两扬声器虚拟声重放的双耳声压控制与定位性能

在两扬声器虚拟声重放中,通过精确重构双耳声压而产生不同的空间听觉感知。其重放的定位性能应该是由双耳声压控制的代价和稳定性所共同决定的。过去研究主要对双耳声压控制的稳定性进行分析,并以此作为扬声器布置和信号处理的依据。该文研究表明仅对双耳声压的稳定性分析是不足以完全衡量扬声器虚拟声重放的定位性能的。进一步采用虚拟声信号处理滤波器响应平均功率对双耳声压控制的代价进行分析。结果表明,缩窄左右对称扬声器布置的张角或采用非对称扬声器布置会明显增加产生侧向目标虚拟源时的双耳声压控制代价。虚拟源(虚拟声像)定位实验表明,双耳声压控制代价增加会引起虚拟源定位缺陷。实际应用中,为了有效产生侧向虚拟源,应避免采用过窄张角(如立体声偶极)和非对称的扬声器布置。

听觉障碍患者的空间听觉

空间听觉是对声音空间属性或特性的主观感觉,包括对声源的定位、对环境反射声的主观感觉等。复杂声学环境下的语言获取也和空间听觉密切相关。听觉障碍通常会包括空间听觉能力的下降甚至缺失,影响语言的获取能力。人工听觉是治疗听觉障碍的手段,理想情况下应能恢复或改善患者的空间听觉能力。该文综述了听觉障碍患者的空间听觉及其人工恢复方面的研究、进展及存在的问题。

混合阶Ambisonics重放的分析与实验验证

Ambisonics是一系列基于声场的空间谐波分解和各阶近似的空间声系统,其重放声场方向信息的精确度与系统的复杂程度均随阶数的增加而增加.由于听觉系统对水平面空间信息的分辨率较垂直方向高,混合阶Ambisonics采用较高阶的空间谐波重构水平面方向声场,而用较低阶的空间谐波重构垂直方向声场,以实现感知性能与系统复杂性的折中.文中以28+1分层式扬声器布置为例,对混合阶Ambisonics重放的双耳重构声压进行分析,并和传统的高阶Ambisonics重放及目标声源的情况进行比较,同时进行了虚拟源定位实验验证.分析和实验结果表明,对实际中可实现的扬声器数目与布置,与传统的高阶Ambisonics相比,合理设计的混合阶Ambisonics在保持非水平方向目标声源重放性能的同时,可改善水平面目标声源的重放性能.

扬声器数目对Ambisonics重放声压误差的影响

由于对Ambisonics重放的自由场声压误差和双耳声压误差的分析会得到不同的结果,而双耳声压的误差更接近实际的听觉感知.为此,文中通过对双耳声压误差的分析,评估了扬声器数目对三维空间Ambisonics重放误差的影响.结果表明:在Shannon-Nyquist空间采样理论给出的上限频率附近,扬声器数目从低限L^2增加至(L+1)~2或者(L+2)~2,能够有效地减小重构误差;继续增加扬声器数目或者在更高的频率,双耳声压误差随扬声器数目的变化关系将变得复杂;文中结果修正和推广了现有文献的结论.

头部平移对局域Ambisonics声重放的影响

在最近的研究中,笔者等提出了产生前方垂直方向虚拟源的扬声器信号馈给法,称为“局域Ambisonics”信号馈给法,并已证明该方法对理想倾听位置,可以产生期望的垂直定位动态因素,较传统的分立—对信号馈给法有较大的改进。在实际倾听过程中,听者的头部平移会导致双耳声压及相应的定位因素的变化,从而影响合成虚拟源的准确性和稳定性。本文将进一步分析局域Ambisonics信号馈给法中,头部移动对合成定位因素的影响,并用心理声学实验验证该信号馈给法的稳定性。结果表明,对于扬声器布置范围内的目标声源,当听者偏离了理想倾听位置,但在侧向移动不大于0.20 m的合理范围内,重放仍能产生合适的定位因素,并得到平均误差基本少于5°的合理的虚拟源定位效果。因而局域Ambisonics信号馈给法对头部平移具有一定的稳定性。

基于个体张量分解与模态的头相关传输函数重构

远场头相关传输函数(HRTF)随声源方向、频率以及个体变化。完整HRTF的数据量很大,且测量或计算每个人的高方向分辨率HRTF是很困难的。本文提出一种从少量方向的测量或计算重构高方向分辨率HRTF的方法。基于HRTF张量分解,远场HRTF可分解为方向模态、频率模态和少量个体模态的张量组合。通过对已有的基线HRTF数据库进行统计分析,可得到与个体无关的方向模态矩阵和频率模态矩阵。而对于任何新的个体,只要少量方向的测量或计算HRTF即可估计出个体模态的变化,并重构出高方向分辨率的HRTF数据。对两个HRTF数据库的计算表明,采用11个个体模态即可表示超过98%的个体相关的HRTF能量变化,并从大约30个方向的测量或计算HRTF重构出高方向分辨率的HRTF幅度。心理声学实验验证了提出的方法。该方法可用于简化个性化HRTF的测量或计算。

Reconstruction of HRTF from individualized tensor decomposition and modes

Far-field head-related transfer functions(HRTFs)vary with source direction,frequency and individual.Correspondingly,the dimensionality of a full set of HRTFs is huge and measurement or calculation of individualized HRTFs with high directional resolution could be difficult.A method to construct HRTFs with high directional resolution from a small set of directional measurements or calculations is proposed in present paper.Based on tensor decomposition of HRTFs,far-field HRTFs are decomposed into a combination of direction,frequency and a few of individual-related modes.The individual-independent matrices of direction and frequency-related modes are derived by statistical analysis on a known baseline HRTF database.For an arbitrary new individual,the individual mode can be estimated from measured or calculated HRTFs at a small set of directions,then the HRTFs with high directional resolution are reconstructed.Calculation from two base-line HRTF databases indicates that 11 individualrelated modes account for more than 98%individual-related energy variation of HRTFs;and HRTFs with high directional resolution can be reconstructed from about 30 directional measurements or calculation.A psychoacoustic experiment validates the analysis.The present method is applicable to simplify individualized HRTF measurement or calculation.

稳态和动态虚拟听觉重放对近场距离感知的影响

虚拟听觉重放是研究距离感知的一种重要工具,它可分为传统的稳态虚拟听觉重放和动态虚拟听觉重放。稳态虚拟听觉重放经常会产生头中定位而影响距离感知的判断,从而影响最终的实验结果。过去的实验大多采用稳态虚拟听觉重放,因此其结论也存在较大的争议性。该文对比研究稳态和动态的虚拟听觉重放对绝对距离感知效果的影响。实验采用非个性化头相关传输函数进行双耳合成,将分布在水平面0.20∼1.00 m的7个距离、0°∼180°的7个方位角下的信号通过耳机进行虚拟重放。心理声学实验结果表明,在稳态虚拟听觉重放条件下,受试者不同程度地报告了存在头中定位。而动态虚拟听觉重放系统能够利用动态因素,从而产生头外化的听觉感知。统计分析表明动态虚拟听觉重放下的距离感知效果与稳态重放存在明显的差异。由于动态虚拟重放更接近真实声源的感知效果,应选用这种方法作为实验工具。同时,在实际的空间声应用中,也应该采用动态虚拟听觉重放产生不同的距离感知。

前方空间环绕声的四扬声器虚拟重放

考虑头部转动带来的动态因素对听觉垂直定位的贡献,提出了前方空间环绕声的四扬声器虚拟重放方法。4个扬声器分别布置在水平面左前、右前以及高仰角的左前上、右前上方向,并采用听觉传输信号处理的方法将多通路空间环绕声信号转换为4个扬声器的重放信号。以9.1通路空间环绕声虚拟重放为例,采用头相关传输函数对双耳声压及其包含的定位因素进行分析表明,该方法可以产生正确的双耳时间差及其随头部转动的变化,从而产生合适的侧向定位双耳因素和垂直定位的动态因素。而心理声学实验结果表明,该方法可以重放稳定的前方空间的水平和垂直虚拟源。因此,四扬声器布置结合听觉传输处理足以重放前方空间环绕声的垂直定位信息,实现多通路空间环绕声的向下混合与简化。

Platform for dynamic virtual auditory environment real-time rendering system

This paper reports the recent works and progress on a PC and C++ language-based virtual auditory environment(VAE) system platform.By tracing the temporary location and orientation of listener's head and dynamically simulating the acoustic propagation from sound source to two ears,the system is capable of recreating free-field virtual sources at various directions and distances as well as auditory perception in reflective environment via headphone presentation.Schemes for improving VAE performance,including PCA-based(principal components analysis) near-field virtual source synthesis,simulating six degrees of freedom of head movement,are proposed.Especially,the PCA-based scheme greatly reduces the computational cost of multiple virtual sources synthesis.Test demonstrates that the system exhibits improved performances as compared with some existing systems.It is able to simultaneously render up to 280 virtual sources using conventional scheme,and 4500 virtual sources using the PCA-based scheme.A set of psychoacoustic experiments also validate the performance of the system,and at the same time,provide some preliminary results on the research of binaural hearing.The functions of the VAE system is being extended and the system serves as a flexible and powerful platform for future binaural hearing researches and virtual reality applications.

Head-related transfer function database and its analyses

Based on the measurements from 52 Chinese subjects (26 males and 26 females), a high-spatial-resolution head-related transfer function (HRTF) database with corre- sponding anthropometric parameters is established. By using the database, cues relating to sound source localization, including interaural time difference (ITD), interaural level difference (ILD), and spectral features introduced by pinna, are analyzed. Moreover, the statistical relationship between ITD and anthropometric parameters is estimated. It is proved that the mean values of maximum ITD for male and female are significantly different, so are those for Chinese and western sub- jects. The difference in ITD is due to the difference in individual anthropometric parameters. It is further proved that the spectral features introduced by pinna strongly depend on individual; while at high frequencies (f≥ 5.5 kHz), HRTFs are left-right asymmetric. This work is instructive and helpful for the research on bin- aural hearing and applications on virtual auditory in future.

重放自由场虚拟源距离信息的动态双耳Ambisonics方法

双耳重放的目标之一是在耳机重放中产生不同方向和距离的虚拟源感知。本文研究了动态双耳Ambisonics重放自由场虚拟源方向和距离信息的简化信号处理方法。该信号处理方法包括两步:第1步是基于目标声场的球谐函数分解,合成采用扬声器的近场Ambisonics重放中逐级重构目标声场的信号;第2步是采用虚拟扬声器重放的方法,用动态头相关函数滤波处理将Ambisonics的扬声器重放信号转换为双耳重放信号并用耳机重放。进一步研究了动态双耳Ambisonics的阶数对定位效果的影响,为简化信号处理提供依据。对重放产生的双耳声压分析表明,5阶动态双耳Ambisonics重放足以提供听觉方向定位和距离感知的重要信息。同时心理声学的实验结果表明,结合声源距离相关的响度因素,5阶动态双耳Ambisonics重放可产生不同方向和1.0 m以下不同近场距离的自由场虚拟源的听觉感知。本文的方法仅需要固定距离的48个均匀空间方向的远场非个性化HRTF处理,实现了信号处理的简化。

Virtual reproduction of surround sound in frontal space using four loudspeakers

By considering the contribution of dynamic cue to auditory vertical localization,a method for virtual reproduction of surround sound in frontal space using four actual loudspeakers is proposed.The four actual loudspeakers are arranged in the left-front and right-front directions in the horizontal plane,as well as left-front-up,right-front-up directions in a higher elevation plane,respectively.Transaural signal processing is used to convert the multichannel sound signals to the signals for the four actual loudspeakers.Virtual reproduction of 9.1 channel sound is taken as an example.The analysis of binaural pressures and corresponding localization cues using head-related transfer functions indicates that the current method is able to recreate correct interaural time difference and its dynamic variation with head turning,and thus able to create appropriate binaural cue for lateral localization and dynamic cue for vertical localization.Results of psychoacoustic experiment indicate that the current method is able to recreate stable horizontal and vertical virtual source within the frontal-hemispherical directions.Therefore,combined with transaural processing,the four loudspeakers arrangement is enough to reproduce the vertical localization information in frontal space and realize the down-mixing and simplification of multichannel spatial surround sound.

A head-related transfer function model for fast synthesizing multiple virtual sound sources

A head-related transfer function （HRTF） model for fast and real-time synthesizing multiple virtual sound sources is proposed. A head-related impulse response （HRIR, time- domain version of HRTF） is first decomposed by a two-level wavelet packet and then represented by a model composed of subband filters and reconstruction filters. The coefficients of the subband filters are the zero interpolation of the wavelet coefficients of the HRIR. The coefficients of the reconstruction filters can be calculated from the wavelet function. The model is simplified by applying a threshold method to reduce the wavelet coefficients. The calculated results indicate that for a model with 30 wavelet coefficients, the error of reconstructed HRIR is about 1%. And the result of a psychoacoustic test shows that a model with 35 wavelet coefficients is perceptually indistinguishable from the original HRIR. When multiple virtual sound sources are synthesized simultaneously, the computational cost of the proposed model is much less than the traditional HRTF filters.

On the low frequency characteristics of head-related transfer function

A method to correct the measured head-related transfer functions （HRTFs） at low frequency was proposed. By analyzing the HRTFs from the spherical head model at low frequency, it is proved that below the frequency of 400 Hz, magnitude of HRTF is nearly constant and the phase is a linear function of frequency both for the far and near field. Therefore, if the HRTFs above 400 Hz are accurately measured by experiment, it is able to correct the HRTFs at low frequency by the theoretical model. The results of calculation and subjective experiment show that the feasibility of the proposed method.

Analysis of the spatial discrimination threshold of head-related transfer function magnitude

A binaural-loudness-model-based method for evaluating the spatial discrimination threshold of magnitudes of head-related transfer function（HRTF） is proposed.As the input of the binaural loudness model,the HRTF magnitude variations caused by spatial position variations were firstly calculated from a high-resolution HRTF dataset.Then,three perceptualrelevant parameters,namely interaural loudness level difference,binaural loudness level spectra,and total binaural loudness level,were derived from the binaural loudness model.Finally,the spatial discrimination thresholds of HRTF magnitude were evaluated according to just-noticedifference of the above-mentioned perceptual-relevant parameters.A series of psychoacoustic experiments was also conducted to obtain the spatial discrimination threshold of HRTF magnitudes.Results indicate that the threshold derived from the proposed binaural-loudness-modelbased method is consistent with that obtained from the traditional psychoacoustic experiment,validating the effectiveness of the proposed method.

Analysis with binaural auditory model and experiment on the timbre of Ambisonics recording and reproduction

A scheme for analyzing the timbre in spatial sound with binaural auditory model is proposed and the Ambisonics is taken as an example for analysis. Ambisonics is a spatial sound system based on physical sound field reconstruction. The errors and timbre colorations in the final reconstructed sound field depend on the spatial aliasing errors on both the recording and reproducing stages of Ambisonics. The binaural loudness level spectra in Ambisonics recon- struction is calculated by using Moore＇s revised loudness model and then compared with the result of real sound source, so as to evaluate the timbre coloration in Ambisonics quantitatively. The results indicate that, in the case of ideal ＇independent signals, the high-frequency limit and radius of region without perceived timbre coloration increase with the order of Ambisonics. On the other hand, in the case of recording by microphone array, once the high-frequency limit of microphone array exceeds that of sound field reconstruction, array recording influences little on the binaural loudness level spectra and thus timbre in final reconstruction up to the high- frequency limit of reproduction. Based on the binaural auditory model analysis, a scheme for optimizing design of Ambisonics recording and reproduction is also suggested. The subjective experiment yields consistent results with those of binaural model, thus verifies the effectiveness of the model analysis.

Design and validation on a multiple sound source fast-measurement system of near-field head-related transfer functions

Near-field head-related transfer functions （HRTFs） are essential to scientific re- searches of binaural hearing and practical applications of virtual auditory display. High ef- ficiency, accuracy and repeatability are required in a near-field HRTF measurement. Hence, there is no reference which intents on solving the measuring difficulties of near-field HRTF for human subjects. In present work, an efficient near-field HRTF measurement system based on computer control is designed and implemented, and a fast calibration method for the system is proposed to first solve the measurement of near-field HRTF for human subjects. The efficiency of measurement is enhanced by a comprehensive design on the acoustic, electronic and mechanical parts of the system. And the accuracy and repeatability of the measurement are greatly im- proved by carefully calibrating the positions of sound source, subject and binaural microphones. This system is suitable for near-field HRTF measurement at various source distances within 1.0 m, for both human subject and artificial head. The time costs of HRTF measurement at a single sound source distance and full directions has been reduced to less than 20 minutes. The measurement results indicate that the accuracy of the system satisfies the actual requirements. The system is applicable to scientific research and can be used to establish an individualized near-field HRTF database for human subjects.

The cross-correlation of signals and spatial impression in surround sound reproduction

The relationship between the cross-correlation coefficients of feeding signals and auditory spatial impression（ASI） which are created by the left,right,left surround and right surround loudspeakers in 5.1 channel surround sound system is investigated by psychoacoustic experiments.The results show that for reproducing by the front left-right or left-right surround loudspeakers pair,the auditory source width（ASW） can be broadened by controlling the crosscorrelation coefficients of feeding signals to some extent.The quantitative relationships between ASW and the cross-correlation coefficients is frequency dependent.For reproducing by a pair of lateral loudspeakers,however,ASW can not be changed by controlling the cross-correlation coefficients of feeding signals.For reproducing by the front and surround loudspeakers pairs simultaneously and for pink noises and octave noises with central frequencies no more than 1kHz,a strong sense of listener envelopment（LEV） can be obtained by controlling the crosscorrelation coefficients of feeding signals properly.For the octave band noises with central frequencies at 2 kHz and 4 kHz,however,LEV can not be obtained by controlling the crosscorrelation coefficients of feeding signals.Further theoretical calculations and measurements show that there is no unique relationship between the inter-aural cross-correlation（IACC） and the ASW in 5.1 channels surround sound reproduction,which may be due to the algorithms of IACC calculation.Further experimental verifications are needed to investigate the applicability of IACC for evaluating ASI.The present results will be helpful to the actual surround sound programming recording and evaluation.

Spatial interpolation of HRTFs and signal mixing for multichannel surround sound

From the point of spatial sampling, spatial interpolation of HRTFs （head-related transfer functions） and signal mixing for multichannel （surround） sound are analyzed. First, it is proved that they are mathematically equivalent. Different methods for HRTFs interpolation are equivalent to different signal mixing methods for multichannel sound. Then, a stricter derivation for the signal mixing of multichannel sound and the law of sine for stereophonic sound is given. It is pointed out that trying to reconstruct lateral HRTFs by adjacent linear interpolation is wrong. And for accurate sound image localization, the conventional equation of adjacent linear interpolation of HRTFs is revised. At last, it is also pointed out that some methods used in the analysis of HRTFs and multichannel sound can be used for reference mutually.