Localization Of Virtual Sources Research Articles

Analysis of binaural sound pressure control and virtual source characteristics in stereophonic reproduction

In stereophonic reproduction, the manipulation of relative magnitude and phase between two loudspeakers enables to create virtual sources within an azimuth region in frontal–horizontal plane. The law of sine, which is based on evaluating the interaural phase difference and a conventional method for stereophonic analysis at low frequencies, fails to explain some characteristic in stereophonic reproduction, including the influence of span angle of loudspeakers on virtual source localization and the stability of virtual source created by out-of-phase loudspeaker signals. Analogy to the method in active sound control, an alternative method for analyzing the characteristics of virtual source in stereophonic reproduction is presented in this work. On the basis of shadowless head model at low frequency, the power effort of loudspeaker signals for creating virtual source in stereophonic reproduction is evaluated. The results indicate that for frontal virtual source within the span of loudspeakers, the power effort of in-phase loudspeaker signals increases with the increase in the span angle of loudspeakers. For lateral virtual source outside the span of loudspeakers, the power effort of out-of-phase loudspeaker signals increases with the target azimuth of virtual source. Increase in power effort makes the control of binaural pressures difficulty and, thus, virtual source instable.

The localization of virtual source reproduced by the crosstalk cancellation system under different direct-to-reverberation energy ratios conditions

This study explores the localization of virtual sources reproduced by the crosstalk cancellation (CTC) system under reverberation environments with different direct-to-reverberation energy ratios (DRRs). Binaural room impulse responses in rectangle rooms are simulated by high-order image source method. Two independent key variables, the reverberation time (RT) and the distance of the sound source to the listener, are employed to manipulate DRR through simulation. Subsequently, a subjective localization experiment based on the CTC system is virtually conducted using earphone reproduction. The results show that the azimuth localization in horizontal plane is almost impervious by RT but is subjected to the change of the source distance. Specifically, as the distance of sound source increases, the perceived azimuth of lateral virtual sources exhibits a bias towards the front, accompanied by larger variances. The results indicate that reflections with small delays have a more detrimental effect on sound localization compared to the late reverberation with large intensity, which may be associated with the inhibition of late reflections based on the precedence effect. Additionally, the localization data are analyzed and explained using binaural cues, including the interaural cross-correlation coefficient, the interaural time difference (ITD), and interaural level difference (ILD).

Exploring the limits of virtual source localization with amplitude panning on a flat panel with actuator array: Implications for future researcha).

Immersive and spatial sound reproduction has been widely studied using loudspeaker arrays. However, flat-panel loudspeakers that utilize thin flat panels with force actuators are a promising alternative to traditional coaxial loudspeakers for practical applications, with benefits in low-visual profiles and diffuse radiation. Literature has addressed the sound quality and applications of flat-panel loudspeakers in three-dimensional sound reproduction, such as wave field synthesis and sound zones. This paper revisits the spatial sound perception of flat-panel loudspeakers, specifically the localization mismatch between the perceived and desired sound directions when using amplitude panning. Subjective tests in an anechoic chamber with 24 subjects result in the mean azimuth direction mismatch within ±6.0° and the mean elevation mismatch within ±10.0°. The experimental results show that the virtual source created by amplitude panning over a flat-panel loudspeaker still achieves spatial localization accuracy close to that of a real sound source, despite not using complex algorithms or acoustic transfer function information. The findings of this study establish a benchmark for virtual source localization in spatial sound reproduction using flat-panel loudspeakers, which can serve as a starting point for future research and optimization of algorithms.

Influence of first-order lateral reflections on the localization of virtual source reproduced by crosstalk cancellation system

This study elucidates the influence of first-order lateral reflections on the localization of virtual sources reproduced by a crosstalk cancellation system. First, an experimental study was conducted to examine how the reflections with a delay within the scope of the summing localization and the precedence effect influence the localization of virtual sources with different frequency bands and target azimuths. The results show that the localization of virtual sources is influenced due to the reflections with a delay below the low limit of the precedence effect (1 ms universally), and the influence degree is related to the frequency band and target azimuth. When the delay is further increased, the effect of reflected sound is almost negligible. Particularly, for a virtual source in the region outside the loudspeaker on the contralateral side of the reflecting surface (at -60° or -90°), the perceived azimuth under reflections with a small delay was more lateral than that without reflections. Then, simulations were conducted under the same conditions as the experiments. Based on the binaural signals modified by the inhibition function of the precedence effect, binaural cues, including interaural phase delay difference (ITDp) and interaural level difference, were calculated to explain the experimental results. Particularly, the average ITDp over frequency can qualitatively predict the localization results below 1.5 kHz from the subjective experiment in most cases.

An Algorithm for Generating Virtual Sources in Dynamic Virtual Auditory Display Based on Tensor Decomposition of Head-Related Impulse Responses

Dynamic virtual auditory displays (VADs) are increasingly used for generating various auditory objects and scenes in virtual and augmented reality. Dynamic VADs are required to generate virtual sources in various directions and distances by using HRTF- or HRIR-based binaural synthesis. In the present work, an algorithm for improving the efficiency and performance of binaural synthesis in dynamic VAD is proposed. Based on tensor decomposition, a full set of near-field HRIRs is decomposed as a combination of distance-, direction-, and time-related modes. Then, binaural synthesis in VAD can be implemented by a common set of time mode-related convolvers or filters associated with direction- and distance-related weights. Dynamic binaural signals are created by updating the weights rather than updating the HRIR-based convolvers, which enables the independent control of virtual source distance and direction and avoids the audible artifact caused by updating the HRIR-based convolvers. An example of implementation indicates that a set of eight common convolvers or filters for each ear is enough to synthesize the binaural signals with sufficient accuracy. The computational efficiency of simultaneously generating multiple virtual sources is improved when the number of virtual sources is larger than eight. A virtual-source localization experiment validates the algorithm.

Dynamic Binaural Rendering: The Advantage of Virtual Artificial Heads over Conventional Ones for Localization with Speech Signals

As an alternative to conventional artificial heads, a virtual artificial head (VAH), i.e., a microphone array-based filter-and-sum beamformer, can be used to create binaural renderings of spatial sound fields. In contrast to conventional artificial heads, a VAH enables one to individualize the binaural renderings and to incorporate head tracking. This can be achieved by applying complex-valued spectral weights—calculated using individual head related transfer functions (HRTFs) for each listener and for different head orientations—to the microphone signals of the VAH. In this study, these spectral weights were applied to measured room impulse responses in an anechoic room to synthesize individual binaural room impulse responses (BRIRs). In the first part of the paper, the results of localizing virtual sources generated with individually synthesized BRIRs and measured BRIRs using a conventional artificial head, for different head orientations, were assessed in comparison with real sources. Convincing localization performances could be achieved for virtual sources generated with both individually synthesized and measured non-individual BRIRs with respect to azimuth and externalization. In the second part of the paper, the results of localizing virtual sources were compared in two listening tests, with and without head tracking. The positive effect of head tracking on the virtual source localization performance confirmed a major advantage of the VAH over conventional artificial heads.

Efficient algorithm and localization experiment on spherical microphone array recording and binaural rendering

Spherical microphone array recording and binaural rendering (SMABR) is a novel spatial sound technique, which records spatial information of sound field, transfers to dynamic binaural signals, and then renders via headphone. To improve the computational efficiency of SMABR, the present work proposes a PCA-based (principal component analysis) algorithm for dynamic binaural synthesis from the beamforming outputs of spherical microphone array, in which only 33 pairs of sharable filters are required. Incorporated the proposed algorithm, a PC-based SMABR system consisting of a 64-channel spherical microphone array, an electromagnetic head-tracker, and a headphone is established. A virtual source localization experiment is carried out to evaluate the system and algorithm. Results indicate that the system yields reasonable localization performance within a target virtual source region near the horizontal plane. However, larger localization error is also observed for high elevation above 30 degrees or low elevation below -30 degrees. Localization error at high or low elevation is caused by the spatial aliasing error of spherical microphone array at high frequency, which spoils the spectral cue for elevation localization. Therefore, further improvement on spherical microphone array recording is needed. [Supported by the National Natural Science Foundation of China, Grant No.11174087.]

Modeling Localization of Amplitude-Panned Virtual Sources in Sagittal Planes.

Vector-base amplitude panning (VBAP) aims at creating virtual sound sources at arbitrary directions within multichannel sound reproduction systems. However, VBAP does not consistently produce listener-specific monaural spectral cues that are essential for localization of sound sources in sagittal planes, including the front-back and up-down dimensions. In order to better understand the limitations of VBAP, a functional model approximating human processing of spectro-spatial information was applied to assess accuracy in sagittal-plane localization of virtual sources created by means of VBAP. First, we evaluated VBAP applied on two loudspeakers in the median plane, and then we investigated the directional dependence of the localization accuracy in several three-dimensional loudspeaker arrangements designed in layers of constant elevation. The model predicted a strong dependence on listeners' individual head-related transfer functions, on virtual source directions, and on loudspeaker arrangements. In general, the simulations showed a systematic degradation with increasing polar-angle span between neighboring loudspeakers. For the design of VBAP systems, predictions suggest that spans up to 40° polar angle yield a good trade-off between system complexity and localization accuracy. Special attention should be paid to the frontal region where listeners are most sensitive to deviating spectral cues.

Virtual auditory display validation using transaural techniques

Validation of headphone-based virtual auditory display technology is inherently difficult, given the potential for display hardware to interfere with normal sound field listening conditions due to occlusion of the pinna. This difficulty is eliminated if loudspeaker-based transaural techniques are instead used for the virtual auditory display. Transaural techniques also offer the advantage of being able to use the display loudspeakers, or other additional loudspeakers, as individual reference locations in order to validate the display under natural listening conditions in the same sound field. Such validation can also include explicit comparison to visual localization of the sound sources. In this way, the accuracy and precision of virtual source localization can be directly compared to real source localization, and optionally to visual localization. Results of a validation experiment of this type using non-individualized head-related transfer functions are reported and compared to analogous data from a headphone-based virtual auditory display. [Work supported by NEI.]

Typical data and cluster analysis on head-related transfer functions from Chinese subjects

Head-related transfer functions (HRTFs) vary with individuals. To obtain consistent results in binaural analysis, typical or representative HRTFs are needed. Moreover, matched or customized HRTFs are also needed to improve the localization performance in virtual auditory display (VAD). Based on a HRTF baseline database with 52 subjects, the present work derives the typical HRTFs of Chinese subjects and sets of matched HRTFs data for scientific researches and VAD applications. Similarities among the HRTF magnitudes of different subjects are first analyzed. Then the typical HRTFs are obtained by searching for the HRTFs of an individual that, in the sense of average, are most similar with the HRTFs of other individuals in the baseline database. Moreover, individualized cluster analysis is applied to the baseline database and the matched HRTFs are selected from the nearest cluster center. The results indicate that the magnitudes of typical HRTFs highly correlate with most of other 51 subjects in the database, exhibiting an average normalized cross-correlation coefficient of 0.888. In addition, cluster analysis indicates that HRTF magnitudes for most subjects can be classified into seven clusters and represented by the corresponding nearest cluster centers, with average normalized cross-correlation coefficient between the nearest cluster center and other subjects within the cluster exceeding 0.882. By comparing the similarity between the Chinese HRTFs and those of Western subjects, it is also proved that Chinese and Western HRTFs differ significantly on statistics, and therefore separately analyzing the similarity and cluster of Chinese HRTFs is necessary and reasonable. Psychoacoustic experiments validate that, the typical HRTFs yield reasonable virtual source localization performance and for four out of seven subjects, matched HRTFs selected from the nearest cluster center reduce the reversal and in-head-localization errors.

Circle Fitting Using a Virtual Source Localization Algorithm in Wireless Sensor Networks

A novel circle fitting algorithm is proposed in this paper. The key points of this paper are given as follows: (i) it formulates the circle fitting problem into the special source localization one in wireless sensor networks (WSN); (ii) the multidimensional scaling (MDS) analysis is applied to the data points, and thus the propagator-like method is proposed to represent the circle center parameters as the functions of the circle radius; (iii) the virtual source localization model can be rerepresented as special nonlinear equations of a unique variable (the circle radius) rather than the original three ones (the circle center and radius), and thus the classical fixed-point iteration algorithm is applied to determine the radius and the circle center parameters. The effectiveness of the proposed circle fitting approach is demonstrated using the simulation and experimental results.

The relative contribution of dynamic and spectral cues in virtual sound source localization

It is well known that the dynamic cues caused by head turning and pinna-based spectral cue are vital to sound source localization, especially for front-back and vertical localization. A series of localization experiment is carried out via virtual auditory display to explore the relative contribution of dynamic and spectral cues to localization. Virtual sources at different intended spatial locations are recreated using various combinations with noise stimuli at full audio or 4 kHz-lowpass frequency ranges, individualized HRTFs or non-individual HRTFs derived from KEMAR and spherical-head model, and static or dynamic binaural synthesis. Furthermore, statistical analyses are performed on localization performance in terms of the percentage of front-back and up-down confusion as well as the mean angle error in virtual source localization. Primary results indicate that both dynamic and spectral cues contribute to front-back and vertical localization; whereas dynamic cue contributes more to front-back localization, while individualized spectral cue at high frequency above 4 kHz contributes more to vertical localization. Dynamic and high-frequency individualized spectral cues are also helpful to reduce the angle error in localization. [Work supported by the National Natural Science Foundation of China,11174087, 50938003, and State Key Lab of Subtropical Building Science, South China University of Technology.]

Virtual Sound Rendering in a Stereophonic Loudspeaker Setup

This paper presents a mathematical analysis of the effects of interchannel amplitude and time differences in two channel (stereophonic) sound systems. The analysis is developed by computing the acoustic conditions at the listener's ears as a function of the stereophonic signal feature. We also present separate approximations of head-related transfer function (HRTF)-based panning according to predefined frequency bands. We attempt to create non-sophisticated models of the stereophonic listening mechanism with approximations in both the time and frequency domains. The models are based upon psychoacoustical theories that present the frequency-dependent relative importance of acoustical cues. Based on the model, we propose new panning methods that can enhance the localization accuracy of conventional panning methods, such as amplitude panning and HRTF-based panning. The localization performances of the new panning techniques are evaluated and compared by means of auditory model simulations and listening tests. Through simulations and listening test results, it is shown that the proposed panning method makes substantial improvements in the localization of virtual sources.

An innovative professional music recording/monitoring space

A new kind of professional recording/monitoring space has been designed and constructed as part of the Blackbird Studio complex in Nashville, Tennessee. The room will be introduced and the early subjective impressions will be discussed. The objective of this new room was threefold: first, to provide a surround mixing space with much-improved virtual source localization over a broader footprint in the space, second, to provide a highly diffuse space for the recording of live music performances by one or more vocalists and/or musicians, and third, to investigate a largely diffuse monitoring environment as it might benefit the quality and/or efficiency of professional music mixing in different formats. This presentation will discuss some of the results and some of the surprise outcomes, including practitioner-reported improvements in quickly attaining comprehensive, surgical equalization of sources, possibly attributable to an environment with somewhat more equal reverb times across the frequency spectrum.

Localization of virtual sources in multichannel audio reproduction

The localization of virtual sources generated with different two-dimensional (2-D) multichannel reproduction systems has been studied by means of auditory model simulations and listening tests. The reproduction was implemented with typical five- and eight-channel loudspeaker setups. The microphone systems used were first- and second-order Ambisonics as well as a spaced microphone technique. Pair-wise panning was also studied. The results show that the auditory model can be used in the prediction of perceived direction in multichannel sound reproduction near the median plane. Some systematic deviations between the model predictions and the listening test results were found farther from the median plane. The frequency-dependent capability to produce narrow-band virtual sources to targeted directions is reported for the studied systems.

Effectiveness of interaural delays alone as cues during dynamic localization of virtual sources

The contribution of interaural time differences (ITDs) to the localization of virtual sound sources with and without head motion was examined. Listeners estimated the apparent azimuth, elevation, and distance of virtual sources presented over headphones. Stimuli (3-s, white noise) were synthesized from minimum-phase representations of nonindividualized head-related transfer functions (HRTFs); binaural magnitude spectra were derived from the minimum phase estimates and ITDs were represented as a pure delay. During dynamic conditions, listeners were encouraged to move their heads; head position was tracked and stimuli were synthesized in real time using a Convolvotron to simulate a stationary external sound source. Two synthesis conditions were tested: (1) both interaural level differences (ILDs) and ITDs correctly correlated with source location and head motion and (2) ITDs correct, no ILDs (flat magnitude spectrum). Head movements reduced azimuth confusions primarily when interaural cues were correctly correlated, although a smaller effect was also seen for ITDs alone. Externalization was generally poor for ITD-only conditions and was enhanced by head motion only for normal HRTFs. Overall, the data suggest that, while ITDs alone can provide a significant cue for azimuth, the errors most commonly associated with virtual sources are reduced by location-dependent magnitude cues.