Binaural Direction Research Articles

Early Studies of Binocular and Binaural Directions.

Understanding how the eyes work together to determine the direction of objects provided the impetus for examining integration of signals from the ears to locate sounds. However, the advantages of having two eyes were recorded long before those for two ears were appreciated. In part, this reflects the marked differences in how we can compare perception with one or two organs. It is easier to close one eye and examine monocular vision than to “close” one ear and study monaural hearing. Moreover, we can move our eyes either in the same or in opposite directions, but humans have no equivalent means of moving the ears in unison. Studies of binocular single vision can be traced back over two thousand years and they were implicitly concerned with visual directions from each eye. The location of any point in visual or auditory space can be described by specifying its direction and distance, from the vantage point of an observer. From the late 18th century experiments indicated that binocular direction involved an eye movement component and experimental studies of binaural direction commenced slightly later. However, these early binocular and binaural experiments were not incorporated into theoretical accounts until almost a century later. The early history of research on visual direction with two eyes is contrasted to that on auditory direction with two ears.

An objective metric for describing the basic acoustics of binaural directivity patterns in humans

It is accepted knowledge that having two ears are better than one when trying to listen to a signal of interest in the presence of spatially-separated noise sources (e.g. Blauert, 1997; Bregman, 1994; Zurek, 1993). Models have been proposed purporting of the benefits of the head shadow effect, binaural interactions, and cognitive factors that explain how one can understand sound with linguistic or other contextual meaning better in the presence of spatially-separate noise sources. However, less discussed is the attribute of human listeners having the ability to also hear and identify sound sources, seemingly on demand, that occur around them, a condition made possible by the fact of having two ears. Zurek (1993) proposed and discussed at length on the directivity effects of binaural listening (e.g., Better Ear Strategy). What is proposed in this study is an extension to this model to include the omni-directional directivity effects of binaural listening to describe the listener’s ability to remain connected and aware of the sound landscape that surrounds them. Where the head shadow effect plays a role in improving the signal-to-noise ratio in one of the two ears, this strategy looks at how the two ears, due to their geometric location on the head, allows for the head to be acoustically transparent and keeps the listener connected to their surrounding sound landscape.

Binaural Range Finding from Synthetic Aperture Computation as the Head is Turned

A solution to binaural direction finding described in Tamsett (Robotics 2017, 6(1), 3) is a synthetic aperture computation (SAC) performed as the head is turned while listening to a sound. A far-range approximation in that paper is relaxed in this one and the method extended for SAC as a function of range for estimating range to an acoustic source. An instantaneous angle λ (lambda) between the auditory axis and direction to an acoustic source locates the source on a small circle of colatitude (lambda circle) of a sphere symmetric about the auditory axis. As the head is turned, data over successive instantaneous lambda circles are integrated in a virtual field of audition from which the direction to an acoustic source can be inferred. Multiple sets of lambda circles generated as a function of range yield an optimal range at which the circles intersect to best focus at a point in a virtual three-dimensional field of audition, providing an estimate of range. A proof of concept is demonstrated using simulated experimental data. The method enables a binaural robot to estimate not only direction but also range to an acoustic source from sufficiently accurate measurements of arrival time/level differences at the antennae.

Synthetic Aperture Computation as the Head is Turned in Binaural Direction Finding

Binaural systems measure instantaneous time/level differences between acoustic signals received at the ears to determine angles λ between the auditory axis and directions to acoustic sources. An angle λ locates a source on a small circle of colatitude (a lamda circle) on a sphere symmetric about the auditory axis. As the head is turned while listening to a sound, acoustic energy over successive instantaneous lamda circles is integrated in a virtual/subconscious field of audition. The directions in azimuth and elevation to maxima in integrated acoustic energy, or to points of intersection of lamda circles, are the directions to acoustic sources. This process in a robotic system, or in nature in a neural implementation equivalent to it, delivers its solutions to the aurally informed worldview. The process is analogous to migration applied to seismic profiler data, and to that in synthetic aperture radar/sonar systems. A slanting auditory axis, e.g., possessed by species of owl, leads to the auditory axis sweeping the surface of a cone as the head is turned about a single axis. Thus, the plane in which the auditory axis turns continuously changes, enabling robustly unambiguous directions to acoustic sources to be determined.

Binaural Directivity Patterns for Normal and Aided Human Hearing

The aim of this brief report was to investigate binaural directivity patterns for normal and aided hearing, as opposed to conventional monaural measures. Head-related transfer functions for an artificial head measured at the entrance to the ear canal and above the pinnae and a binaural loudness model for directional sounds were used to estimate binaural directivity patterns for normal human hearing and behind-the-ear (BTE) hearing devices in the horizontal plane. The results show that binaural directivity patterns are smoother than the corresponding monaural patterns, and that there are clear frequency-dependent differences in binaural directivity between the two measurement positions. The data can be used in the signal processing of BTE hearing aid systems to mimic the binaural directivity of normal, unoccluded ears.

A low cost, non-individualized surround sound system based upon head related transfer functions: An ergonomics study and prototype development

This paper reports on the types and magnitudes of localization errors of simulated binaural direction cues generated using non-individualized, head-related transfer functions (HRTFs) with different levels of complexity. Four levels of complexity, as represented by the number of non-zero coefficients of the associated HRTF filters (128, 64, 32, 18 non-zero coefficients), were studied. Experiment 1 collected 1728 data runs that were exhaustive combinations of the four levels of complexity, nine simulated directions of sound (no direction (i.e., diotical-mono), 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° azimuth angles at 0° elevation), two repetitions, and 24 participants). Binaural cues generated from HRTFs of reduced complexity (from 128 to 18 non-zero coefficients) produced significantly higher localization errors for the directions of 45°, 135°, 225°, and 315° azimuth angles ( p < 0.01 ). From the directions of 0°, 90°, and 270° azimuth angles, the cues produced by HRTFs with reduced complexity did not affect the localization error ( p > 0.2 ) . Surprisingly, cues produced by HRTFs of 128 non-zero coefficients did not have the lowest number of errors. From 45°, 135°, 225°, and 315°, the lowest numbers of errors were obtained from cues produced by HRTFs of 64, 32, 32, and 64 non-zero coefficients, respectively. Based on these findings, a prototype virtual headphone-based surround-sound (VHSS) system was developed. A double-blind usability experiment with 32 participants indicated that the prototype VHSS system received significantly better surround-sound ratings than did a Dolby TM stereo system ( p < 0.02 ) . This paper reports results from an original ergonomics study and the application of these results to the design of a consumer product.

Monaural and binaural hearing directivity in the bottlenose dolphin: Evoked-potential study

Hearing thresholds as a function of sound-source azimuth were measured in bottlenose dolphins using an auditory evoked potential (AEP) technique. AEP recording from a region next to the ear allowed recording monaural responses. Thus, a monaural directivity diagram (a threshold-vs-azimuth function) was obtained. For comparison, binaural AEP components were recorded from the vertex to get standard binaural directivity diagrams. Both monaural and binaural diagrams were obtained at frequencies ranging from 8 to 128 kHz in quarter-octave steps. At all frequencies, the monaural diagram demonstrated asymmetry manifesting itself as: (1) lower thresholds at the ipsilateral azimuth as compared to the symmetrical contralateral azimuth and (2) ipsilateral shift of the lowest-threshold point. The directivity index increased with frequency: at the ipsilateral side it rose from 4.7 to 17.8 dB from 11.2 to 128 kHz, and from 10.5 to 15.6 dB at the contralateral side. The lowest-threshold azimuth shifted from 0 degrees at 90-128 kHz to 22.5 degrees at 8-11.2 kHz. The frequency-dependent variation of the lowest-threshold azimuth indicates the presence of two sound-receiving apertures at each head side: a high-frequency aperture with the axis directed frontally, and a low-frequency aperture with the axis directed laterally.

Phase-locked onset detectors for monaural sound grouping and binaural direction finding

Locating sound sources is an important task for animals. IIDs and ITDs are normally used to provide information about the instantaneous direction of sound received at the ears. In a reverberating environment, this may differ from the direction of the sound source. However the IID and ITD always provide information about sound source direction at onset, since onsets always arrive from the shortest, direct path. Binaural recordings were filtered using a gammatone filterbank, converted to a phase-locked spike code, and passed to a leaky integrate-and-fire neuron through a rapidly depressing synapse. This provides a phase-locked onset detector in each bandpassed channel. Nearly coincident onsets from different channels in each ear were grouped. IIDs and ITDs were computed when grouped onsets in both ears occur at almost the same time. ITDs were converted to azimuth geometrically: IIDs were converted using the impulse response at each ear. The results show that even in a reverberating environment, sound direction can be found from a single onset. Wideband and long sounds provide better results. Multiple sound sources can be accommodated. The system exhibits the precedence effect since a second onset (without intermediate offset) will be ignored because the depressing synapses will not have recovered.

Mechanism of Binaural Fusion in the Hearing of Speech

The mechanism of binaural fusion is here discussed as a form of statistical operation based upon the brain's execution of running cross-correlation of the two ear signals. The technique of measurement of the degree of binaural fusion, outlined in our earlier paper, is here extensively applied to sine waves, to chords of sine waves, to intoned vowels and to running speech; such technique results in curves which assess listener's judgments as to whether a source of sound lies in the right or left hemisphere of subjective space. An extended theory is also developed here explaining these judgment curves in detail. Finally, a discussion is made of the importance of having an understanding of the mechanism of binaural fusion, in any consideration of our binaural directivity sense and our speech discrimination sense (the “cocktail, party problem”).

Some Further Experiments upon the Recognition of Speech, with One and with Two Ears

The experiments described here continue a series reported in this Journal earlier. A number of objective tests are described, directed towards the general problem concerning the faculty we possess of perceiving only one acoustic world under normal conditions (in spite of having two ears) and yet, if our ears be stimulated by different signals, artificially, we can attend to one or the other of them. The tests have been made with continuous speech (readings from light fiction) and the results assessed statistically. The first tests aim at measuring the reaction time r required to “switch the attention” from one ear to the other, as assessed by perception of the words of the message. A second set of tests show that we perceive only one speaker, as a “gestalt,” when the ears are stimulated by similar messages but with a delay between them exceeding 20 times that ever experienced in real life by virtue of binaural directivity.