Individual Head-related Transfer Functions Research Articles

Approximately calculate individual near-field head-related transfer function using an ellipsoidal head and pinnae model

Head-related transfer functions (HRTFs) describe the acoustic transmission from a point source to ears and are individual-dependent. Measurement is an effective way to obtain individual HRTFs, but the workload is very heavy. This is particularly difficult in the near-field HRTFs measurement due to their source distance-dependence within 1.0 m. Numerical calculation is another way to obtain HRTFs. To reduce the calculation load, an ellipsoid head and pinnae (HAP) model is proposed in the present work for approximately calculating the individual near-field HRTFs via the fast multipole boundary method (FMBEM). The dimension of ellipsoid head is defined by head width, head height, head depth, which is selected to approximately fit the individual interaural time difference (ITD) and interaural level difference (ILD) below about 5 kHz. The individual pinna geometry obtained via a 3D laser scanner provides individual HRTF spectral cues above 5 kHz. To validate the proposed method, the HAP of KEMAR and human is constructed, and calculating results are compared with the measurements. Psychoacoustic experiment is also carried out to evaluate the model and results. [Work supported by the National Natural Science Foundation of China for Young Scholars (No 11104082) and the Natural Science Foundation of Guangdong Province.

Recovery of individual head-related transfer functions from a small set of measurements

Head-related transfer functions (HRTFs) vary with individuals, and in practice, measuring HRTFs with high directional resolution for each individual is tiresome. Based on a basis functions representation of HRTFs, the present work proposes a method for recovering individual HRTFs from a small set of measurements. The HRTFs are represented by a combination of a small set of spatial basis functions (SBFs) with frequency- and individual-dependent weights. The SBFs are derived by applying spatial principal component analysis to a baseline HRTF dataset with high directional resolution. The individual weights for any subject outside the dataset are estimated from measurements at a few source directions, and then the HRTFs with high directional resolution are recovered by combining the SBFs and the individual weights. In an illustrative case, the SBFs derived from a baseline dataset that includes 20 subjects are used to recover the HRTF magnitudes for six subjects outside the baseline dataset. Results show that individual HRTF magnitudes can be recovered from measurements at 73 directions with a mean signal-to-distortion ratio of 19 dB. The proposed method is also applicable to recovering head-related impulse responses. The results of psychoacoustic experiments indicate that in most cases the recovered and measured HRTFs are indistinguishable.

Discovering Your Inner Bat: Echo–Acoustic Target Ranging in Humans

Echolocation is typically associated with bats and toothed whales. To date, only few studies have investigated echolocation in humans. Moreover, these experiments were conducted with real objects in real rooms; a configuration in which features of both vocal emissions and perceptual cues are difficult to analyse and control. We investigated human sonar target-ranging in virtual echo-acoustic space, using a short-latency, real-time convolution engine. Subjects produced tongue clicks, which were picked up by a headset microphone, digitally delayed, convolved with individual head-related transfer functions and played back through earphones, thus simulating a reflecting surface at a specific range in front of the subject. In an adaptive 2-AFC paradigm, we measured the perceptual sensitivity to changes of the range for reference ranges of 1.7, 3.4 or 6.8 m. In a follow-up experiment, a second simulated surface at a lateral position and a fixed range was added, expected to act either as an interfering masker or a useful reference. The psychophysical data show that the subjects were well capable to discriminate differences in the range of a frontal reflector. The range-discrimination thresholds were typically below 1 m and, for a reference range of 1.7 m, they were typically below 0.5 m. Performance improved when a second reflector was introduced at a lateral angle of 45°. A detailed analysis of the tongue clicks showed that the subjects typically produced short, broadband palatal clicks with durations between 3 and 15 ms, and sound levels between 60 and 108 dB. Typically, the tongue clicks had relatively high peak frequencies around 6 to 8 kHz. Through the combination of highly controlled psychophysical experiments in virtual space and a detailed analysis of both the subjects' performance and their emitted tongue clicks, the current experiments provide insights into both vocal motor and sensory processes recruited by humans that aim to explore their environment by echolocation.

Localization of virtual sound sources with bilateral hearing aids in realistic acoustical scenes

Sound localization with hearing aids has traditionally been investigated in artificial laboratory settings. These settings are not representative of environments in which hearing aids are used. With individual Head-Related Transfer Functions (HRTFs) and room simulations, realistic environments can be reproduced and the performance of hearing aid algorithms can be evaluated. In this study, four different environments with background noise have been implemented in which listeners had to localize different sound sources. The HRTFs were measured inside the ear canals of the test subjects and by the microphones of Behind-The-Ear (BTEs) hearing aids. In the first experiment the system for virtual acoustics was evaluated by comparing perceptual sound localization results for the four scenes in a real room with a simulated one. In the second experiment, sound localization with three BTE algorithms, an omnidirectional microphone, a monaural cardioid-shaped beamformer and a monaural noise canceler, was examined. The results showed that the system for generating virtual environments is a reliable tool to evaluate sound localization with hearing aids. With BTE hearing aids localization performance decreased and the number of front-back confusions was at chance level. The beamformer, due to its directivity characteristics, allowed the listener to resolve the front-back ambiguity.

Estimating head-related transfer functions of human subjects from pressure–velocity measurements

Direct measurements of individual head-related transfer functions (HRTFs) with a probe microphone at the eardrum are unpleasant, risky, and unreliable and therefore have not been widely used. Instead, the HRTFs are commonly measured from the blocked ear canal entrance, which excludes the effects of the individual ear canals and eardrums. This paper presents a method that allows obtaining individually correct magnitude frequency responses of HRTFs at the eardrum from pressure-velocity (PU) measurements at the ear canal entrance with a miniature PU sensor. The HRTFs of 25 test subjects with nine directions of sound incidence were estimated using real anechoic measurements and an energy-based estimation method. To validate the approach, measurements were also conducted with probe microphones near the eardrums as well as at blocked ear canal entrances. Comparisons between the different methods show that the method presented is a valid and reliable technique for obtaining magnitude frequency responses of HRTFs. The HRTF filters designed using the PU measurements are also shown to yield more correct frequency responses at the eardrum than the filters designed using measurements from the blocked ear canal entrance.

Similarity and cluster analysis on magnitudes of individual head-related transfer functions

Although head-related transfer functions (HRTFs) vary with individuals, non-individualized HRTFs are typically employed in virtual auditory display (VAD) due to the difficulty in measuring individual HRTFs. Similarity among the HRTFs of different individuals allows for customizing a matched set of HRTFs for a user from an existing database and thereby improving performance of VADs. The first step of HRTF customization is evaluation of the similarities among individual HRTFs. In the present work, based on a HRTF database of 52 Chinese subjects, the similarities are evaluated by calculating the directional-mean of normalized cross-correlation coefficient (MCC) of HRTF magnitudes between each pair of subjects and then applying cluster analysis to the resultant MCC. The results indicate that MCC depends on subject pair with values ranging from 0.934 to 0.562 (mean 0.842) and from 0.948 to 0.635 (mean 0.863) for the left and right ear, respectively. HRTF magnitudes for most subjects can be classified into six to eight clusters and represented by the corresponding cluster centers. Some singleton clusters are also observed on a few subjects, which reflect the diversity in HRTFs and should be careful in practice. Psychoacoustic experiment also validates above analyses.

차세대 디지털 TV 방송을 위한 오디오 규격 비교 분석 및 제언

3D 디지털 시험 방송 개시와 함께, 다가오는 UHDTV 시대에 대비 한 차세대 디지털 방송 방식에 관한 연구가 활발히 진행되고 있다. 본 논문에서는 현행 사용 중이거나 연구되고 있는 주요 서라운드 오디오 규격을 비교 분석하고 차세대 디지털 방송용 오디오 규격을 제시한다. 현재 주목받고 있는 손실 및 비손실 압축 방식을 채용한 디지털 서라운드 오디오 규격인 Dolby True HD와 DTS HD MA (Master Audio) 규격과 함께 일본 NHK 연구소가 제안한 UHDTV용 22.2 채널 서라운드 규격에 대해 비교 검토한다. 이를 기초로 하여 우리나라의 주택 사정을 감안한 3D 서라운드 7.1 손실 압축 오디오 규격과 하이파이 오디오와의 호환성을 중시하는 2.0, 4.0 비손실 압축규격을 차세대 디지털 방송용 규격으로 제시한다. 이와 함께 개인별 HRTF (Head Related Transfer Function) 생성을 통하여 홀로그래픽 사운드에 근접하는 3차원 입체 음장 제공을 해줄 수 있는 바이노럴 (binaural) 헤드폰용 2 채널 오디오 데이터를 부음성 규격으로 별도로 전송 방안도 함께 제시한다. 각 전송 규격 별 소요 비트 레이트 율도 함께 산출하여 제시하였다. With commencing trial 3D digital broadcasting, the studies on next generation digital broadcasting technology for coming UHDTV era is being actively progressing. In this paper, I propose surround audio formats for next-generation digital TV broadcasting, along with comparative study of major surround audio formats in use or under development. I did comparative study on current major competing surround formats such as Dolby True HD and DTS HD MA, along with NHK proposed 22.2 channel surround format for UHDTV system. Upon this comparative study and our housing situation consideration, I propose lossy compression 3D surround 7.1 channel surround format along with loosless 2.0 and 4.0 hi-fi format as next generation digital TV broadcasting standard. In lieu with this, I also propose transmitting binaural 2 channel audio data as sub-audio. It will give holographic sound experience when properly processed with individual HRTF (Head Related Transfer Function) with headphone. The table for data rate of each proposed audio format is also presented.

Efficient Individualization Method of HRTFs Using Critical-band Based Spectral Cue Control

Recently, 3-D audio technologies are commonly implemented through headphones. A major problem of the headphone-based 3-D audio is in-the-head localization, which occurs due to the inaccurate Head-Related Transfer Function (HRTF). Since the individual measurements of HRTFs are impractical, there have been several researches for HRTF customization. In this paper, an efficient method of customizing HRTFs for the sound externalization is proposed. Firstly, it is determined which part will be customized in HRTF through psychoacoustical experiments. Then, the method controlling spectral notches and envelopes to provide individual localization cues are described. Since the proposed method is based on a critical-band rate, the structure is much simpler than that of previous studies, but still effective. The performance was evaluated through a series of subjective tests, and the results confirmed that the customized HRTF using proposed method could replace the measured individual HRTF successfully.

Toward orthogonal non-individualised head-related transfer functions for forward and backward directional sound: cluster analysis and an experimental study

Individualised head-related transfer functions (HRTFs) have been shown to accurately simulate forward and backward directional sounds. This study explores directional simulation for non-individualised HRTFs by determining orthogonal HRTFs for listeners to choose between. Using spectral features previously shown to aid forward–backward differentiation, 196 non-individualised HRTFs were clustered into six orthogonal groups and the centre HRTF of each group was selected as representative. An experiment with 15 listeners was conducted to evaluate the benefits of choosing between six centre-front and six centre-back directional sounds rather than the single front/back sounds produced by MIT-KEMAR HRTFs. Sound localisation error was significantly reduced by 22% and 65% of listeners reduced their front–back confusion rates. The significant reduction was maintained when the number of HRTFs was reduced from six to five. This represents a preliminary success in bridging the gap between individual and non-individual HRTFs for applications such as spatial surround sound systems. Statement of Relevance:Due to different pinna shapes, directional sound stimuli generated by non-individualised HRTFs suffer from serious front–back confusion. The reported work demonstrates a way to reduce front–back confusion for centre-back sounds generated from non-individualised HRTFs.

Effects of individualized headphone equalization on front/back discrimination of virtual sound sources.

Individualized head related transfer functions (HRTFs) were used to process brief noise bursts for a two-interval forced choice front/back shift between virtual sound source locations presented via two models of headphones, the frequency responses of which could be made nearly flat for each of 21 listeners using their own individualized headphone equalization filters. In order to remove tone coloration differences between virtual sources processed using individualized HRTFs measured in front or in back of each listener, the spectral centroid of the “back” source was adjusted to more nearly match that of the source processed using the “front” HRTF. This manipulation resulted in chance levels of discrimination performance for 12 out of 21 listeners. For the remaining nine listeners showing good discrimination, the virtual sources presented using individualized headphone equalization supported significantly better front/back discrimination rates than did virtual sources presented without correction to headphone responses.

Intelligibility comparison of Japanese speech with competing noise spatialized in real and virtual acoustic environments

We investigated the effect of a competing noise source on the intelligibility of target speech in various acoustic environments. To spatialize these sources in virtual acoustic space, we used head-related transfer functions (HRTFs) measured using the KEMAR dummy head, as well as HRTFs measured individually for each subject. We also compared the intelligibility when these sound sources were generated from loudspeakers located at the target and competing source positions in real acoustic space. The speech intelligibility of the target speech was evaluated using the Japanese diagnostic rhyme test (DRT). The target speech was placed directly in front of the listener, and a single competing source was placed on the same horizontal plane at various azimuths and distances. Individual HRTFs showed slightly better intelligibility than those with KEMAR HRTFs, but the difference was small in most cases. Intelligibility in real acoustic space still outperformed both types of HRTFs, especially when the competing sound source was closer to the listener than the target speech and the competing source was located at 180°. However, this difference was small in most of the other competing source locations tested.

Errors in the measurements of individual head-related transfer function.

Head-related transfer function (HRTF) describes the transmission process from a point sound source to ears in free field and thus includes the effect of sound diffraction imposed by human anatomical structures. Because each person owns a unique configuration of anatomical structures, HRTF is individual-dependent. Individual HRTF plays an important role in guaranteeing authenticity in virtual auditory display. Measurement is a common way to obtain individual HRTF. In practice, measurement error is unavoidable. Moreover, measurement error and individual characteristic are often blended together, especially at high frequencies where both the measurement error and the individual characteristic are obvious. The object of this work is to evaluate the errors in the measurements of individual HRTF, and extract individual characteristic of HRTF from measured results. HRTFs from six human subjects and KEMAR were measured at 24 spatial positions with six repetitions. Significance of the individual characteristic was examined using analysis of variance. In addition, psychoacoustic experiments were conducted to investigate the audibility of the measurement error and individual characteristic, respectively. Results show that the individual characteristic of HRTF dominates the measured results. [Work supported by Nature Science Fund of Guangdong province (07300617).]

Spatial principal components analysis on head-related transfer functions and individualized customization from fewer measurements.

Head-related transfer function (HRTF) is an individual-dependent and continuous function of sound source position and frequency. Measurement is a common way to obtain individual HRTFs. It is difficult or even impractical to measure HRTFs with high directional resolution for individual, however. To solve the problem, spatial principal components analysis (SPCA) of HRTFs and resulting customization of individual HRTF from fewer measurements were proposed in present paper. In SPCA, HRTF is decomposed as a weighted combination of common spatial basis functions (CSBFs) which are only direction-dependent, while the weights are frequency- and individual-independent. The CSBFs were derived from the measured HRTFs of 20 subjects with high directional resolution. Based on the calculated CSBFs, for any new subject, the weight coefficients can be estimated from fewer measured HRTFs data, and subsequently the individual HRTFs with high directional resolution can be customized. SPCA were applied to HRTF magnitudes and head-related impulse responses, respectively, and the individual data were customized for six subjects. Psychoacoustic experiments were also conducted to evaluate the customization performance. Theoretical and experimental results validate the performance of proposed method. [Work supported by of National Nature Science Fund of China Grant No. 10774049.]

State-space models of head-related transfer functions for virtual auditory scene synthesis

This study investigates the use of reduced-order state-space models of collections of head-related transfer functions (HRTFs). Recent head-phone applications have motivated interest in binaural displays that can render multiple simultaneous virtual sound sources, acoustic reflections, and source and listener motion. In the present study, a multi-direction framework is considered that can render such phenomena by filtering source signals with a collection of HRTFs rather than individual HRTFs. The collection of HRTFs is implemented in the state-space, and approximation techniques are applied to construct low-order approximants that are indiscriminable from full-order HRTFs. Two experiments are described in which five observers are asked to discriminate between state-space and full-order renderings. Depending on the stimulus conditions and discrimination task, order thresholds of 7<or=N<or=24 are sufficient for an array of 50 HRTFs surrounding the listener. For comparison, a reference system of truncated minimum-phase impulse responses is also considered.

Real‐virtual equivalent auditory localization with head motion.

Several researchers have shown that individualized head related transfer functions (HRTFs) can be used to produce virtual sounds that are equivalent in terms of localization to free field sounds. Thus far, however, these results have only been shown in studies that have required listeners to keep their heads stationary during the playback of the virtual sounds. In this study, we investigated the performance limits of a virtual auditory display system that incorporated individualized HRTFs but allowed free head motion during the localization process. One key aspect of the system is a high‐speed HRTF measurement process that allowed a full set of individualized HRTFs to be measured in less than 4 min. This made it possible to make an HRTF recording and complete a localization task using the resulting HRTFs within the same 30‐min experimental session. The results show that equivalent free‐field and virtual localization performance was achieved when the virtual sounds were generated in the same session using specially‐designed open‐ear headphones that did not need to be removed during the headphone equalization process. This indicates that equivalent real‐virtual closed‐loop localization is possible even with the truncated, interpolated, minimum‐phase HRTF filters that are required for practical, real‐world virtual audio systems.

Identification of Anthropometric Measurements for Individualization of Head-Related Transfer Functions

As one of the core technologies to generate a three-dimensional (3D) virtual sound, Head-Related Transfer Function (HRTF) can be appropriately estimated using a listeneraŕs physical features. In this paper, a weighted correlation method (WCM) was proposed to identify key anthropometric measurements (KAMs) for HRTF individualization. Then, local KAMs (dependent on positions) and global KAMs (independent of positions) were identified by this method. An objective evaluation tool, spectral distortion (SD), was used to evaluate the HRTF estimation errors. No significant differences of SDs when using local and global KAMs were found. They could satisfy rough sound localization at almost all sampled positions. The global KAMs cover measurements of pinna, head, and torso. Compared with using average HRTFs, the SDs when using local and global KAMs could be averagely reduced by 9.4% and 9.7% respectively. The reduction of SD scores is significant at most of the positions of interest (p < 0.05).

Individualized head-related transfer functions based on population grouping

A method is proposed to divide a population into different groups for partial individualization of head-related transfer functions (HRTFs). Borrowing the basic idea in sizing system design, factor analysis is used to identify the most representative measurements which are then in a case study used to group the population. The comparison between the group mean HRTFs and the population mean HRTFs shows that the group mean HRTFs could greatly reduce spectral distortion at most sampled positions.

Reconstructing head models from photographs for individualized 3D‐audio processing

AbstractVisual fidelity and interactivity are the main goals in Computer Graphics research, but recently also audio is assuming an important role. Binaural rendering can provide extremely pleasing and realistic three‐dimensional sound, but to achieve best results it's necessary either to measure or to estimate individual Head Related Transfer Function (HRTF). This function is strictly related to the peculiar features of ears and face of the listener. Recent sound scattering simulation techniques can calculate HRTF starting from an accurate 3D model of a human head. Hence, the use of binaural rendering on large scale (i.e. video games, entertainment) could depend on the possibility to produce a sufficiently accurate 3D model of a human head, starting from the smallest possible input. In this paper we present a completely automatic system, which produces a 3D model of a head starting from simple input data (five photos and some key‐points indicated by user). The geometry is generated by extracting information from images and accordingly deforming a 3D dummy to reproduce user head features. The system proves to be fast, automatic, robust and reliable: geometric validation and preliminary assessments show that it can be accurate enough for HRTF calculation.

Circumaural transducer arrays for binaural synthesis

Binaural cues such as the interaural time and level differences are the primary cues for estimation of the lateral position of a sound source, but are not sufficient to determine elevation or the exact position on the "cone of confusion". Spectral content of the head-related transfer function (HRTF) provides cues that permit this discrimination, notably high frequency peaks and notches created by diffraction effects within the pinnae. For high quality binaural synthesis, HRTFs need to be individualized, matching the morphology of the listener. Typical means for this are to measure or calculate the HRTF of the listener, but these lengthy and costly methods are not feasible for general public applications. This paper presents a novel approach for HRTF individualization, separating the head and torso effect from that of the pinnae. The head/torso component is numerically modelled while the pinnae component is created using a multiple transducer array placed around each pinna. The philosophy of this method consists in trying to excite the correct localization cues provided by the diffraction of the reconstructed wave front on the listener's own pinnae. Simulations of HRTF reconstruction with various array sizes and preliminary auditory localization tests are presented.

Processing of sound location in human cortex

This functional magnetic resonance imaging study was focused on the neural substrates underlying human auditory space perception. In order to present natural-like sound locations to the subjects, acoustic stimuli convolved with individual head-related transfer functions were used. Activation foci, as revealed by analyses of contrasts and interactions between sound locations, formed a complex network, including anterior and posterior regions of temporal lobe, posterior parietal cortex, dorsolateral prefrontal cortex and inferior frontal cortex. The distinct topography of this network was the result of different patterns of activation and deactivation, depending on sound location, in the respective voxels. These patterns suggested different levels of complexity in processing of auditory spatial information, starting with simple left/right discrimination in the regions surrounding the primary auditory cortex, while the integration of information on hemispace and eccentricity of sound may take place at later stages. Activations were identified as being located in regions assigned to both the dorsal and ventral auditory cortical streams, that are assumed to be preferably concerned with analysis of spatial and non-spatial sound features, respectively. The finding of activations also in the ventral stream could, on the one hand, reflect the well-known functional duality of auditory spectral analysis, that is, the concurrent extraction of information based on location (due to the spectrotemporal distortions caused by head and pinnae) and spectral characteristics of a sound source. On the other hand, this result may suggest the existence of shared neural networks, performing analyses of auditory 'higher-order' cues for both localization and identification of sound sources.