Binaural Image Research Articles

Dynamic binaural sound localization based on variations of interaural time delays and system rotations.

This work develops the mathematical model for a steerable binaural system that determines the instantaneous direction of a sound source in space. The model combines system angular speed and interaural time delays (ITDs) in a differential equation, which allows monitoring the change of source position in the binaural reference frame and therefore resolves the confusion about azimuth and elevation. The work includes the analysis of error propagation and presents results from a real-time application that was performed on a digital signal processing device. Theory and experiments demonstrate that the azimuthal angle to the sound source is accurately yielded in the case of horizontal rotations, whereas the elevation angle is estimated with large uncertainty. This paper also proves the equivalence of the ITD derivative and the Doppler shift appearing between the binaurally captured audio signals. The equation of this Doppler shift is applicable for any kind of motion. It shows that weak binaural pitch differences may represent an additional cue in localization of sound. Finally, the paper develops practical applications from this relationship, such as the synthesizing of binaural images of pure and complex tones emitted by a moving source, and the generation of multiple frequency images for binaural beat experiments.

Effect of mismatched place-of-stimulation on the salience of binaural cues in conditions that simulate bilateral cochlear-implant listening

Although bilateral cochlear implantation has the potential to improve sound localization and speech understanding in noise, obstacles exist in presenting maximally useful binaural information to bilateral cochlear-implant (CI) users. One obstacle is that electrode arrays may differ in cochlear position by several millimeters, thereby stimulating different neural populations. Effects of interaural frequency mismatch on binaural processing were studied in normal-hearing (NH) listeners using band-limited pulse trains, thereby avoiding confounding factors that may occur in CI users. In experiment 1, binaural image fusion was measured to capture perceptual number, location, and compactness. Subjects heard a single, compact image on 73% of the trials. In experiment 2, intracranial image location was measured for different interaural time differences (ITDs) and interaural level differences (ILDs). For larger mismatch, locations perceptually shifted towards the ear with the higher carrier frequency. In experiment 3, ITD and ILD just-noticeable differences (JNDs) were measured. JNDs increased with decreasing bandwidth and increasing mismatch, but were always measurable up to 3 mm of mismatch. If binaural-hearing mechanisms are similar between NH and CI subjects, these results may explain reduced sensitivity of ITDs and ILDs in CI users. Large mismatches may lead to distorted spatial maps and reduced binaural image fusion.

Binaural image position distributions for phase-shifted low frequency tone bursts

This experiment was designed to yield precise measures of the statistical properties of perceived sound images. Results are reported for listeners' judgments of intracranial sound image lateral positions in response to binaural tone burst stimuli (250 Hz, 50 ms) with varying interaural phase differences, conditional on the absence or presence of a (left or right) reference monaural tone burst (also 250 Hz, 50 ms) ending 500 ms prior to the test signal. The monaural-reference shifted the position distributions toward the opposite side of the head. The position distribution variance and skewness depended on the mean of the position distribution, not on the interaural phase difference of the stimulus. The standard deviation increased as the mean moved laterally from midline. Near the midline the position distributions were skewed ipsilaterally. Near either ear they were skewed toward the midline. The results suggest that the most important noise limiting performance originates central to brainstem coincidence detector networks.

Monaural level discrimination under dichotic conditions

The ability to make judgments about the stimulus at one ear when a stimulus is simultaneously presented to the other ear was tested. Specifically, subjects discriminated the level of a 600 Hz target tone presented at the left ear while an identical-frequency distractor was simultaneously presented at the other ear. When there was no distractor, threshold was 0.7 dB. Threshold increased to 1.1 dB when a distractor with a fixed phase and level was introduced contra-aurally to the target. Further increases in threshold were observed when an across-presentation variability was introduced into the distractor phase (threshold of 1.6 dB) or level (threshold of 5.8 dB). When both the distractor level and phase varied, the largest threshold of 7.3 dB was obtained. These increases in threshold cannot be predicted by common binaural models, which assume that a target stimulus at one ear can be processed without interference from the stimulus at the nontarget ear. The measured thresholds are consistent with a model that utilizes two binaural dimensions that roughly correspond to the loudness and the position of a fused binaural image. The results show that, with binaurally fused tonal stimuli, subjects are unable to listen to one ear.

Near-field binaural synthesis, experimental progress report

A methodology was previously presented for displaying high quality binaural images of near-field complex sources, using wave reconstruction. Multipole representations of objects are transformed to Fourier-Bessel and plane wave expansions at the listener, before conversion to binaural signals. One advantage of this approach is that does not require special HRTF information other than the planewave HRTFs, and can fully render the complex field of a near object. As a first step towards a full working system, a real-time implementation is described here for displaying a monopole source using a six degrees-of-freedom infrared head tracking device.

Cepstral processing for bat biosonar

Bats routinely discriminate multiple glint targets whose interglint time spacings are much less than the integration time of the bat auditory system. This suggests that the bats are exploiting spectral cues in the received echos to resolve the glints. Closely spaced glints cause notches and peaks in the received signal spectrum with frequency spacings characteristic of the glint separations. A biologically inspired signal processing algorithm for resolving these echos into target images is described. The algorithm combines a coarse energy detector to locate targets with a windowed short‐time cepstral analysis, or cepstrogram, to resolve the fine structure of the detected targets. The algorithm’s performance is demonstrated using data recorded in a laboratory experiment. The experiments transmit an ultrasonic chirp and record the echos from a target consisting of a pair of cylinders spaced 5 cm apart, presented at aspect angles ranging from 0 to 90 deg. These recordings are processed with the cepstrogram algorithm using parameter choices commensurate with the performance of the bat auditory system. The cepstrogram algorithm accurately estimates the spacing of the cylinders. Combining the results from the left and right receiver channels produces a binaural image of the target. [Work supported by ONR.]

Temporal pitch perception and the binaural system.

Two experiments examined the relationship between temporal pitch (and, more generally, rate) perception and auditory lateralization. Both used dichotic pulse trains that were filtered into the same high (3,900-5,400-Hz) frequency region in order to eliminate place-of-excitation cues. In experiment 1, a 1-s periodic pulse train of rate Fr was presented to one ear, and a pulse train of rate 2Fr was presented to the other. In the "synchronous" condition, every other pulse in the 2Fr train was simultaneous with a pulse in the opposite ear. In each trial, subjects concentrated on one of the two binaural images produced by this mixture: they matched its perceived location by adjusting the interaural level difference (ILD) of a bandpass noise, and its rate/pitch was then matched by adjusting the rate of a regular pulse train. The results showed that at low Fr (e.g., 2 Hz), subjects heard two pulse trains of rate Fr, one in the "higher rate" ear, and one in the middle of the head. At higher Fr (>25 Hz) subjects heard two pulse trains on opposite sides of the midline, with the image on the higher rate side having a higher pitch than that on the "lower rate" side. The results were compared to those in a control condition, in which the pulses in the two ears were asynchronous. This comparison revealed a duplex region at Fr > 25 Hz, where across-ear synchrony still affected the perceived locations of the pulse trains, but did not affect their pitches. Experiment 2 used a 1.4-s 200-Hz dichotic pulse train, whose first 0.7 s contained a constant interaural time difference (ITD), after which the sign of the ITD alternated between subsequent pulses. Subjects matched the location and then the pitch of the "new" sound that started halfway through the pulse train. The matched location became more lateralized with increasing ITD, but subjects always matched a pitch near 200 Hz, even though the rate of pulses sharing the new ITD was only 100 Hz. It is concluded from both experiments that temporal pitch perception is not driven by the output of binaural mechanisms.

Perception of modulations in pitch and lateralization

The ability to perceive low‐frequency sinusoidal modulations of monaurally and dichotically created pitch was compared to the perception of modulations of the subjective lateral position of a binaural image. The stimuli used in the dichotic pitch experiments had low‐pass spectra that were flat between 0 and 2000 Hz, and they were created by modulating the time delay of a multiple‐phase‐shift filter [F. A. Bilsen, J. Acoust. Soc. Am. 59, 467–468 (1976)]. Stimuli with similar spectra and sinusoidally modulated interaural time delays (ITD's) were used for the latralization experiments [D. W. Grantham and F. L. Wightman, J. Acoust. Soc. Am. 63, 511–523 (1978)]. Also examined was the perception of monaural frequency‐modulated pure tones, and monaural low‐pass stimuli with power spectra similar in shape to the “internal spectra” of the dichotic pitch stimuli. Subjects discriminated between sinusoidally modulated and unmodulated stimuli using two‐cue, two‐alternative 4IFC paradigms, and we determined the threshold frequency deviation or ITD as a function of modulation frequency. Results were similar in form for all experiments: threshold frequency deviation or ITD was constant for modulation frequencies up to a particular “corner” frequency, and then increased as a power function of modulation frequency up to at least 32 Hz. This corner frequency was 4 Hz or less for the binaural lateralization and dichotic pitch experiments, and approximately 10 Hz for the two monaural pitch experiments. [Work supported by NIH.]

Perception of modulations in pitch and lateralization

We compare the ability to perceive low-frequency sinusoidal modulations of monaurally and dichotically created pitch to the perception of modulations of the subjective lateral position of a binaural image. The stimuli used in the dichotic pitch experiments had a 0- to 2000-Hz lowpass spectrum, and they were created by modulating the time delay of a multiple-phase-shift filter [F. A. Bilsen, J. Acoust. Soc. Am. 59, 467–468 (1976)]. Stimuli with similar spectra and sinusoidally modulated interaural time delays (ITDs) were used for the lateralization experiments [D. W. Grantham and F. L. Wightman, J. Acoust. Soc. Am. 63, 511–523 (1978)]. We also examined the perception of monaural frequency-modulated pure tones, and monaural lowpass stimuli created by summing the signals to the two ears from the dichotic pitch stimuli. Subjects discriminated between sinusoidally modulated and unmodulated stimuli using 2IFC paradigms, and we determined the threshold frequency deviation or ITD as a function of modulation frequency. Preliminary results were similar for all three pitch experiments: Frequency deviation at threshold was constant for modulation frequencies up to approximately 10 Hz, and then increased as a power function of modulation frequency up to at least 32 Hz. Threshold ITDs in the lateralization experiments were constant for frequencies up to about 4 Hz, and then increased with increasing modulation frequency. [Work supported by NIH.]

Lateral-position-based models of interaural discrimination.

This letter investigates the hypothesis that lateral position is the only cue available for interaural discrimination experiments using 500-Hz stimuli. The discussion of this hypothesis is in the context of comparisons of the experimental data to predictions of the "position-variable" model of binaural interaction. The model predicts the mean and variance of the subjective lateral position of stimuli used in the discrimination experiments, assuming that discrimination performance is based on optimal processing of this subjective position. To the extent that the laterality predictions of the model are accurate, data that are inconsistent with its predictions would also be problematical for any model based on the subjective laterality of a single binaural image. The predictions (at least qualitatively) describe much of the observed experimental data, including a number of results that have not been addressed by any previous theory. Nevertheless, the observed performance is significantly better than the corresponding predictions for three types of experiments in which the utility of the position cue has been eliminated by experimental design. We believe that our results indicate that changes in lateral position are the primary cue in most interaural discrimination experiments, but that secondary attributes of the perceptual images can be useful when performance based on position alone would be poor.

The lateralization of carrier‐delayed and modulation‐delayed pulse‐modulated signals

An anomalous result has been reported in the lateralization of pulse‐modulated signals presented with a modulation delay: For a fixed interaural modulation delay the spatial position of the binaural image varies with the carrier frequency, and for some carrier frequencies the image is heard to be closer to the ear presented with the delayed modulation waveform. A cross‐correlation model has been proposed to account for these results [Hafter, Shelton, and Green, J. Acoust, Soc. Am. 68, S16 (1980)]. The lateralization of pulse‐modulated signals presented with an interaural carrier delay is reported here. The data do not contain “illusory” results, that is, the binaural image is always heard closer to the ear presented with a carrier advance. However, variations in performance as a function of carrier frequency correspond to the dependency of modulation‐delay performance on carrier frequency. The assumption that a cross‐correlation process is involved in the binaural analysis of both modulation delay and carrier delay provides a means of predicting the carrier‐delay performance from the modulation‐delay data. The model provides a good prediction of carrier‐delay lateralization.

Cue-reversal points and interaural time JND's at various frequencies

Previous research on the lateralization of a 500-Hz tone as a function of interaural delay T has suggested that there exists a delay T0 (usually in the vicinity of 700 μs) that divides the interval 0 ⩽ T ⩽ 1 ms into two regions: the region 0 ⩽ T ⩽ T0, where an incremental increase in T causes the binaural image to move farther to the side, and the region T0 ⩽ T ⩽ 1 msec where an incremental increase in T causes the image to move back towards the center [Hershkowitz and Durlach, J. Acoust. Soc. Am. 46, 1464–1467 ( 1969)]. This critical delay To is referred to as the “cue-reversal” point. We are conducting experiments employing 21, 2AFC procedures and acoustic monitoring to explore the dependence of cue-reversal points on frequency f. In these experiments, no feedback is given and the subjects are required to judge whether the addition of an increment ΔT to T causes the image to move toward or away from center. Preliminary results indicate that (1) the values of T0 on the right and left are often unequal and (2) the cue-reversal phenomenon is incomplete in the sense that lateralization performance in the region T0⩽T⩽12f is less consistent than performance in the region 0 ⩽ T ⩽ T0. We also plan to investigate, using a similar paradigm with feedback, interaural time jnd's in the vicinity of the cue-reversal point. [Work supported by NIH.]

Current problems in binaural hearing research

We will conduct an informal open discussion of issues related to stimulus control, choice and design of experiments, theoretical models, and general research strategy in binaural hearing. Topics to be considered include acoustical monitoring of stimuli, variability and drift in subject performance, techniques for determining the shape of binaural images, time‐intensity trading, the relation of lateralization to detection, similarities and differences in current theoretical models, the inadequacy of many previous experiments to critically test and separate models, the incorporation of multiple sampling of inputs into models, and the effects of long‐term previous experience in natural environments on the perception of laboratory stimuli (and the possibility of taking explicit account of such effects in the construction of models. [Work supported by NIH.]

Lateralisation of Binaural Images in A Computer-Controlled Experiment

Binaural fusion and the ability to resolve or separate coexisting binaural images greatly facilitates speech communication in the presence of competing auditory cues. However, the study of these two processes has been somewhat neglected by clinical audiologists. In this paper quantitative methods for assessing lateralisation (fusion) ability are discussed; together with a review of schemes for presenting experimental subjects with competing binaural images. Current understanding of the physiological basis of image formation, based essentially upon knowledge of peripheral auditory mechanisms, allows the theoretical prediction of perceived image location to be attempted. These predictions have been tested using a computer-controlled lateralisation experiment; and departures from predicted and judged locations found. The significance of these departures is briefly discussed.

Tracking of complex binaural images.

Subjects were presented with an initially stationary binaural image formed by the fusion of two identical pulses, without interaural time difference (ITD), at the two ears. The image was then made to traverse the subject's auditory perceptual space by introducing ITD, varying linearly with time, under computer control. The direction of movement, i.e. towards the right or left ear, could be reversed by the listener, by pressing a button. Subjects were requested to keep the image central, by pressing the button when they judged that deviation from subjective centre had occurred. Experiments of this type can be considered as analogous to Békésy audiometry, where the subject automatically traces his threshold of hearing, in that here the listener traces out his auditory perceptual centre as it varies with time. Hence, equivalent analyses to those employed for Békésy audiometry are possible. Subsequent to the initial part of the experiment, an additional pulse was added to one channel, preceding the original pulse, to form a pulse pair. The monaural masking of the original pulse by this additional pulse thus acts to shift the pre-existing binaural image. The effect of varying the amplitude and onset time of the masking pulse, relative to the original pulse, on the Békésy-type trace was examined.

Detection of shift in binaural images: A rating method approach

Two practiced listeners were run in a lateralization task in which a rating method was employed. The detection of right and left shifts from the center position was determined. The effect of frequency of the signal was also assessed in the lateralization task. In addition, nine naive listeners were run in a similar situation in order to determine the feasibility of the use of the rating method approach with untrained Ss. First-half, second-half comparisons were determined to check reliability of the technique and also to evaluate the effect of number of trials. The experimental results indicate the procedure is a promising one for the study of directional shifts in binaural stimuli with practiced listeners.

Interaural amplitude effects in binaural hearing.

The interaural amplitude difference, which will compensate the effect of interaural time difference for lateralization of a binaural image, has been further investigated with binaural transients low-pass filtered in order to minimize the risk of interference from multiple images arising with wide-band transients. Arguing from the effects observed, it is shown that if properties of cochlear neural responses found in the experimental animal are applicable in the human, a simple basis for explaining the main effects of amplitude difference in binaural listening would be available.