Eberhard Zwicker

Mechanical and acoustical influences on spontaneous oto-acoustic emissions

Spontaneous, tone-like emissions produced by normal ears and measured in the closed outer ear canal can be affected by mechanical and acoustical events. Such effects can be measured in steady-state conditions as well as for transient stimulation, and are seen in response to the stapedius reflex, to ear canal air pressure changes, and to the presentation of external tones. Frequency and level of the emissions follow certain characteristics which are described and discussed. The emissions seem to react with 2 ms delay and with an exponential rise and decay, the time constant of which is about 13 ms.

Delayed evoked oto-acoustic emissions and their suppression by Gaussian-shaped pressure impulses

The sound pressure of delayed evoked oto-acoustic emissions was measured as a function of the sound pressure of the stimulating sound impulse. They are directly proportional for sensation levels of the stimuli lower than about 20 dB; above that level, the emission saturates. Spontaneous emissions lying in the same frequency range as evoked emissions seem to influence this simple relation. Within the linear range, delayed emissions superimpose linearly even throughout long lasting delayed emissions. Short test tone bursts were used as test sound, to produce masking-period patterns, and as stimulus, to produce suppression-period patterns, respectively, while low-frequency Gaussian-shaped pressure impulses served as masker and as suppressor. The very close relation of the two patterns is indicated by their mirrored forms. This is considered directly relevant to the phenomenon of masking.

Acoustical responses and suppression-period patterns in guinea pigs

Acoustic responses to short sound stimuli have been measured in the external ear canal of guinea pigs. Unequivocal responses had frequencies between 1.8 and 3.2 kHz, latencies (to their peak pressures) of 2.8 to 4.5 ms and sound pressures up to -10 dB SPL. A set of criteria was developed to clearly distinguish acoustical responses from after-resonances of the stimuli. Added phase-locked, very-low-frequency tones were able under some conditions to totally suppress the acoustical responses. The suppression-period pattern thus produced strongly resemble those obtained for man and the masking-period patterns obtained with human subjects.

Temporal Resolution in Hard-of-Hearing Patients

Temporal resolution is, next to frequency resolution, the most important ability of the ear for perceiving speech. A simplified method for measuring temporal resolution clinically is described. It is based on masking period patterns, but measures only their maximum and minimum through the use of continuous and square-wave amplitude-modulated masking noises. Test tones and filtered masking noises are presented at 500, 1 500, and 4 000 Hz. Temporal resolution data of a normally hearing group and of seven pathological groups are given: Patients with conductive hearing loss show normal resolution; those with noise-induced, age-induced, and sudden hearing loss show normal resolution at low and middle frequencies, but reduced resolution at 4 000 Hz; patients with retrocochlear hearing loss and Menières disorder show a tendency for reduced resolution at all frequencies, but reduction at 4 000 Hz, while ototoxic hearing loss shows the greatest reduction at 1 500 Hz. The method used also gives an indication for reduced frequency selectivity.

Cochlear preprocessing in analog models, in digital models and in human inner ear.

Data of spontaneous, delayed, and simultaneously evoked otoacoustic emissions produced in human subjects are compared with data produced in an analog and in a digital wave-parameter realization of our model. The high probability of the frequency distance of the emissions extreme values at 0.4 Bark, corresponding to a local distance of 0.5 mm along the basilar membrane is found also in the models. The influence of the lateral feedback coupling in the models on the frequency selectivity and on the appearance of emissions show the high quality of simulating human peripheral signal processing by the models.

A model of loudness summation applied to noise-induced hearing loss

The main contention of this paper is that Zwickers model of loudness summation is applicable to observers with noise-induced hearing loss when certain parameters of the model are modified. Two types of measurement were obtained in observers with normal hearing and noise-induced hearing loss: loudness summation as a function of level and narrow-band masking. These measurements provided a basis for modifying the parameters of the model. Results suggest that the model of loudness summation is applicable to observers with noise-induced hearing loss when the presence of recruitment and reduced frequency selectivity is taken into account.

Delayed Evoked Otoacoustic Emissions - An Ideal Screening Test for Excluding Hearing Impairment in Infants

A screening test is badly needed with which cochlear hearing impairment in small children and newborns can be detected. Delayed evoked otoacoustic emissions (DEOAEs) have been found in laboratory research to be a very useful tool to test the normality of cochlear preprocessing. The characteristics of a simple apparatus for clinical use to measure DEOAEs are described together with typical examples of emissions. Many case studies have already demonstrated the clinical usefulness of DEOAEs in neonates and small children. Five cases are explained in detail and the advantages of such an early screening test discussed.

Binaural masking-level differences with tones masked by noises of various bandwidths and levels

Binaural masking-level differences (BMLDs) were measured for tones at frequencies below and above masking noises centred on 250 Hz, having 10, 31.6 or 100 Hz bandwidths and either 40 or 60 dB spectral density levels. The BMLD drops rapidly when masker and signal have no frequency components in common. The magnitude of the decrease in the BMLD is not merely a result of the reduced masking effect and is not to be predicted by the models of Durlach [2] or Schenkel [12].

Binaural masking-level difference as a function of masker and test-signal duration

The binaural masking-level difference (BMLD) is measured as a function of duration of the masker (uniform masking noise) and of the test tone (400 and 800 Hz). Four observers used a Békésy tracking method which proved to be a time-saving procedure for BMLD measurements. Although the masked threshold depends on the test-tone duration, the BMLD does not. The latter, however, increases from about 5 to 10 dB as the masker duration increases from 10 to 200 ms.

On the influence of acoustical probe impedance on evoked otoacoustic emissions

Using two different probes representing different acoustical impedances, simultaneously evoked otoacoustic emissions, thresholds in quiet, and delayed evoked emissions were measured in the frequency region of 900 to 1100 Hz for one subject who has extremely large emissions at these frequencies. All data show that the sound pressure produced in the closed ear canal clearly depends on two contributions, that of the activity of the inner ear, and that of the acoustical impedance of the probe. These effects can be simulated in an analog model of peripheral preprocessing.