoacoustic emissions with a more complete frequency content (more high frequencies) than click evoked OAEs. The chirp stimulus could reduce current acquisition and processing time, yielding a more robust clinical screening tool for audition