Localization of tones in a room by moving listeners.

It is difficult to localize the source of a tone in a room because standing waves lead to complicated interaural differences that become uninterpretable localization cues. This paper tests the conjecture that localization improves if the listener can move to explore the complicated sound field over space and time. Listener head and torso movements were free and uninstructed. Experiments at low and high frequencies with eight human listeners in a relatively dry room indicated some modest improvement when listeners were allowed to move, especially at high frequencies. The experiments sought to understand listener dynamic localization strategies in detail. Head position and orientation were tracked electronically, and ear-canal signals were recorded throughout the 9 s of each moving localization trial. The availability of complete physical information enabled the testing of two model strategies: (1) relative null strategy, using instantaneous zeros of the listener-related source angle; and (2) inferred source strategy, using a continuum of apparent source locations implied by the listener's instantaneous forward direction and listener-related source angle. The predicted sources were given weights determined by the listener motion. Both models were statistically successful in coping with a great variety of listener motions and temporally evolving cues.

Matched transaural synthesis with probe microphones for psychoacoustical experiments.

Transaural synthesis using loudspeaker signals determined through contemporaneous ear canal calibration is proposed as an alternative to headphone presentation for critical psychoacoustical experiments. The proposed technique can afford greater accuracy, improved reproducibility, and continuous signal monitoring. It allows the experimenter to compare listener responses to real and virtual presentations. In this article, the advantages of transaural (three or four loudspeakers) compared to crosstalk cancellation (two loudspeakers) are shown through computer modeling and manikin measurements in a moderately reverberant room. Measurements employ binaurally challenging signals and speech from a distant source. Transaural synthesis is shown to be a better solution to the essential inverse problem resulting in reduced average synthesis amplitudes, fewer large-amplitude outliers, improved amplitude and phase accuracy for real and imagined sources, and improved noise immunity. Immunity to inadvertent listener head rotation depends sensitively on loudspeaker placement and is not an advantage in general. Appendixes review the relevant mathematical foundation and extend it to the relationship between ear canal signals and eardrum signals.

Noise edge pitch and models of pitch perception.

Monaural noise edge pitch (NEP) is evoked by a broadband noise with a sharp falling edge in the power spectrum. The pitch is heard near the spectral edge frequency but shifted slightly into the frequency region of the noise. Thus, the pitch of a lowpass (LP) noise is matched by a pure tone typically 2%-5% below the edge, whereas the pitch of highpass (HP) noise is matched a comparable amount above the edge. Musically trained listeners can recognize musical intervals between NEPs. The pitches can be understood from a temporal pattern-matching model of pitch perception based on the peaks of a simplified autocorrelation function. The pitch shifts arise from limits on the autocorrelation window duration. An alternative place-theory approach explains the pitch shifts as the result of lateral inhibition. Psychophysical experiments using edge frequencies of 100 Hz and below find that LP-noise pitches exist but HP-noise pitches do not. The result is consistent with a temporal analysis in tonotopic regions outside the noise band. LP and HP experiments with high-frequency edges find that pitch tends to disappear as the edge frequency approaches 5000 Hz, as expected from a timing theory, though exceptional listeners can go an octave higher.

On the localization of high-frequency, sinusoidally amplitude-modulated tones in free field.

Previous headphone experiments have shown that listeners can lateralize high-frequency sine-wave amplitude-modulated (SAM) tones based on interaural time differences in the envelope. However, when SAM tones are presented to listeners in free field or in a room, diffraction by the head or reflections from room surfaces alter the modulation percentages and change the shapes of the envelopes, potentially degrading the envelope cue. Amplitude modulation is transformed into mixed modulation. This article presents a mathematical transformation between the six spectral parameters for a modulated tone and six mixed-modulation parameters for each ear. The transformation was used to characterize the stimuli in the ear canals of listeners in free-field localization experiments. The mixed modulation parameters were compared with the perceived changes in localization attributable to the modulation for five different listeners, who benefited from the modulation to different extents. It is concluded that individual differences in the response to added modulation were not systematically related to the physical modulation parameters themselves. Instead, they were likely caused by individual differences in processing of envelope interaural time differences.

Transaural experiments and a revised duplex theory for the localization of low-frequency tones.

The roles of interaural time difference (ITD) and interaural level difference (ILD) were studied in free-field source localization experiments for sine tones of low frequency (250-750 Hz). Experiments combined real-source trials with virtual trials created through transaural synthesis based on real-time ear canal measurements. Experiments showed the following: (1) The naturally occurring ILD is physically large enough to exert an influence on sound localization well below 1000 Hz. (2) An ILD having the same sign as the ITD modestly enhances the perceived azimuth of tones for all values of the ITD, and it eliminates left-right confusions that otherwise occur when the interaural phase difference (IPD) passes 180°. (3) Increasing the ILD to large, implausible values can decrease the perceived laterality while also increasing front-back confusions. (4) Tone localization is more directly related to the ITD than to the IPD. (5) An ILD having a sign opposite to the ITD promotes a slipped-cycle ITD, sometimes with dramatic effects on localization. Because the role of the ITD itself is altered by the ILD, the duplex processing of ITD and ILD reflects more than mere trading; the effect of the ITD can be reversed in sign.

Computing interaural differences through finite element modeling of idealized human heads.

Acoustical interaural differences were computed for a succession of idealized shapes approximating the human head-related anatomy: sphere, ellipsoid, and ellipsoid with neck and torso. Calculations were done as a function of frequency (100-2500 Hz) and for source azimuths from 10 to 90 degrees using finite element models. The computations were compared to free-field measurements made with a manikin. Compared to a spherical head, the ellipsoid produced greater large-scale variation with frequency in both interaural time differences and interaural level differences, resulting in better agreement with the measurements. Adding a torso, represented either as a large plate or as a rectangular box below the neck, further improved the agreement by adding smaller-scale frequency variation. The comparisons permitted conjectures about the relationship between details of interaural differences and gross features of the human anatomy, such as the height of the head, and length of the neck.

Testing, correcting, and extending the Woodworth model for interaural time difference.

The Woodworth model and formula for interaural time difference is frequently used as a standard in physiological and psychoacoustical studies of binaural hearing for humans and other animals. It is a frequency-independent, ray-tracing model of a rigid spherical head that is expected to agree with the high-frequency limit of an exact diffraction model. The predictions by the Woodworth model for antipodal ears and for incident plane waves are here compared with the predictions of the exact model as a function of frequency to quantify the discrepancy when the frequency is not high. In a second calculation, the Woodworth model is extended to arbitrary ear angles, both for plane-wave incidence and for finite point-source distance. The extended Woodworth model leads to different formulas in six different regions defined by ear angle and source distance. It is noted that the characteristic cusp in Woodworth's well-known function comes from ignoring the longer of the two paths around the head in circumstances when the longer path is actually important. This error can be readily corrected.

Interaural time difference thresholds as a function of frequency.

Different models of the binaural system make different predictions for the just-detectable interaural time difference (ITD) for sine tones. To test these models, ITD thresholds were measured for human listeners focusing on high- and low-frequency regions. The measured thresholds exhibited a minimum between 700 and 1,000 Hz. As the frequency increased above 1,000 Hz, thresholds rose faster than exponentially. Although finite thresholds could be measured at 1,400 Hz, experiments did not converge at 1,450 Hz and higher. A centroid computation along the interaural delay axis, within the context of the Jeffress model, can successfully simulate the minimum and the high-frequency dependence. In the limit of medium-low frequencies (f), where f . ITD << 1, mathematical approximations predict low-­ frequency slopes for the centroid model and for a rate-code model. It was found that measured thresholds were approximately inversely proportional to the frequency (slope = ­1) in agreement with a rate-code model. However, the centroid model is capable of a wide range of predictions (slopes from 0 to ­2).

Human interaural time difference thresholds for sine tones: the high-frequency limit.

The smallest detectable interaural time difference (ITD) for sine tones was measured for four human listeners to determine the dependence on tone frequency. At low frequencies, 250-700 Hz, threshold ITDs were approximately inversely proportional to tone frequency. At mid-frequencies, 700-1000 Hz, threshold ITDs were smallest. At high frequencies, above 1000 Hz, thresholds increased faster than exponentially with increasing frequency becoming unmeasurably high just above 1400 Hz. A model for ITD detection began with a biophysically based computational model for a medial superior olive (MSO) neuron that produced robust ITD responses up to 1000 Hz, and demonstrated a dramatic reduction in ITD-dependence from 1000 to 1500 Hz. Rate-ITD functions from the MSO model became inputs to binaural display models-both place based and rate-difference based. A place-based, centroid model with a rigid internal threshold reproduced almost all features of the human data. A signal-detection version of this model reproduced the high-frequency divergence but badly underestimated low-frequency thresholds. A rate-difference model incorporating fast contralateral inhibition reproduced the major features of the human threshold data except for the divergence. A combined, hybrid model could reproduce all the threshold data.

Temporal predictability as a grouping cue in the perception of auditory streams.

This study reports a role of temporal regularity on the perception of auditory streams. Listeners were presented with two-tone sequences in an A-B-A-B rhythm that was either regular or had a controlled amount of temporal jitter added independently to each of the B tones. Subjects were asked to report whether they perceived one or two streams. The percentage of trials in which two streams were reported substantially and significantly increased with increasing amounts of temporal jitter. This suggests that temporal predictability may serve as a binding cue during auditory scene analysis.

Generating partially correlated noise--a comparison of methods.

There are three standard methods for generating two channels of partially correlated noise: the two-generator method, the three-generator method, and the symmetric-generator method. These methods allow an experimenter to specify a target cross correlation between the two channels, but actual generated noises show statistical variability around the target value. Numerical experiments were done to compare the variability for those methods as a function of the number of degrees of freedom. The results of the experiments quantify the stimulus uncertainty in diverse binaural psychoacoustical experiments: incoherence detection, perceived auditory source width, envelopment, noise localization/lateralization, and the masking level difference. The numerical experiments found that when the elemental generators have unequal powers, the different methods all have similar variability. When the powers are constrained to be equal, the symmetric-generator method has much smaller variability than the other two.

Localization of sound in rooms. V. Binaural coherence and human sensitivity to interaural time differences in noise.

Binaural recordings of noise in rooms were used to determine the relationship between binaural coherence and the effectiveness of the interaural time difference (ITD) as a cue for human sound localization. Experiments showed a strong, monotonic relationship between the coherence and a listener's ability to discriminate values of ITD. The relationship was found to be independent of other, widely varying acoustical properties of the rooms. However, the relationship varied dramatically with noise band center frequency. The ability to discriminate small ITD changes was greatest for a mid-frequency band. To achieve sensitivity comparable to mid-band, the binaural coherence had to be much larger at high frequency, where waveform ITD cues are imperceptible, and also at low frequency, where the binaural coherence in a room is necessarily large. Rivalry experiments with opposing interaural level differences (ILDs) found that the trading ratio between ITD and ILD increasingly favored the ILD as coherence decreased, suggesting that the perceptual weight of the ITD is decreased by increased reflections in rooms.

Interaural coherence for noise bands: waveforms and envelopes.

This paper reports the results of experiments performed in an effort to find a formulaic relationship between the interaural waveform coherence of a band of noise gamma(W) and the interaural envelope coherence of the noise band gamma(E). An interdependence described by gamma(E)=pi/4+(1-pi/4)(gamma(W))(2.1) is found. This relationship holds true both in a computer experiment and for binaural measurements made in two rooms using a KEMAR manikin. Room measurements are used to derive a measure of reliability for the formula. Ultimately, a user who knows the waveform coherence can predict the envelope coherence with a small degree of uncertainty.

The acoustical bright spot and mislocalization of tones by human listeners.

Listeners attempted to localize 1500-Hz sine tones presented in free field from a loudspeaker array, spanning azimuths from 0 degrees (straight ahead) to 90 degrees (extreme right). During this task, the tone levels and phases were measured in the listeners' ear canals. Because of the acoustical bright spot, measured interaural level differences (ILD) were non-monotonic functions of azimuth with a maximum near 55 degrees . In a source-identification task, listeners' localization decisions closely tracked the non-monotonic ILD, and thus became inaccurate at large azimuths. When listeners received training and feedback, their accuracy improved only slightly. In an azimuth-discrimination task, listeners decided whether a first sound was to the left or to the right of a second. The discrimination results also reflected the confusion caused by the non-monotonic ILD, and they could be predicted approximately by a listener's identification results. When the sine tones were amplitude modulated or replaced by narrow bands of noise, interaural time difference (ITD) cues greatly reduced the confusion for most listeners, but not for all. Recognizing the important role of the bright spot requires a reevaluation of the transition between the low-frequency region for localization (mainly ITD) and the high-frequency region (mainly ILD).

Phase effects on the perceived elevation of complex tones.

Free-field source localization experiments with 30 source locations, symmetrically distributed in azimuth, elevation, and front-back location, were performed with periodic tones having different phase relationships among their components. Although the amplitude spectra were the same for these different kinds of stimuli, the tones with certain phase relationships were successfully localized while the tones with other phases led to large elevation errors and front-back reversals, normally growing with stimulus level. The results show that it is not enough to have a smooth, broadband, long-term signal spectrum for successful sagittal-plane localization. Instead, temporal factors are important. A model calculation investigates the idea that the tonotopic details that mediate localization need to be simultaneously, or almost simultaneously, accessible in the auditory system in order to achieve normal elevation perception. A qualitative model based on lateral inhibition seems capable in principle of accounting for both the phase effects and level effects.

Matching the waveform and the temporal window in the creation of experimental signals.

When a periodic waveform with a discrete-harmonic spectrum is temporally windowed to make a signal, its spectrum becomes a continuous function of frequency. However, there are discrete-frequency representations for windowed signals such as the Fourier series representation of a periodically extended signal. This article introduces the concept of matching between the temporal window and the periodic waveform. Matching leads to a discrete-frequency representation in which the Fourier transform of the windowed signal preserves the amplitudes and phases of the waveform on the set of original waveform frequencies. Generating signals with matched window and waveform leads to important control of experiments.

Release from speech-on-speech masking in a front-and-back geometry.

Informational masking of a target female talker by female distracters was measured with target and distracters presented from directly in front of the listener as a baseline condition. Next, it was found that if the distracters were also presented from directly in back of the listener, advanced or delayed by a few milliseconds with respect to the distracters in front, release from informational masking occurred. Release from informational masking was found for all delays within the Haas region of +/-50 ms, with peak release of about 3.5 dB. This peak occurred for a delay of +/-2 ms and it was shown to be the result of delay-and-add filtering. Release from energetic masking was also found, but only for delays of +/-0.5 ms or less.

Lateralization of Huggins pitch.

The Huggins pitch is a sensation of pitch generated from a broadband noise having a narrowband boundary region where the interaural phase difference varies rapidly as a function of frequency. Models of binaural hearing predict that the pitch image should be well lateralized. A direct psychophysical experimental method was used to estimate the lateral positions of Huggins pitch images with two different forms of phase boundaries, linear phase and stepped phase. A third experiment measured the lateral positions of sine tones with controlled interaural phase differences. The results showed that the lateralization of Huggins pitch stimuli was similar to that of the corresponding sine tones and that the lateralizations of the two forms of Huggins pitch phase boundaries were even more similar to one another. Both Huggins pitches and sine tones revealed strong laterality compression (exponent approximately 0.5). Ambiguous stimuli, with an interaural phase difference of 180 degrees , were consistently lateralized on one side or the other according to individual asymmetries-an effect called "earedness." An appendix to this article develops a new first-order lateralization model, the salient phase density model, which combines attributes of previous models of dichotic pitch lateralization.

A framework for testing and comparing binaural models.

Auditory research has a rich history of combining experimental evidence with computational simulations of auditory processing in order to deepen our theoretical understanding of how sound is processed in the ears and in the brain. Despite significant progress in the amount of detail and breadth covered by auditory models, for many components of the auditory pathway there are still different model approaches that are often not equivalent but rather in conflict with each other. Similarly, some experimental studies yield conflicting results which has led to controversies. This can be best resolved by a systematic comparison of multiple experimental data sets and model approaches. Binaural processing is a prominent example of how the development of quantitative theories can advance our understanding of the phenomena, but there remain several unresolved questions for which competing model approaches exist. This article discusses a number of current unresolved or disputed issues in binaural modelling, as well as some of the significant challenges in comparing binaural models with each other and with the experimental data. We introduce an auditory model framework, which we believe can become a useful infrastructure for resolving some of the current controversies. It operates models over the same paradigms that are used experimentally. The core of the proposed framework is an interface that connects three components irrespective of their underlying programming language: The experiment software, an auditory pathway model, and task-dependent decision stages called artificial observers that provide the same output format as the test subject.

Interaural fluctuations and the detection of interaural incoherence. III. Narrowband experiments and binaural models.

In the first two articles of this series, reproducible noises with a fixed value of interaural coherence (0.992) were used to study the human ability to detect interaural incoherence. It was found that incoherence detection is strongly correlated with fluctuations in interaural differences, especially for narrow noise bandwidths, but it remained unclear what function of the fluctuations best agrees with detection data. In the present article, ten different binaural models were tested against detection data for 14- and 108-Hz bandwidths. These models included different types of binaural processing: independent-interaural-phase-difference/interaural-level-difference, lateral-position, and short-term cross-correlation. Several preprocessing transformations of the interaural differences were incorporated: compression of binaural cues, temporal averaging, and envelope weighting. For the 14-Hz bandwidth data, the most successful model postulated that incoherence is detected via fluctuations of interaural phase and interaural level processed by independent centers. That model correlated with detectability at r=0.87. That model proved to be more successful than short-term cross-correlation models incorporating standard physiologically-based model features (r=0.78). For the 108-Hz bandwidth data, detection performance varied much less among different waveforms, and the data were less able to distinguish between models.