Joachim Neumann

Detection of the Acoustic Reflex below 80 dB HL

A new method for detecting the acoustic reflex that utilizes standard otoacoustic emissions recording techniques is introduced and discussed. Two successive identical tone bursts of 100 ms duration and 10 ms interstimulus interval are presented in the occluded ear canal at a repetition rate of one per second. If the acoustic reflex is elicited, the contraction of the stapedius muscle is delayed with respect to the onset of the first stimulus. Hence, the acoustic compliance in the ear canal decreases primarily during the second stimulus. The difference of the microphone signals produced by the two stimuli is computed and averaged across a certain number of repetitions of the sequence. The presentation level is increased until this difference is larger than -40 dB (with respect to the stimulus level) and if its signal-to-noise ratio exceeds 20 dB. For normal-hearing subjects, the acoustic reflex threshold measured with this method is on average 8 dB lower than in a standard clinical setup. In 5 out of the 10 tested hearing-impaired subjects, the new method could detect an acoustic reflex at one or more frequencies where no reflex was detected in the clinical setup.

Chirp evoked otoacoustic emissions

The principles of short frequency sweeps (chirps) and their application to evoke transiently evoked otoacoustic emissions (TEOAE) are developed in comparison to using standard click stimuli. In contrast to click stimuli, chirp signals have the advantage of stimulating a freely selectable frequency range. In addition, chirp signals contain more energy than a click stimulus with the same maximum amplitude. The effects of different stimuli on TEOAE were investigated in normal hearing and hearing-impaired subjects. Using wide-band chirp signals yields a better signal-to-noise ratio compared to click stimulation. In addition, the stimulation of selected regions of the basilar membrane with frequency-limited chirps evokes TEOAE with frequency components that lie within the stimulated frequency range. The characteristic fine structure of this spectrum was found to be independent of the stimulus applied. The utilization of chirp stimuli appears to be useful for evoking TEOAE in, e.g., clinical applications.

Suppression of transiently evoked otoacoustic emissions

The influence of additional continuous tones on transiently evoked otoacoustic emissions (TEOAE) is studied with normal hearing subjects. If a single continuous tone is present, nearby frequency components of the TEOAE are synchronized to this tone in a similar way as spontaneous OAE can be synchronized. This effect can be deducted from the reduction (‘‘suppression’’) of the corresponding OAE component since the additional continuous tone is designed to exhibit a phase shift of pi between successive averaging frames and thus cancels out. To suppress several spectral components of the TEOAE, a continuous tone complex was employed. The frequency difference between adjacent components of this tone complex yields an optimum value at about a critical bandwidth. An application of these findings is the stimulation and evaluation of TEOAE from a predefined region on the basilar membrane: While the frequency range of the evoking signal can be limited by using chirp stimuli (i.e., short frequency sweeps), the TEOAE...