头相关传输函数幅度谱的听觉空间分辨阈值的分析

提出一种分析头相关传输函数(head-related transfer function,HRTF)幅度谱的听觉空间分辨阈值模型。采用数值计算得到的高空间分辨率HRTF数据,计算了声源空间位置变化引起的HRTF幅度谱的变化,进一步利用Moore响度模型分析双耳响度级差、双耳响度级谱和总响度级等三个听觉感知量的变化。根据现有的3个听觉感知量最小可察觉差异,模型利用双耳响度级差和双耳响度级谱的变化得到的估计结果与心理声学实验一致,因此是一种有效预测听觉空间分辨阈值的方法,可用于为简化虚拟听觉信号处理和数据储存。     

头相关传输函数与虚拟听觉

头相关传输函数(HRTF)是自由场情况下点声源到双耳的声学传输函数,它包含了有关声源的主要空间信息,因而在双耳空间听觉的研究方面有非常重要的意义.作为HRTF的一个重要的应用,虚拟听觉则是近二十年发展起来的新技术,它利用HRTF进行信号处理,模拟出声波从声源到双耳的传输,从而在耳机或扬声器重放中虚拟出相应的空间听觉.虚拟听觉技术在有关听觉的科学实验、通信、多媒体与虚拟现实、家用声重放、室内声学设计等科学研究、工程技术、消费电子领域都有重要的应用价值.近十多年来,国际上有关HRTF和虚拟听觉技术的研究发展很快,已成为声学、信号处理、听觉等研究领域的热门与前沿课题,并已在众多的领域得到应用.     

头相关传输函数空间对称性的分析

提出了分析头相关传输函数空间对称性的方法。利用对KEMAR人工头/躯干系统和真人受试者进行测量得到的头相关传输函数,分析了生理结构对头相关传输函数空间对称性的影响。结果表明,在5 kHz至6 kHz以上的频率,耳廓破坏了头相关传输函数的前后对称性;而受耳道入口位置的影响,真人受试者的头相关传输函数在2.5 kHz以上的频率就开始出现前后不对称。另一方面,在中、低频的情况下,头相关传输函数近似左右对称,然而随着频率的升高,生理外形在细微结构上的左右差异将导致头相关传输函数的左右不对称。这种左右不对称的起始频率和程度存在个体差异。上述分析可以反映出头相关传输函数空间对称性的规律和现有双耳听觉模型的适用范围。     

近场头相关传输函数的测量与分析

设计了测量距离可调整的头相关传输函数的实验测量方法，并采用人工头进行近场头相关传输函数的测量，建立了高空间分辨率的近场头相关传输函数数据库，为进一步开展双耳听觉的研究和虚拟听觉的应用提供了数据基础。根据实验数据，初步分析了远、近场情况下，距离、仰角、方位角等参量对头相关传输函数的影响规律。     

头相关传输函数的低频特性

提出了一种对测量得到头相关传输函数(HRTF)的低频特性进行修正的方法。通过头部的刚球模型对HRTF的低频特性进行分析,证明了无论对远场或近场HRTF,大约在400 Hz以下,其幅度趋于恒定,而相位接近线性。因而只要通过实验测量得到400 Hz以上的HRTF特性,即可采用理论的结果对其进行修正。理论计算和主观实验证明,所提出的方法是切实可行的。     

头相关传输函数空间采样、插值与环绕声重放

从空间方向采样的角度对头相关传输函数(HRTF)空间插值、多通路环绕声重放进行了分析,证明了它们在数学上是完全等价的,不同的HRTF空间插值方法对应于不同的多通路环绕声信号馈给,并给出了多通路环绕声信号馈给以及立体声的正弦定理更严格的数学推导。分析指出企图用相邻线性插值的方法得到侧向的HRTF是错误的,并从保证声像定位的角度,对现有的HRTF相邻线性插值公式进行了修正。分析最后指出,HRTF以及虚拟声的许多分析方法可与多通路环绕声相互借鉴。     

不同测量对头相关传输函数的听觉影响

头相关传输函数(HRTF)是虚拟听觉重放的核心·目前,HRTF的实验室测量缺乏统一的规范。本文研究了不同测量对HRTF的听觉影响。首先提出了扩散场均衡的预处理方法,并对来自5个不同数据库的KEMAR假人的HRTF数据进行了扩散场均衡;然后,采用谱差异评估了不同数据库HRTF测量的频谱差异;最后,采用HRTF合成的虚拟声信号开展了一系列的主观听音实验,包括定位实验和区分实验·结果表明,扩散场均衡是一种有效的HRTF预处理方法,可以减小不同测量对HRTF频谱的影响;不同测量基本上不影响HRTF在12 kHz以下的定位效果,但对音色的影响较大,从而导致听觉上的可区分.     

推介谢菠荪教授的专著《头相关传输函数与虚拟听觉》

非常欣喜地向大家推介谢菠荪教授的专著《头相关传输函数与虚拟听觉》一书(国防工业出版社出版,北京,2008年1月)。这是我国的第一部系统论述"头相关传输函数(HRTF)"与"虚拟听觉(Virtual Audi- tory)"的专著,也是近15年来国际上第一本全面论述这些领域研究和应用的著述,它包含了国际上在这个领域的理论、实验与应用的主要内容。     

人工头近场头相关传输函数及其特性

采用球形正十二面体声源及其空间定位系统,测量并建立了KEMAR人工头的近场头相关传输函数(HRTF)数据库。基于数据库分析了近场HRTF在频域和时域随声源距离变化的规律;讨论了用近场HRTF算得的双耳声级差(TLD)和双耳时间差(ITD)所包含的声源距离定位信息。结果表明,测量系统和所得数据具有较好的重复性和准确性,保留了1 kHz以下的低频定位信息。并且,近场HRTF幅度谱和ILD随声源距离的变化明显;用相关法算得2 kHz以下频段的ITD随声源距离略有变化。本文数据库及其分析结果将为声源距离定位的应用提供基础。     

采用无耳壳头相关传输函数的虚拟声信号处理

本文提出采用无耳壳、封闭耳道的头相关传输函数（HRTF）进行虚拟声信号处理的方法。理论分析表明，在包含功率均衡的扬声器虚拟声信号处理中，采用无耳壳、封闭耳道的头相关传输函数可有效地减少信号处理函数的谷点，从而减少声重发的音色改变。心理声学的实验结果表明，新处理方法的声像定位与传统的带耳壳、耳道的HRTF处理无显著区别，但音色较后者有所改善。因而新的方法更适合实际的虚拟声信号处理应用。     

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.     

Infinite-impulse-response models of the head-related transfer function

Head-related transfer functions (HRTFs) measured from human subjects were approximated using infinite-impulse-response (IIR) filter models. Models were restricted to rational transfer functions (plus simple delays) so that specific models are characterized by the locations of poles and zeros in the complex plane. The all-pole case (with no nontrivial zeros) is treated first using the theory of linear prediction. Then the general pole-zero model is derived using a weighted-least-squares (WLS) formulation of the modified least-squares problem proposed by Kalman (1958). Both estimation algorithms are based on solutions of sets of linear equations and result in efficient computational schemes to find low-order model HRTFs. The validity of each of these two low-order models was assessed in psychophysical experiments. Specifically, a four-interval, two-alternative, forced-choice paradigm was used to test the discriminability of virtual stimuli constructed from empirical and model HRTFs for corresponding locations. For these experiments, the stimuli were 80 ms, noise tokens generated from a wideband noise generator. Results show that sounds synthesized through model HRTFs were indistinguishable from sounds synthesized from original HRTF measurements for the majority of positions tested. The advantages of the techniques described here are the computational efficiencies achieved for low-order IIR models. Properties of the all-pole and pole-zero estimators are discussed in the context of low-order HRTF representations, and implications for basic and applied contexts are considered.     

Spatial symmetry of head-related transfer function

Methods for analyzing the spatial symmetry of head-related transfer function (HRTF) are proposed. The influences of anatomical structures on the symmetry of HRTF are investigated using HRTFs measured on KEMAR mannequin and human subjects. Results show that for KEMAR mannequin, pinnae destroy the front-back symmetry of HRTF above 5 to 6 kHz, while for human subjects the frequency reduces to 2.5 kHz because of the locations of ears. Furthermore, at low and median frequencies, HRTF is approximately left-right symmetrical. While as frequencies increase, the asymmetry caused by the fine anatomical leftright differences appears. The starting frequency and the extent of the left-right asymmetry in HRTF depend on individuals. The analyses demonstrate the spatial symmetrical characteristics of HRTF and the frequency ranges in which the current binaural models are valid.     

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。     

Perceptually based head-related transfer function database optimization

In the context of binaural audio rendering, choosing the best head-related transfer function (HRTF) for an individual from large databases poses several problems. This study proposes a method to reduce the size of a given HRTF database. Participants, 45 in total, were asked to rate the quality of binaural synthesis for 46 HRTFs. The lack of reciprocity in the ratings was noted. Results were used to create a perceptually optimized HRTF subset which satisfied all participants' judgments. The subset was validated using localization tests on a separate group of subjects with results showing reduced errors when subjects were given their best choice, rather than their worst choice HRTF.     

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.     

Fast head-related transfer function measurement via reciprocity

An efficient method for head-related transfer function (HRTF) measurement is presented. By applying the acoustical principle of reciprocity, one can swap the speaker and the microphone positions in the traditional (direct) HRTF measurement setup, that is, insert a microspeaker into the subject's ear and position several microphones around the subject, enabling simultaneous HRTF acquisition at all microphone positions. The setup used for reciprocal HRTF measurement is described, and the obtained HRTFs are compared with the analytical solution for a sound-hard sphere and with KEMAR manikin HRTF obtained by the direct method. The reciprocally measured sphere HRTF agrees well with the analytical solution. The reciprocally measured and the directly measured KEMAR HRTFs are not exactly identical but agree well in spectrum shape and feature positions. To evaluate if the observed differences are significant, an auditory localization model based on work by J. C. Middlebrooks [J. Acoust. Soc. Am. 92, 2607-2624 (1992)] was used to predict where a virtual sound source synthesized with the reciprocally measured HRTF would be localized if the directly measured HRTF were used for the localization. It was found that the predicted localization direction generally lies close to the measurement direction, indicating that the HRTFs obtained via the two methods are in good agreement.