Biomedical subjects
Publications and source records attributed to P M Zurek.
An analysis is presented of the detectability of the two types of otoacoustic emissions being considered for use in clinical tests of hearing. It is estimated that, for equal emission levels and equal probability of error, measurement of distortion-product emissions can be made at least 30 times faster than transient-evoked emissions using current techniques.
In this paper evaluations of a two-microphone adaptive beamforming system for hearing aids are presented. The system, based on the constrained adaptive beamformer described by Griffiths and Jim [IEEE Trans. Antennas Propag. AP-30, 27-34 (1982)], adapts to preserve target signals from straight ahead and to minimize jammer signals arriving from other directions. Modifications of the basic Griffiths-Jim algorithm are proposed to alleviate problems of target cancellation and misadjustment that arise in the presence of strong target signals. The evaluations employ both computer simulations and a real-time hardware implementation and are restricted to the case of a single jammer. Performance is measured by the spectrally weighted gain in the target-to-jammer ratio in the steady state. Results show that in environments with relatively little reverberation: (1) the modifications allow good performance even with misaligned arrays and high input target-to-jammer ratios; and (2) performance is better with a broadside array with 7-cm spacing between microphones than with a 26-cm broadside or a 7-cm endfire configuration. Performance degrades in reverberant environments; at the critical distance of a room, improvement with a practical system is limited to a few dB.
Some current single-microphone hearing aids employ techniques for adaptively varying the frequency-gain characteristics in an attempt to improve speech reception in noise. The potential benefit of this strategy depends on the spectral spread of masking and the degree to which it can be reduced by changing the frequency-gain characteristic. In this study these benefits were examined for subjects with normal hearing under static listening conditions. In the unprocessed condition, subjects were presented with nonsense syllables in an octave-band noise centered on 0.5, 1, or 2 kHz. The frequency-gain characteristic was then modified with the goal of reducing the intensity of the frequency region containing the octave-band noise. This processing resulted in increases as large as 60 percentage points in consonant-correct scores with the low- and mid-frequency octave noise bands, and a small increase with the high-frequency noise. Masking patterns produced by the octave noises were also measured and were related to the intelligibility results via an analysis based on Articulation Theory. The Articulation Index was also used to compare the effectiveness of three adaptive rules. A simple multiband volume control is expected to provide much of the benefit of more sophisticated systems without the need for separate estimation of input speech and noise spectra.
Probability distributions of interaural phase and level differences are derived for the stimuli used in many studies of binaural detection and interaural-correlation discrimination. Example distributions are shown and summary plots give the standard deviations of these distributions as functions of signal-to-noise ratio.
We describe the results of computer simulations of a multimicrophone adaptive-beamforming system as a noise reduction device for hearing aids. Of particular concern was the system's sensitivity to violations of the underlying assumption that the target signal is identical at the microphones. Two- and four-microphone versions of the system were tested in simulated anechoic and modestly-reverberant environments with one and two jammers, and with deviations from the assumed straight-ahead target direction. Also examined were the effects of input target-to-jammer ratio and adaptive-filter length. Generally, although the noise-reduction performance of the system is degraded by target misalignment and modest reverberation, the system still provides positive advantage at input target-to-jammer ratios up to about 0 dB. This is in contrast to the degrading target-cancellation effect that the system can have when the equal-target assumption is violated and the input target-to-jammer ratio is greater than zero.
Lateralization responses to noise targets were obtained in a diotic noise background. On each trial, a noise target was added to the background noise in one earphone. Subjects were required to identify the earphone containing the target. Noise targets were either coherent or incoherent with the background. The long-term power spectrum of the incoherent target was identical to that of the background noise, and its amplitude was adjusted so that the addition of either the coherent or incoherent target to the background produced the same average increment in power. When 50-ms noise targets were presented in the middle of a 750-ms diotic background, lateralization thresholds were lower for the incoherent targets. The advantage with incoherent targets was shown to depend on the temporal relationships between target and masker. Superior performance with incoherent targets is inconsistent with predictions based on an analysis of interaural phase and amplitude differences. Models based on interaural subtraction, on the other hand, are able to account for the lateralization advantage shown by incoherent targets but are unable to account for the variations in threshold produced by altering the temporal relationships between target and masker.
Measurements of the just-noticeable change in resonance frequency delta Fr of a second-order filter are reported. The source signal was either periodic, with or without a smooth change in fundamental frequency, or it was random white noise. These differences in the nature of the source had little effect on delta Fr. Over the investigated ranges of reference resonance frequency (Fr = 300 to 2000 Hz) and filter selectivity (Q = 1 to 36), the results are well summarized by delta Fr = 0.079 Fr/square root Q. The data were used to evaluate filter-bank models employing different filter shapes and performance-prediction schemes. A good fit to the resonance-discrimination data was obtained with filters derived in a masking study by Patterson [J. Acoust. Soc. Am. 55, 802-809 (1974)] and with performance based on use of the maximum level difference over all bands. The latter finding indicates that listeners may not make optimal use of small level differences distributed over multiple bands.
Two aspects of the intermodulation distortion product at 2f1-f2 generated by normal human ears and measured acoustically in the ear canal were studied: (1) its relation to tone-evoked and spontaneous otoacoustic emissions, and (2) its relation to the perceived combination tone at the same frequency. With regard to (1), substantial differences among ears in the detectability of emissions were observed; ears tended to exhibit all or none of the emission types that were sought. Within ears possessing emissions, the magnitudes of tone-evoked emissions and acoustic distortion showed a similar dependence on frequency. With regard to (2), a three-primary-tone stimulus was employed to ask whether the ear canal acoustic distortion tone is canceled under the same stimulus conditions that produce perceptual cancellation. Simultaneous cancellation of perceptual and acoustic distortion was produced rarely. Results are interpreted qualitatively with a model in which primary tones produce distortion at their interaction region within the cochlea; this distortion propagates to the distortion-frequency place where it mediates perception. This same distortion wave produces emission components at additional locations, including the primary-tone interaction region, which sum vectorially to mediate the emitted acoustic distortion product.
This research is concerned with the ability of normal-hearing listeners to discriminate broadband signals on the basis of spectral shape. The signals were six broadband noises whose spectral shapes were modeled after the spectra of unvoiced fricative and plosive consonants. The difficulty of the discriminations was controlled by the addition of noise filtered to match the long-term speech spectrum. Two-interval discrimination measurements were made in which loudness cues were eliminated by randomizing (roving) the overall stimulus level between presentation intervals. Experimental results, examined as a function of intensity rove width, stimulus duration, and stimulus pair, were related to the predictions of a simple filter-bank model whose fitting parameter provides an estimate of internal noise. Most results, with the notable exception of duration effects, were predicted by the model. Estimates of internal noise in each frequency channel averaged roughly 7 dB for long-duration stimuli and 13 dB for short-duration stimuli. Results and predictions are compared to results of other studies concerned with the discrimination of spectral shape.
Thresholds for the detection of 500-ms tones in noise at either 0.25 or 4 kHz and either interaurally in-phase (NoSo) or out-of-phase (NoS pi) were measured as a function of the bandwidth of a diotic masking noise (with total noise power held constant). NoSo thresholds followed the classic trend indicative of an effective critical band in noise masking. NoS pi thresholds also indicated critical-band filtering, but with a wider effective critical bandwidth. At subcritical masker bandwidths, for both 0.25 and 4 kHz, NoS pi thresholds increased with an increase in noise bandwidth, despite the fact that total noise power was constant. This latter finding is attributed to binaural insensitivity to rapid fluctuations in the interaural cues that subserve detection in the NoS pi condition. These results suggest a new interpretation for the small difference between NoSo and NoS pi thresholds measured with high-frequency tones in a broadband noise masker. This interpretation is based partly on the inability to utilize rapidly varying interaural cues, partly on out-of-band interference effects, and partly on loss of information related to stimulus fine structure.
The goal of this study was to determine the extent to which the difficulty experienced by impaired listeners in understanding noisy speech can be explained on the basis of elevated tone-detection thresholds. Twenty-one impaired ears of 15 subjects, spanning a variety of audiometric configurations with average hearing losses to 75 dB, were tested for reception of consonants in a speech-spectrum noise. Speech level, noise level, and frequency-gain characteristic were varied to generate a range of listening conditions. Results for impaired listeners were compared to those of normal-hearing listeners tested under the same conditions with extra noise added to approximate the impaired listeners' detection thresholds. Results for impaired and normal listeners were also compared on the basis of articulation indices. Consonant recognition by this sample of impaired listeners was generally comparable to that of normal-hearing listeners with similar threshold shifts listening under the same conditions. When listening conditions were equated for articulation index, there was no clear dependence of consonant recognition on average hearing loss. Assuming that the primary consequence of the threshold simulation in normals is loss of audibility (as opposed to suprathreshold discrimination or resolution deficits), it is concluded that the primary source of difficulty in listening in noise for listeners with moderate or milder hearing impairments, aside from the noise itself, is the loss of audibility.
To reduce interference in monaural hearing aids from sound sources that are spatially separated from a target source, we are investigating methods for combining information from multiple microphones. In this paper, we describe an adaptive beamforming method that functions to preserve target signals arriving from straight-ahead of a microphone array while minimizing output power from off-axis interference sources. In a preliminary evaluation of a two-microphone system, sentence intelligibility tests were administered to normal-hearing subjects using processed and unprocessed materials from simulated environments in which the target was on-axis, the interference (speech babble) was 45 degrees off-axis, and the reverberation mimicked that of a living room, a conference room, and anechoic space. Compared to listening through a single microphone, the two-microphone beamformer reduced the target-to-interference ratio required to achieve 50 percent keyword intelligibility by 30, 14, and 0 dB in the anechoic, living-room, and conference-room conditions, respectively. The corresponding improvements over binaural listening (one microphone to each ear) were 24, 9, and 0 dB. Further tests in the living-room environment using the same beam-forming system but with filter impulse responses shortened by a factor of four (which would decrease the adaptation time by a factor of four) decreased the improvement by 5 dB. These results are sufficiently encouraging to warrant further tests involving more realistic reverberant conditions, multiple sources of interference, and time-varying acoustic environments.
Studies of persons with conductive hearing loss have revealed substantial degradations in binaural hearing abilities. Described here is a model based on the hypothesis that this degradation is due to high levels of bone-conducted signals relative to air-conducted signals. The model makes quantitative predictions for the effects of conductive impairment on measurements of interaural discrimination. Qualitative predictions for binaural advantages in detection and speech intelligibility are also made by employing auxiliary models. Generally, available data are consistent with the models, although strong tests have not yet been performed.
As mentioned in Saunders et al. [J. Acoust. Soc. Am. 78, 299-311 (1985)] an understanding of the transduction of sound into the auditory neural code is a major goal of the hearing scientist. A variety of tools has been and is being used to achieve that goal. The recent discovery of physical acoustic emissions from the ear provides both new insight into possible transduction mechanisms and a useful new tool for research. The existence of narrow-band spontaneous acoustic emissions from the ear suggests the possibility of active cochlear processes that were not seriously considered a decade ago. Acoustic emissions, both spontaneous and evoked, also provide a simple and noninvasive means of monitoring a by-product of cochlear-mechanical activity. In the present paper, results of studies of acoustic emissions from human ears are briefly summarized and compared to results obtained with animals. Differences between human and animal emissions are noted and the implications of these differences and hypotheses to explain them are discussed.
Two cases of spontaneous otoacoustic emissions (SOAEs) have been found among a sample of 28 chinchilla ears after noise exposure, and no cases of SOAEs have been found among 28 unexposed ears. Further observations of the characteristics of SOAEs recorded in the ear canals of two chinchillas after noise exposure are described. These signals were tonal, robust, and could be suppressed by presenting external tones to the ear. Histopathological evaluation of the cochlea of emitting ears revealed discrete basal-turn lesions near the positions corresponding to the frequencies of the emissions. Behavioral threshold shifts measured in one animal after noise exposure and acoustic intermodulation distortion product behavior in the other both suggest a region of increased vibratory response of the cochlear partition near the location of the SOAE. Results from these emitting ears support a hypothesis that a punctate loss of the organ of Corti (OC) may facilitate the occurrence of an SOAE. We further hypothesize that the following conditions are both necessary and sufficient for an SOAE to occur: (1) functional disruption of a normally present biomechanical control mechanism in a region of the OC; (2) presence of functionally active OC (especially outer hair cells) adjacent to the region. The observations from ears possessing SOAEs provide strong, though indirect support for active and nonlinear models in interpreting cochlear biomechanical phenomena.
The existence of audible combination tones at frequencies greater than the primary tones that generate them has long been problematic. With primary tones at frequencies f1 and f2, combination tones at f1 + f2 - f1, and other frequencies can be demonstrated and measured by using a contralateral probe tone to establish a binaural interaction with a given combination tone. The estimated amplitudes of these higher-frequency combination tones are generally 20 to 40 decibels below the amplitude of the primary tones.
The notion of binaural echo suppression that has persisted through the years states that when listening binaurally, the effects of reverberation (spectral modulation or coloration) are less noticeable than when listening with one ear only. This idea was tested in the present study by measuring thresholds for detection of an echo of a diotic noise masker with the echo presented with either a zero or a 500-musec interaural delay. With echo delays less than 5-10 msec, thresholds for the diotic echo were about 10 dB lower than for the dichotic signal, a finding opposite that of the usual binaural masking-level difference but consistent with the notion of binaural echo suppression. Additional echo-threshold measurements were made with echoes of interaurally reversed polarity, producing out-of-phase spectral modulations. The 10-15 dB increase in thresholds for the reverse-polarity echo, over those for the same-polarity echo, indicated that the apparent "hollowness" associated with spectral modulations can be partially canceled centrally. Overall, the results of this study are consistent with a model in which: (1) the monaural representations of spectral magnitude are nonlinearly compressed prior to being combined centrally; and (2) neither monaural channel can be isolated in order to perform the detection task.
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