Binaural System Research Articles

Binaural models and the strength of dichotic pitches.

Modern physiologically based models of the binaural system incorporate internal delay lines in the pathways from left and right peripheries to central processing nuclei. Different binaural models for the formation of dichotic pitch employ these delay lines in different ways. Consequently, the different models make different predictions for the relative strengths of dichotic pitches made with particular phase conditions. The differences are magnified for dichotic pitches at low frequencies where especially long delay lines may be required. Data from four low-frequency pitch strength experiments on pure-tone-like dichotic pitches (two on Huggins pitch and two on binaural coherence edge pitch) are consistent with models of the equalization-cancellation type and not consistent with the central activity pattern model.

Microsecond temporal resolution in monaural hearing without spectral cues?

The auditory system encodes the timing of peaks in basilar-membrane motion with exquisite precision, and perceptual models of binaural processing indicate that the limit of temporal resolution in humans is as little as 10-20 microseconds. In these binaural studies, pairs of continuous sounds with microsecond differences are presented simultaneously, one sound to each ear. In this paper, a monaural masking experiment is described in which pairs of continuous sounds with microsecond time differences were combined and presented to both ears. The stimuli were matched in terms of the excitation patterns they produced, and a perceptual model of monaural processing indicates that the limit of temporal resolution in this case is similar to that in the binaural system.

Monaural and diotic detection of tones in wideband and narrow-band reproducible noise maskers

In order to test monaural and binaural processing components of binaural detection, diotic and monaural detection were studied using reproducible noises. Correlations across masker bandwidth and interaural configuration were computed. Preliminary results reveal high correlations for probability of detection across N0S0 and NmSm configurations for both wideband and narrow-band maskers, suggesting that the binaural system does not influence results across reproducible noises for diotic detection. As reported previously [Evilsizer et al., J. Acoust. Soc. Am. 111, 336–345 (2001)], significant differences in correlations between wideband and narrow-band results were observed for the diotic case, suggesting cross-frequency interactions. Preliminary findings indicate that the same result holds for the monaural case. These data will be used to test models that incorporate monaural processing based on the phase-opponency model [Carney et al., Acta Acustica United with Acustica 88, 334–347 (2002)], and binaural processing based on the Jeffress model and on the two-channel model [McAlpine et al., Nature Neurosci. 4, 396–401 (2001)]. Predictions will be made for NmSm, N0S0, and N0Sπ interaural configurations, and for wideband and narrow-band maskers. [Work supported by NIDCD.]

Binaural digital hearing aid system

A binaural digital hearing aid system comprises two hearing aid units (1, 2) for arrangement in a user's left and right ear, respectively. Each unit comprises input signal transducer means (3r, 3l), A/D conversion means (4r, 4l), digital signal processing means (5r-13r, 5l-13l), D/A conversion means (14r, 14l) and output signal transducer means (15r, 15l). A bi-directional communication link (7) is provided between the units. The digital signal processing means of each unit is arranged to affect a substantially full digital signal processing including individual processing of signals from the input transducer means of the actual unit and simulated processing of signals from the input transducer means of the other unit as well as binaural signal processing and includes at least a first digital signal processor part (5r, 5l) for processing said internally supplied signal, a second digital signal processor part (6l, 6r) for processing the signal supplied via said communication link (7) and a third digital signal processor part (9r, 9l) to effect common binaural digital signal processing of information derived from the signals processed in said first and second digital signal processor parts, said second digital signal processor part (6l, 6r) in each unit simulating the first digital signal processor part (5l, 5r) in the other unit with respect to adjustment parameters controlling the performance of said first signal processor part in said other unit.

Features of echoes bottlenose dolphins use to perceive object properties

Echolocating bottlenose dolphins can discriminate among objects that vary in size, shape, and structure. Our purpose in this project is to investigate the features of echoes dolphins may use to determine each of these object properties (e.g., target strength, number, and position of highlights). A dolphin performed a cross-modal matching task where he was presented with an object in one modality (e.g., vision) and then asked to choose the same object from among a group of three objects using another modality (e.g., echolocation). The dolphin was presented with object sets in which the objects within the set varied along one feature (size or shape or material or texture). These objects were later ensonified with dolphin-like clicks and the object echoes were collected with a binaural (two hydrophone) system. To mimic the dolphin’s ability to scan across objects, echoes were captured as the objects were rotated. Differences in echoic features within object sets were examined along with the dolphin’s error. In addition, an artificial neural network (ANN) was used to classify objects using the object echoes. The ANN was utilized to explore the importance of spectral versus time domain features, and single versus multiple object orientations in object classification.

Interaural coherence and the lateralization of noise by interaural level differences

Coherent noise, with an interaural cross correlation of unity, and incoherent noise, where signals to left and right ears are independent, sound very different. The former produces a compact lateralized image. The latter produces a diffuse image that fills the head. Nevertheless, listening experiments show that coherent and incoherent noise images seem to be equally affected by small interaural level differences (ILD). The ILD threshold for lateralizing incoherent noise is less than half a dB greater than the threshold for lateralizing coherent noise. In this sense, the human binaural system appears to behave like an ideal level meter, insensitive to the wave form and envelope fine structures that determine coherence. The small discrepancy (less than 0.5 dB) can be understood from a standard model of loudness perception—incorporating critical band filtering, half-wave rectification, amplitude compression (0.6 power law), and temporal integration (300 ms). The same model can also account for the small observed difference in lateralization ILD thresholds for coherent noise (N0) and antiphasic noise (Nπ). It can further account for the observed noise bandwidth dependence of lateralization ILD thresholds. [Work supported by the NIDCD.]

Interaural correlation sensitivity.

Sensitivity to differences in interaural correlation was measured for 1.3-ERB-wide bands of noise using a 2IFC task at six frequencies: 250, 500, 750, 1000, 1250, and 1500 Hz. The sensitivity index, d', was measured for discriminations between a number of fixed pairs of correlation values. Cumulative d' functions were derived for each frequency and condition. The d' for discriminating any two values of correlation may be recovered from the cumulative d' function by the difference between cumulative d''s for these values. Two conditions were employed: the noisebands were either presented in isolation (narrow-band condition) or in the context of broad, contiguous flanking bands of correlated noise (fringed condition). The cumulative d' functions showed greater sensitivity to differences in correlation close to 1 than close to 0 at low frequencies, but this difference was less pronounced in the fringed condition. Also, a more linear relationship was observed when cumulative d' was plotted as a function of the equivalent signal-to-noise ratio (SNR) in dB for each correlation value, rather than directly against correlation. The equivalent SNR was the SNR at which the interaural correlation in an NoS(pi) stimulus would equal the interaural correlation of the noise used in the experiment. The maximum cumulative d' declined above 750 Hz. This decline was steeper for the fringed than for the narrow-band condition. For the narrow-band condition, the total cumulative d' was variable across listeners. All cumulative d' functions were closely fitted using a simple two-parameter function. The complete data sets, averaged across listeners, from the fringed and narrow-band conditions were fitted using functions to describe the changes in these parameters over frequency, in order to produce an interpolated family of curves that describe sensitivity at frequencies between those tested. These curves predict the spectra recovered by the binaural system when complex sounds, such as speech, are masked by noise.

Temporal resolution in an interaural temporal difference discrimination task with distracter stimuli

Interaural temporal difference (ITD) thresholds were determined for brief (14-ms) 500-Hz tone bursts in a two-interval forced-choice paradigm in which the interstimulus interval was 36 ms. Preliminary data indicate that the ITD threshold can be affected by the presence of a third (distracter) burst temporally centered between the two primary bursts on each trial: When the distracter has the same ITD as the final burst of the pair, the ITD threshold is unchanged or decreases relative to in the no-distracter condition; when it has the same ITD as the first burst, the threshold increases. The results are consistent with the hypothesis that, when presented with a rapid three-burst triad, the binaural system processes the ITD of the first burst normally (i.e., as if presented in isolation), but confuses or averages the ITDs of the latter two bursts. Follow-up experiments will measure the effects of the discrete nature of the distracter (i.e., by removing the silent gap between the distracter and the burst that shares its ITD), and the frequency of the distracter on the magnitude of its effect. Results will be discussed in terms of binaural temporal resolution of onset versus post-onset information. [Supported by NIDCD.]

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.

Binaural sluggishness in the perception of tone sequences and speech in noise.

The binaural system is well-known for its sluggish response to changes in the interaural parameters to which it is sensitive. Theories of binaural unmasking have suggested that detection of signals in noise is mediated by detection of differences in interaural correlation. If these theories are correct, improvements in the intelligibility of speech in favorable binaural conditions is most likely mediated by spectro-temporal variations in interaural correlation of the stimulus which mirror the spectro-temporal amplitude modulations of the speech. However, binaural sluggishness should limit the temporal resolution of the representation of speech recovered by this means. The present study tested this prediction in two ways. First, listeners' masked discrimination thresholds for ascending vs descending pure-tone arpeggios were measured as a function of rate of frequency change in the NoSo and NoSpi binaural configurations. Three-tone arpeggios were presented repeatedly and continuously for 1.6 s, masked by a 1.6-s burst of noise. In a two-interval task, listeners determined the interval in which the arpeggios were ascending. The results showed a binaural advantage of 12-14 dB for NoSpi at 3.3 arpeggios per s (arp/s), which reduced to 3-5 dB at 10.4 arp/s. This outcome confirmed that the discrimination of spectro-temporal patterns in noise is susceptible to the effects of binaural sluggishness. Second, listeners' masked speech-reception thresholds were measured in speech-shaped noise using speech which was 1, 1.5, and 2 times the original articulation rate. The articulation rate was increased using a phase-vocoder technique which increased all the modulation frequencies in the speech without altering its pitch. Speech-reception thresholds were, on average, 5.2 dB lower for the NoSpi than for the NoSo configuration, at the original articulation rate. This binaural masking release was reduced to 2.8 dB when the articulation rate was doubled, but the most notable effect was a 6-8 dB increase in thresholds with articulation rate for both configurations. These results suggest that higher modulation frequencies in masked signals cannot be temporally resolved by the binaural system, but that the useful modulation frequencies in speech are sufficiently low (<5 Hz) that they are invulnerable to the effects of binaural sluggishness, even at elevated articulation rates.

Mechatronics applied to auditory localisation for telepresence

This paper describes the importance of the sense of hearing for immersive telepresence systems, and a fundamental approach to designing and implementing a binaural sensor system. This approach relies on knowledge of how the human auditory localisation mechanisms work to locate sounds in space, in order that a synthetic system can be derived. The most important of these mechanisms are explained. The Strathclyde auditory system is then described including ideas drawn from the literature to define a binaural approach. This novel system consists of a set of artificial pinnae and microphones mounted on a binocular stereo sensor platform capable of pan, tilt and roll which is slaved to a head tracking system. A set of preliminary experiments is described which test the performance of the system in practice, as a basis for further development.

Localization of sounds from a paired source in the dolphin and fur seal

There is a notion that the dolphin’s spatial hearing operates similarly to that of humans, even though the dolphin’s ability to localize a source in the vertical plane is practically the same as in the horizontal one. Binaural phenomena are used to explain localization in the dolphin; however, there are no interaural differences in the vertical plane. Given the dolphin’s very short echolocation click and high time resolution, one would find it difficult to describe the dolphin’s hearing in terms of binaural phenomenon. The dolphin’s sonar system performs as a monaural rather than a binaural system [G. Zaslavskiy and V. Ryabov, in Marine Mammal Sensory System (1991)]. In fact, very directional sonar click transmission and reception seem to account well for the dolphin’s spatial hearing. In the present study, the way in which dolphins and fur seals respond to a sum of the signals from two sources will be discussed. There are different hearing phenomena in the human associated with the binaural perception of the sound coming from two different directions. Some of these phenomena were found in the fur seal but none of them was present in the dolphin.

Spectral integration in binaural processing

Results are reported from human psychophysical and animal neurobehavioral experiments that examined the ability of the binaural system to integrate information across frequency channels. The psychophysical experiments measured the lateralization of low-frequency (&amp;lt;1200 Hz) multitone complexes as a function of number of components (five or fewer) and frequency spacing between these components (25 to 400 Hz). A pointing task was used to match the perceived lateral position of a broadband noise burst to that of the complex. The complex contained a fixed interaural delay of 1500 μs leading to the left, and the pointer’s perceived lateral position was adjusted by varying its interaural level difference. Parallel neurophysiological experiments were conducted in the optic tectum (superior colliculus) of the barn owl, using narrow-band dichotic noise bursts and multitone stimuli; the localization behavior of the owl in response to these same stimuli was also measured. Results show complex patterns of responses with implications for models of frequency interaction in the binaural system. Linear multiplicative or additive models customarily used in cross-correlation analysis do not explain the entire data set. [Work supported by NIH.]

Directional hearing or binaural phenomenon?

Binaural phenomenon is known to be responsible for sound localization in humans. A similar mechanism is thought to provide the dolphin with the ability to localize targets and sound source. At the same time, the dolphin possesses quite good directional hearing, let alone very directional click transmission, which is absent in humans. The directional hearing in the dolphin, at least in the horizontal plane, appears to be consistent with two ears’ reception. Does it signify that the binaural phenomenon coexists in dolphins with directional hearing? As far as humans are concerned, the spatial hearing does not involve any substantial hearing directionality. The directionality seems to negate the main advantage of the binaural hearing, that is instantaneous determination of a sound source direction, and complicate the use of binaural cues. It was found that the lesser intensity of high-frequency clicks the smaller an angle between their sources should be for the dolphin to hear them simultaneously. The directivity pattern of the dolphin’s auditory system was measured as a dependence of the threshold angle between the sources on the click’s intensity. At high frequencies, the dolphin’s auditory system performed as a directional system rather than an omnidirectional binaural system known for humans.

A series of experiments exploring the spatial separation of multiple talkers with 3-D audio technology

Most auditory displays are monaural and do not take advantage of the capacity of the binaural system for spatial separation of multiple simultaneous talkers. A program of research has been developed at the Air Force Research Laboratory to investigate the enhanced communication effectiveness obtained with 3-D audio displays. Factors that are being manipulated include angular separation, the number of competing messages, and the sex of the target talker. Angular separation has been studied in both the horizontal and median planes, with between two and eight competing messages. The effect of duration of temporal overlap has also been explored. The goal of this research is to discover the maximum number of speech channels that a listener can effectively monitor, and whether this number is augmented by 3-D audio technology. Potential applications of 3-D audio technology for communications separation will be discussed.

Four-dimensional acoustical audio system

A four-dimensional acoustical audio system utilizes both spatial and temporal signal processing to maximize the depth, width, and perceived directionality of the acoustic field with respect to the binaural auditory system. Transducers are critically placed within the confines of a reverberant enclosure, and each transducer is assigned a specific bandwidth of sound either in mono or stereo, depending upon the spatial location of the transducer to take advantage of the binaural auditory system of the intended listener. The system minimizes the number of transducers and electronics required to achieve the effect of a live performance, regardless of the dynamics of the enclosure in which it is placed. The loudspeaker system can be used in small enclosed volumes such as an automobile or extended to volumes the size of a motion picture theater or concert hall and even outdoors.

Inverse filtering of headphones for binaural sound reproduction

Binaural recordings can be made at a blocked ear canal entrance and can be conveniently played back over headphones. The total transmission is then characterized by a transfer function that includes the responses of the headphone and the microphone as well as the transmission between them. The transfer function can be broken up into a minimum phase and an excess phase component, of which the latter can again be broken up into an all-pass and a linear phase component. In order to make the binaural system acoustically transparent, an equalization filter must be designed as the inverse of the total transfer function. The suitability of various filters for inverting each of the transfer function components is evaluated using data from real ear measurements. The robustness of the equalization is discussed with respect to slight changes in headphone and microphone placement. For the investigation 2892 measurements were made on a range of people wearing various types of headphones and wearing different samples of the same type of headphone.

Anticipated effectiveness of active noise control in propeller aircraft interiors as determined by sound quality tests

Active noise control technology offers the potential for weight-efficient aircraft interior noise reduction, particularly for propeller airplanes. However, there is little information on how passengers respond to this type of interior noise treatment. This paper presents results of two experiments which use sound quality engineering practices to determine the subjective effectiveness of hypothetical active noise control systems in a range of propeller airplanes. Binaural recordings were made in five different propeller airplanes and a commercial jet using an acoustic mannequin and a digital recording system. The noise stimuli for the tests were prepared by digitally modifying the propeller tones in recordings of each airplane type to simulate active noise control systems with a range of effectiveness and complexity. The noise stimuli were presented to the test subjects through electrostatic headphones to preserve the realism, spatiality, and directionality provided by the binaural system. The two tests differed by the type of judgments made by the subjects: pair comparisons in the first test and numerical category scaling in the second. Changes in subjective response relative to changes in measured sound characteristics afforded by the hypothetical active noise control schemes are presented and are contrasted between the two test methods.

Binaural sluggishness in the masked discrimination of signals with different patterns of frequency change

The binaural system helps listeners to understand speech at low signal-to-noise ratios, presumably via time-varying binaural cues. However, the binaural system is often described as sluggish, because listeners cannot follow rapidly changing binaural cues. The speech in such experiments may have been articulated slowly enough for the binaural system to temporally resolve critical formant transitions. Alternatively, changing signal spectra may be analyzed by a nonsluggish aspect of the binaural system. In the former case, binaural unmasking of speech will depend upon speech rate. In a preliminary study, listeners’ discrimination of changing signals in noise was measured as a function of the rate of frequency change. Masked discrimination thresholds were taken for NoSo and NoSπ using a 2I-2AFC task. The signals consisted of ascending or descending three-tone glissandos (400, 500, and 625 Hz), presented repeatedly and continuously during a 1.6-s broadband white noise burst. Rate of frequency change was controlled by altering the duration of each individual tone (32 to 100 ms). The difference between NoSo and NoSπ was smaller for rapidly changing signals. Thus the binaural unmasking of speech should be dependent upon speech rate. However, the rate of articulation sufficient to abolish masking release is not known.

Microphone-array hearing aids with binaural output. II. A two-microphone adaptive system

For pt.I see ibid., vol.5, no.6, p.529-42, 1997. This work is aimed at developing a design for the use of a microphone array with binaural hearing aids. The goal of such a hearing aid is to provide both the spatial-filtering benefits of the array and the natural benefits to sound localization ability and speech intelligibility that result from binaural listening. The present study examines a design in which two ear-level omnidirectional microphones constitute the array. The merging of array processing with binaural listening is accomplished by dividing the frequency spectrum, devoting the lowpass part to binaural processing and the highpass part to adaptive array processing. Acoustic and behavioral measurements were made in an anechoic chamber and in a moderately reverberant room to assess the trade-off between sound localization and speech reception as the cutoff frequency was varied. A lowpass/highpass cutoff frequency of 500 Hz provided an improvement of 40 percentage points in sentence intelligibility over unaided listening for normal-hearing listeners, while still allowing adequate localization performance. Comparison of this binaural adaptive system to traditional amplification configurations with normal-hearing listeners showed improvements in speech reception in noise in a mildly reverberant room of approximately 3 dB over simple binaural amplification and 5 dB over monaural amplification.