Binaural Experiments Research Articles

Towards a simplified and generalized monaural and binaural auditory model for psychoacoustics and speech intelligibility

Auditory perception involves cues in the monaural auditory pathways, as well as binaural cues based on interaural differences. So far, auditory models have often focused on either monaural or binaural experiments in isolation. Although binaural models typically build upon stages of (existing) monaural models, only a few attempts have been made to extend a monaural model by a binaural stage using a unified decision stage for monaural and binaural cues. A typical prototype of binaural processing has been the classical equalization-cancelation mechanism, which either involves signal-adaptive delays and provides a single channel output, or can be implemented with tapped delays providing a high-dimensional multichannel output. This contribution extends the (monaural) generalized envelope power spectrum model by a non-adaptive binaural stage with only a few, fixed output channels. The binaural stage resembles features of physiologically motivated hemispheric binaural processing, as simplified signal-processing stages, yielding a 5-channel monaural and binaural matrix feature “decoder” (BMFD). The back end of the existing monaural model is applied to the BMFD output and calculates short-time envelope power and power features. The resulting model accounts for several published psychoacoustic and speech-intelligibility experiments and achieves a prediction performance comparable to existing state-of-the-art models with more complex binaural processing.

Advancement of Individualized Head-Related Transfer Functions (HRTFs) in Perceiving the Spatialization Cues: Case Study for an Integrated HRTF Individualization Method

Head-related transfer function (HRTF), which varies across individuals at the same direction, has grabbed widespread attention in the field of acoustics and been used in many scenarios. In order to in-depth investigate the performance of individualized HRTFs on perceiving the spatialization cues, this study presents an integrated algorithm to obtain individualized HRTFs, and explores the advancement of such individualized HRTFs in perceiving the spatialization cues through two different binaural experiments. An integrated method for HRTF individualization on the use of Principle Component Analysis (PCA), Multiple Linear Regression (MLR) and Partial Least Square Regression (PLSR) was presented first. The objective evaluation was then made to verify the algorithmic effectiveness of that method. Next, two subjective experiments were conducted to explore the advancement of individualized HRTFs in perceiving the spatialization cues. One was auditory directional discrimination degree based on semantic differential method, in which the azimuth information of sound sources was told to the listeners before listening. The other was auditory localization, in which the azimuth information was not told to the listeners before listening. The corresponding statistical analyses for the subjective experimental results were made. All the experimental results support that individualized HRTFs obtained from the presented method achieve a preferable performance in perceiving the spatialization cues.

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.

Binaural release from masking with single- and multi-electrode stimulation in children with cochlear implants.

Cochlear implants (CIs) provide children with access to speech information from a young age. Despite bilateral cochlear implantation becoming common, use of spatial cues in free field is smaller than in normal-hearing children. Clinically fit CIs are not synchronized across the ears; thus binaural experiments must utilize research processors that can control binaural cues with precision. Research to date has used single pairs of electrodes, which is insufficient for representing speech. Little is known about how children with bilateral CIs process binaural information with multi-electrode stimulation. Toward the goal of improving binaural unmasking of speech, this study evaluated binaural unmasking with multi- and single-electrode stimulation. Results showed that performance with multi-electrode stimulation was similar to the best performance with single-electrode stimulation. This was similar to the pattern of performance shown by normal-hearing adults when presented an acoustic CI simulation. Diotic and dichotic signal detection thresholds of the children with CIs were similar to those of normal-hearing children listening to a CI simulation. The magnitude of binaural unmasking was not related to whether the children with CIs had good interaural time difference sensitivity. Results support the potential for benefits from binaural hearing and speech unmasking in children with bilateral CIs.

Squelch of room effects in everyday conversation

When a conversation is recorded and then played back, listeners are aware of effects of the room that are not perceived in face-to-face conversation. The room effects, which are physically present, are said to be “squelched” under face-to-face conditions. Room effects include reverberation and coloration caused by reflections. The squelch effect has been attributed to the binaural nature of natural listening, based on binaural experiments with headphones [W. Koenig, J. Acoust. Soc. Am., {22}, 61–62 (1950)]. We report experiments on binaural and diotic listening to recordings of speech made at conversational distances in a room with normal frequency dependence of reverberation and a direct to reverberant power ratio between 1/2 and 1/3 at speech fundamental frequencies. Recordings were made with and without a head between the microphones. Listeners ranked the recordings in order of increasing perceived room effects. The data revealed a strong effect of distance, a weak effect of head diffraction, and an advantage for binaural listening somewhat smaller than the advantage of a factor of 2 in direct power. For a few listeners, binaural listening enhanced the room effects. The latter listeners apparently found that binaural differences produced especially prominent spatial effects. [Work supported by the AFOSR.]

Fine‐structure processing, frequency selectivity and speech perception in hearing‐impaired listeners

Hearing‐impaired people often experience great difficulty with speech communication when background noise is present, even if reduced audibility has been compensated for. Other impairment factors must be involved. In order to minimize confounding effects, the subjects participating in this study consisted of groups with homogeneous, symmetric audiograms. The perceptual listening experiments assessed the intelligibility of full‐spectrum as well as low‐pass filtered speech in the presence of stationary and fluctuating interferers, the individual's frequency selectivity and the integrity of temporal fine‐structure processing. The latter was addressed in a binaural and a monaural experiment. In the binaural experiment, the lateralization threshold was measured for low‐frequency tones with ongoing interaural phase delays. In the monaural experiment, detection thresholds for low‐rate frequency modulation were obtained. In addition, these binaural and monaural thresholds were measured in a stationary background noise in order to assess the persistence of the fine‐structure processing to interfering noise. Apart from elevated speech reception thresholds, the hearing impaired listeners showed poorer performance than the normally hearing in terms of frequency selectivity and fine‐structure processing, despite normal audiometric thresholds at the test frequencies. However, the binaural fine‐structure processing was not found to be particularly vulnerable to interfering noise in these listeners.

The perceptual organization of complex sounds by birds

Birds have proven to be ideal models for the perceptual organization of complex sounds because, like humans, they produce, learn, and use complex acoustic signals for communication. Although conducted in laboratory settings, measures of auditory abilities in birds are usually designed to parallel the acoustic problems faced in their natural habitats, including the location of conspecifics, discrimination among potential mates, prey localization, predator avoidance, and territorial defense. As a result, there is probably more known about hearing in birds under both natural and laboratory conditions than in any other nonhuman organism. Behavioral and/or physiological experiments on complex sound perception in birds have revealed that they exhibit serial pattern perception, can discriminate frequency changes in tones embedded within tonal patterns regardless of stimulus uncertainty conditions, segregate signals into auditory streams, and exhibit comodulation masking release. In addition, binaural experiments have revealed that birds exhibit both the cocktail party effect and the precedence effect. Taken together, these results suggest that, like humans, auditory scene analysis plays a general role in auditory perception in birds and probably other animals that must parse the world into auditory objects. [Work supported by NIH DC006124.]

Modeling spectral integration in binaural signal detection

Estimates of critical bandwidths in binaural experiments vary considerably across experimental conditions. For masking experiments using signals of varying bandwidths (i.e., spectral integration), and experiments with a frequency-dependent interaural masker correlation, bandwidth estimates agree with those from monaural experiments. However, if auditory bandwidths are estimated from binaural band-widening masking experiments, the effective bandwidth is usually a factor of 2 to 3 times larger than monaural estimates. One often ignored detail is that this difference between monaural and binaural estimates decreases with decreasing masker level [Hall et al., J. Acoust. Soc. Am. 73, 894–898 (1983)]. Using the binaural model described by Breebaart et al., we have simulated the binaural experiments described above. It can be shown that, without any parameter change in the model, basically all relevant conditions can be modeled accurately, including the band-widening and spectral integration experiments. In agreement with the scheme discussed by Kohlrausch et al., detection in binaural narrow-band-noise conditions is improved by analyzing the internal representation in several adjacent filters. It is concluded that the absence of this detection advantage in monaural random-noise conditions is the primary cause for the so-called ‘‘wider binaural critical band.’’

The influence of masker variability on estimates of monaural and binaural critical bandwidths

If integration bandwidths are derived from band-widening masking experiments, binaural estimates are usually a factor 2 to 3 larger than monaural estimates, at least at high levels. It is proposed that this difference does not reflect ‘‘different critical bandwidths,’’ but that it is a consequence of detection statistics. In monaural experiments, the variability in the masker energy limits detectability of the signal. Therefore with increasing subcritical bandwidths, the S/N ratio at threshold decreases. In binaural NoSπ experiments, the S/N ratio at subcritical bandwidths remains constant because the masker correlation is always 1 without uncertainty. Due to this absence of external variability in the masker correlation, detection must be limited by internal noise. Binaural detection in narrow-band conditions can gain from the spread of excitation across several critical bands, assuming that the internal noises are uncorrelated. The wider binaural critical band is then caused by masking the off-frequency spread through masker components outside the central critical band (for binaural model simulations, see Breebaart et al.). In this study it is shown that this scheme also applies to monaural frozen-noise maskers, which do not have external variability. In agreement with results from binaural experiments, the integration bandwidths increase considerably with masker level.

A Psychoacoustic and Electrophysiological Study

von Wedel H, von Wedel U—Ch, Streppel M. Monaural and binaural time resolution ability in the aged; A psychoacoustic and electrophysiological study. Acta Otolaryngol (Stockh) 1991; Suppl. 476: 161—166.Time resolution ability is an important factor of auditory processing, especially in binaural hearing. In psychoacoustical and electrophysiological studies the time resolution of brief modulations of signal amplitude and frequency in monaural hearing, including gap detection ability, was analysed. To reveal the time resolution ability of the binaural hearing system, brief interaural modulations of signal amplitude, short changes of interaural time delay or noise coherence in broadband noise bursts were generated. The sensation levels of these short—lasting changes in the noise structure and the cortical evoked potentials were investigated in young normal hearing subjects and in the elderly. Thus age—dependent changes in monaural and binaural time resolution ability could be detected. For the monaural experiments, lengthened time constants of between 2—20 ms, depending on the degree of intensity or frequency modulation could be derived for older subjects. The binaural experiments were characterized by prolongation between 4—26 ms for short interaural signal delay changes. For both groups the time constants derived from the electrophysiological investigations were more prolonged, which might be explained by masking effects or habituation. Further electrophysiological investigations will be required to differentiate between time resolution processes of brief signal changes within the peripheral and central parts of the auditory system.

On the use of adaptive procedures in binaural experiments

Adaptive psychophysical procedures have been routinely used in monaural experiments for many years, but only sparsely used in binaural experiments. In this letter, (1) the increasing use of adaptive procedures in binaural experiments is documented; (2) factors that determine their appropriateness are discussed; and (3) data that attest to their usefulness are presented.

Binaural hearing with a bilateral, high‐frequency loss

Results from four binaural experiments are presented for subjects with bilateral, high‐frequency hearing losses. The experiments are interaural time discrimination, interaural amplitude discrimination, interaural correlation discrimination, and binaural detection of tones masked by noise. The stimuli are third‐octave noises and tones at six different center frequencies. Preliminary data with noise at a center frequency in the normal range (500 Hz) and at a center frequency in the loss range (3 kHz) of the audiogram show abnormal performance for all tests, at both frequencies. Specifically, reduced MLDs (∼6 dB), increased interaural correlation jnds (∼0.25 from a reference correlation of 1 and ∼0.7 from a reference correlation of 0), increased interaural amplitude jnds (∼3 dB), and increased interaural time jnds (⩾60 μs). [Work supported by NIH.]

Observations with a method of acoustic stimulus monitoring

The purpose of this study was to evaluate the utility of an electret microphone mounted on an carphone muff [R. H. Domnitz, “Headphone monitoring system for binaural experiments below 1 kHz,” J. Acoust. Soc. Am. 58, 510–511 (1975)1 to monitor stimulus waveform in auditory evoked potential studies. Click stimuli were used to provide a broadband stimulus, and intrasubject and intersubject variability in the stimulus waveform was examined. Stimulus waveforms obtained in a given subject are repeatable from session to session, although some care must be taken in placement of the earphone on the ear. Amplitude measurements are not as repeatable. Significant intersubject variability in the recorded stimulus waveform is observed, and it is concluded that this variability is due to differences in transducer output on different ears rather than to differences related to the monitoring technique. The monitoring microphone can provide a reasonably reliable record of the acoustic waveform produced by the transducer, but amplitude measurements are not reliable. It is also noted that the microphone gives the stimulus waveform in the outer ear, not at the eardrum. [Work supported in part by a grant from NSF.]

Adaptation effects in binaural experiments

Experiments were conducted to examine some of the effects of imbalanced interaural parameters on binaural lateralization. We examined adaptation both to stimuli with interaural amplitude differences and to stimuli with interaural time differences. To measure the adaptation over time, 30‐trial runs were made with each run lasting about 15 min. On each trial, the subject was required to adjust the interaural time delay from an initial random value to a value that resulted in a centered image. For a low‐pass filtered pulse train with an interaural amplitude difference of 10 dB, the amount of interaural time delay needed to center the image changed from 250 μs at the outset to 100 μsec at the end of the run. These results are consistent with earlier results using a paradigm nearly identical to ours [M. P. Bertrand, Proc. R. Soc. Med. 65, 3–4 (Sept. 1972)]. For the same low‐pass filtered pulse train with no interaural amplitude differences, but with the initial random delay restricted to one ear, the amount of time delay needed to center the image changed by as much as 50 μs during the course of a run, i.e., at the end of a run the subjects no longer heard a diotic presentation (zero delay) in the center of their heads. Two possible mechanisms of adaptation will be discussed. [Work supported by NIH.]

Effects of lateralization on selective and divided attention

In most two-channel selective attention studies, each earphone has been assumed to represent a separate perceptual input channel. However, at least for the detection and recognition of relatively simple acoustic signals, separate earphones do not appear to represent separate perceptual input channels. In an effort to clarify the concept of “channel,” we ran a set of monaural and binaural experiments to investigate the effects of differences in earphone, frequency, and perceived lateralized location on the adequate definition of input channel. Observers monitored increments in intensity of two simultaneously presented frequencies (2000 and 4200 Hz). In the first of two binaural conditions, the two signals were equivalent in loudness and thus, theoretically, both occupied central locations. The second of the binaural conditions spatially separated the lateralized locus of the frequencies by reducing the intensity of each in one (but not the same) earphone so that the 2000- and 4200-Hz tones appeared to be lateralized to the left and right, respectively. Analogous monaural conditions were run. Observers ran monaural and binaural conditions involving signal stimuli and two stimulus presentations with selective and divided attention tasks. Preliminary results show equivalent performance across the two binaural conditions, with the binaural conditions tending to be poorer than the monaural conditions. This suggests that channel cannot be defined in terms of earphone or location of perceived image. Frequency appears to provide a more adequate definition for channel. [Research supported in part be a grant from NINCDS.]

Some properties of narrow-band high-frequency waveforms

Since waveforms whose spectra are limited to frequencies above 2000 Hz have been used extensively in recent binaural experiments, we decided to generate waveforms of this type, to filter them through critical-band-like filters, and to study the properties of the resulting waveforms with some simple visual displays. For binaural processing, interaural comparisons between envelopes and carriers of two monaural waveforms are especially relevant as are the effects of the interaural relations of component waveforms (as in a binarual detection stimulus, for example) on the interaural differences in the total waveforms. We believe that people who use narrowband waveforms in experimental or theoretical studies should understand them on both a mathematical and an intuitive level. Our displays should be helpful in both these areas. [Work supported by NIH.]

Letter: Headphone monitoring system for binaural experiments below 1 kHz.

Binaural experiments are often confounded by variability in the acoustic coupling of the headphones. A simple and convenient system is described for detecting and electrically compensating acoustic imbalances while the headphones are worn by the listener. Application of the system in some binaural experiments is discussed. Subject Classification: 65.80; 65.62; 85.76.

Lateralization of Combination Tones

Experiments were performed to compare lateralization of combination tones with that of pure tones under comparable masking conditions. Two primary tones f1 and f2 at 70 dB SPL were employed. The frequency of f1 was 620 Hz, and f2 ranged from 1.1 f1 to 1.5 f1. Phase and amplitude of the cubic difference tone, 2 f1 − f2, were first determined monaurally using an objective forced-choice procedure. These monaural measurements were then used to specify the phase and amplitude of the lateralization tone in the binaural experiment. The two primaries were introduced to one ear and the externally produced lateralization tone of frequency 2 f1 − f2 to the other. Subjects were required to discriminate a change in phase of the lateralization tone in an oddity procedure. In the control condition, in order to simulate the combination tone, an externally generated tone of frequency 2 f1 − f2 was administered in the presence of a 70-dB masking tone of frequency f1. The amplitude of the simulated combination tone was estimated from the initial monaural measurements. Discrimination of changes in phase of the lateralization tone was again observed. Results for both combination and external tone lateralization were very similar. The jnd's for interaural phase ranged from 2° to 20°. [This research was supported by an NIH grant.]

Crosscorrelation Model for Binaural Unmasking

A model for predicting binaural release from masking is described. The model utilizes peripheral filtering followed by nonlinear distortion and short-time crosscorrelation of left and right signals. A comparison is made of the predictions of the model with measurements from a number of binaural experiments.

New Interpretation of the Stereophonic Effect in Lauridsen's and Similar Binaural Experiments

In 1954, H. Lauridsen reported an experiment which gave a remarkable stereophonic effect using a single input signal. The signal was fed to both ears once directly in phase and a second time (delayed by 50–150 milli-seconds) in antiphase. In a recent paper [Akust. Beib. 6, 482–488 (1956)], G. R. Schodder gave an explanation of the resulting stereophonic effect which is based on the analogy between Lauridsen's (and similar) experiments and hearing conditions in rooms. In this paper a different interpretation will be offered. These experiments are recognized as resulting in two different intensity-versus-frequency responses in the paths to the two earn (two interlaced comb-filters in Lauridsen's case). The presence of such different (but indiscriminable by the ear) intensity-versus-frequency responses is postulated as the essential prerequisite that stereophonic effects be obtained from a single input signal. This theory accounts for most of the observed phenomena which could not or not easily be explained by Schodder's interpretation such as his own observation of a strong effect at very high frequencies. To further support this view a number of differential filtering experiments have been conducted. A magnetic tape recording demonstrating some of the results will be played. An earlier view held by this author, namely, that all that is needed to produce a stereophonic effect are two physically different wave forms which would sound alike individually, could not be sustained. Several experiments with pairs of all-pass filters having different phase-versus-frequency characteristics did not give the strong stereophonic illusion observed with differential intensity filtering.