Near-field Head-related Transfer Functions Research Articles

Reconstruction of individualized near-field head-related transfer functions from a small set of far-field data based on tensor decomposition

Head-related transfer functions (HRTFs) are essential for virtual auditory display. Generally, near-field HRTFs vary with source direction, distance, frequency, and individual. The huge dimensionality of data makes the measurement or calculation of individualized near-field HRTFs difficult. In the present work, a method to estimate individualized near-field HRTFs with high directional and distance resolution from a small set of directional measured or calculated data at a far-field distance is proposed. Based on tensor decomposition, the near-field HRTFs are decomposed into a weighted combination of direction-, distance-, frequency-, and a few of individual-related modes. The universal direction-, distance-, and frequency-modes as well as the weights are evaluated from a baseline near-field HRTFs dataset. For an arbitrary new individual outside the baseline dataset, the individual modes are estimated from a small set of directional measured or calculated of far-field HRTFs, and then the individualized near-field HRTFs with high directional resolution are estimated. An example of analysis indicates that 15 individual modes account for more than 98 % individual-related energy variation of HRTFs; and near-field HRTFs with high directional resolution can be estimated from far-field data of 30 directional measurements or calculations. Two psychoacoustic experiments validate the proposed method.

The contributions of level and near-field head-related transfer functions cues on auditory distance perception in a free field

In a free field, distance-dependent level (or loudness) and near-field HRTF (head-related transfer functions) are considered to be two primary cues for auditory distance perception. However, some recent experiments using virtual auditory display (VAD) have exhibited inconsistent or even controversial results, especially regarding with the contribution of near-field HRTF cues to distance perception. Possible reasons for the inconsistency may be that static VAD with coarse approximation of near-field HRTFs were used in these experiments, which cause poor perception of externalization and error in HRTF-related cues. To address this problem, a set of psychoacoustic experiment using dynamic VAD with accurate near-field HRTFs is conducted in present work. The perceived distances of virtual source at target distance less than 1.0 m and with or without level normalization are evaluated. The results indicate that HRTF cue alone enable distance perception in the lateral region. In contrast, HRTF cue alone is insufficient for distance perception in the front region. A collaboration of HRTF and level cue enhances distance perception in all directions. Therefore, the present experiment yields results similar to previous experiment using real source and validate the contribution of level and near-field HRTF cues to distance perception.

Evaluation of photogrammetry-based near-field head related transfer function estimation for immersive sound reproduction

Binaural reproduction algorithms are commonly applied in virtual and augmented reality applications to create immersive audio environments. Nonetheless, flexible near-field binaural rendering remains challenging. In this work, we explore the acoustic accuracy of photogrammetry-based, near-field head-related transfer function estimation. Photogrammetry measurements of the Head and Torso Simulator (HATS) are taken using a photogrammetry rig consisting of 44 synchronized Canon 1300D DSLR cameras mounted on ten aluminium poles arranged in a circle of 0.8m radius. An Apple iPhone 14 Pro mobile phone is used to acquire the ear data. Commercial photogrammetry software is then used to create a 3D mesh from the still images. Finally, numerical acoustic simulations using the fast multipole boundary element method (FM-BEM) technique are applied to the mesh to generate the near-field HRTFs. We present comparisons of acoustically-measured and photogrammetry-based near-field HRTFs.

A database of near-field head-related transfer functions based on measurements with a laser spark source

This paper presents a database of near-field head-related transfer functions (HRTFs) of an artificial head, measured at four distances (0.2, 0.3, 0.4 and 0.5 m), with 49 positions recorded at each distance, for a total of 196 measurement points. The HRTFs were recorded using an acoustic pulse created by a laser-induced breakdown of air (LIB), which realizes a close to ideal, massless, monopole sound source. While the LIB produces a high amplitude pressure pulse, the amplitude decays toward low frequencies, which introduced a low frequency limit of about 200Hz for this particular setup. Thus, a spherical head model based on the analytical expression for scattering by a rigid sphere was fitted to the measured data, and used to extend the low frequency range of the measurements. A brief evaluation of the processed dataset was undertaken, considering interaural time and level differences. The measured and processed database, as well as the low frequency extension procedure are made publicly available to support future research into nearby sound localization, and virtual/augmented reality applications.

The combined effect of source directivity and binaural listening on near-field binaural speech transmission index evaluation

The binaural speech transmission index (BSTI) is mainly affected by the effect of binaural listening, background noise, and acoustic conditions in a given space, as well as source characteristics such as directivity. However, little attention has been paid to the combined effect of source directivity and binaural listening on the BSTI in near-field regions. This study thus investigates this issue using BSTI measurements in an anechoic environment to exclude the influence of acoustic conditions. Three directional loudspeakers are adopted as the sources, and the impulse responses under different radiation directions in the horizontal plane are measured in an anechoic chamber. These impulse responses are then convolved with the near-field head-related transfer functions of a KEMAR manikin to obtain the binaural impulse responses, which are further used to calculate the BSTIs. The results show that the BSTI decreases considerably with an increasing deviation of the axial direction of the source from the listener (i.e., an increasing radiation angle) due to the directivity pattern of the source, which is almost independent of the azimuth and the distance between the source and listener. Finally, the BSTI correction functions of the source radiation angles are established with third-order polynomial regression for the three sources and can be integrated into the azimuth-dependent BSTI model to yield a comprehensive model that considers the source radiation angle as well as the azimuth and distance between the source and listener.

Ear Centering for Accurate Synthesis of Near-Field Head-Related Transfer Functions

The head-related transfer function (HRTF) is a major tool in spatial sound technology. The HRTF for a point source is defined as the ratio between the sound pressure at the ear position and the free-field sound pressure at a reference position. The reference is typically placed at the center of the listener’s head. When using the spherical Fourier transform (SFT) and distance-varying filters (DVF) to synthesize HRTFs for point sources very close to the head, the spherical symmetry of the model around the head center does not allow for distinguishing between the ear position and the head center. Ear centering is a technique that overcomes this source of inaccuracy by translating the reference position. Hitherto, plane-wave (PW) translation operators have yield effective ear centering when synthesizing far-field HRTFs. We propose spherical-wave (SW) translation operators for ear centering required in the accurate synthesis of near-field HRTFs. We contrasted the performance of PW and SW ear centering. The synthesis errors decreased consistently when applying SW ear centering and the enhancement was observed up to the maximum frequency determined by the spherical grid.

Azimuth-dependent model of binaural speech transmission index based on near-field head-related transfer functions

As well as the background noise and acoustic conditions of a given enclosed space, the binaural effect has a significant influence on the binaural speech transmission index (BSTI), especially for nearby sources. This effect can be quantitatively described by head-related transfer functions (HRTFs). As HRTFs are highly individual, the BSTIs from one person’s HRTF should differ from those of other people. However, this issue has rarely been studied. In the current work, we used the near-field HRTFs of 56 people to obtain the corresponding BSTIs, and analyzed the individual differences among them. The results show that the average standard deviation among the 56 individual BSTIs is almost within the just-noticeable-difference level of 0.03, and decreases with increasing sound source distance. We then acquired BSTIs from the KEMAR manikin and found that there were no significant differences with the median BSTIs across the 56 subjects. Thus, using the BSTIs of the KEMAR manikin, we developed regression BSTI models as a function of the azimuth based on seventh-order polynomial regression at different source distances.

Do near-field cues enhance the plausibility of non-individual binaural rendering in a dynamic multimodal virtual acoustic scene?

It is commonly believed that near-field head-related transfer functions (HRTFs) provide perceptual benefits over far-field HRTFs that enhance the plausibility of binaural rendering of nearby sound sources. However, to the best of our knowledge, no study has systematically investigated whether using near-field HRTFs actually provides a perceptually more plausible virtual acoustic environment. To assess this question, we conducted two experiments in a six-degrees-of-freedom multimodal augmented reality experience where participants had to compare non-individual anechoic binaural renderings based on either synthesized near-field HRTFs or intensity-scaled far-field HRTFs and judge which of the two rendering methods led to a more plausible representation. Participants controlled the virtual sound source position by moving a small handheld loudspeaker along a prescribed trajectory laterally and frontally near the head, which provided visual and proprioceptive cues in addition to the auditory cues. The results of both experiments show no evidence that near-field cues enhance the plausibility of non-individual binaural rendering of nearby anechoic sound sources in a dynamic multimodal virtual acoustic scene as examined in this study. These findings suggest that, at least in terms of plausibility, the additional effort of including near-field cues in binaural rendering may not always be worthwhile for virtual or augmented reality applications.

Measurement of Head-Related Transfer Functions: A Review

A head-related transfer function (HRTF) describes an acoustic transfer function between a point sound source in the free-field and a defined position in the listener’s ear canal, and plays an essential role in creating immersive virtual acoustic environments (VAEs) reproduced over headphones or loudspeakers. HRTFs are highly individual, and depend on directions and distances (near-field HRTFs). However, the measurement of high-density HRTF datasets is usually time-consuming, especially for human subjects. Over the years, various novel measurement setups and methods have been proposed for the fast acquisition of individual HRTFs while maintaining high measurement accuracy. This review paper provides an overview of various HRTF measurement systems and some insights into trends in individual HRTF measurements.

Numerical simulations of near-field head-related transfer functions: Magnitude verification and validation with laser spark sources

Despite possessing an increased perceptual significance, near-field head-related transfer functions (nf-HRTFs) are more difficult to acquire compared to far-field head-related transfer functions. If properly validated, numerical simulations could be employed to estimate nf-HRTFs: the present study aims to validate the usage of wave-based simulations in the near-field. A thorough validation study is designed where various sources of error are investigated and controlled. The present work proposes the usage of a highly-omnidirectional laser-induced breakdown (LIB) of air as an acoustic point source in nf-HRTF measurements. Despite observed departures from the linear regime of the LIB pressure pulse, the validation results show that asymptotically-estimated solutions to a lossless model (wave-equation and rigid boundaries) agree in magnitude with the LIB-measured nf-HRTF of a rigid head replica approximately within 1-2 dB up to about 17 kHz. Except a decreased reliability in notch estimation, no significant shortcoming of the continuous model is found relative to the measurements below 17 kHz. The study also shows the difficulty in obtaining accurate surface boundary impedance values for accurate validation studies.

Near-field head-related transfer-function measurement and database of human subjects.

Near-field head-related transfer functions (HRTFs) of human subjects are essential to those researching spatial hearing. By using a carefully designed measurement system, near-field HRTFs of human subjects were measured and a database was constructed. The database includes 56 Chinese human subjects, seven source distances from 0.2 to 1.0 m, and 685 directions at each distance for each subject. In the present work, the technique of near-field HRTF measurement is outlined, the performance of the measurement system is assessed and validated, and the resultant database is reported. The database can provide fundamental data for future research.

Perceptual evaluation on the influence of individualized near-field head-related transfer functions on auditory distance localization

It is desired that spatial audio for virtual reality is able to reproduce various auditory localization information so as to recreate virtual source at different directions and distances. Distance-dependent binaural cue (binaural level difference (ILD), loudness, as well as environmental reflections are considered as auditory distance localization cues. In the case of free and near-field within about 1.0 m, the binaural cue which is encoded in near-field head-related transfer function (HRTFs) is an absolute and dominant distance localization cue [D. S. Brungart, JASA, 1999]. However, HRTFs depending on individualized and non-individualized HRTFs are usually used in spatial audio synthesis. In the present work, the perceptual influence of individualized HRTFs distance localization is evaluated by a psychoacoustic experiment. The binaural signals with various bandwidths are synthesized by filtering the input stimuli with individualized and non-individualized near-field HRTFs and then reproduced by headphone. Preliminary results of virtual source localization experiment indicate that individualized HRTFs influences little on distance localization at low frequencies but have some influence at mid and high frequency. [This work was supported by the Natural Science Foundation of China, Grant No. 11574090.]

Modification of the head-related transfer functions for auditory proximity in rooms

The human hearing uses two signals entering the ears to interpret surrounding spaces and sound sources, and certain acoustic impressions are often regarded favorably by the listeners. At times, music consumers may consider auditory proximity and intimacy as a desired sensation. In some scenarios, the acoustic reflections, often arriving from lateral angles, succeed in enhancing the perceived proximity, even though all acoustic events occur at far distances. In contrast, natural sounds emanating from a very close proximity are instinctively perceived as intimate. The perception of spatial sound is based on the binaural cues, which can be modeled with head-related transfer functions (HRTF). Earlier research has demonstrated that the interaural level difference for lateral incidence is the foremost difference between near and far-distance HRTFs. In the context of room acoustics, only lateral reflections arrive from angles which at high frequencies create traces of interaural level difference otherwise characteristic to near-field HRTF. This paper explores altering the auditory proximity of auralizations by introducing near-field-type interaural level differences to widely available far-field HRTFs. Analysis of the application of these modified HRTF to reflected sound provides more insight into the sources of auditory proximity.

FINITE ELEMENT DETERMINATION OF THE HEAD-RELATED TRANSFER FUNCTION

The auditory peripheral system, which takes care of collection and enlargement in hearing processing, is an important part of the overall auditory system. This system is the key of sound source identification and location, and an important basis for a variety of audio equipment design and sound quality evaluation. In order to better recognize the acoustic characteristics of the auditory peripheral system, and effectively guide the audio equipment design as well as the sound quality evaluation, this study presents a numerical model based on the finite element/infinite element method (FEM/IFEM) to compute the near-field head-related transfer function (HRTF) on a reversed head model. A spherical sound source is positioned at different locations in the horizontal, median and lateral planes. The HRTFs and sound field distributions are computed for a frequency range up to 6 kHz. The present model is qualitatively validated against experimental data from MIT database. The results confirm the potentiality of the proposed approach which can efficiently detect the diffraction region, diffraction angle and tails in the sound field.

Head-related transfer function interpolation in azimuth, elevation, and distance.

Although distance-dependent head-related transfer function (HRTF) databases provide interesting possibilities, e.g., for rendering virtual sounds in the near-field, there is a lack of algorithms and tools to make use of them. Here, a framework is proposed for interpolating HRTF measurements in 3-D (i.e., azimuth, elevation, and distance) using tetrahedral interpolation with barycentric weights. For interpolation, a tetrahedral mesh is generated via Delaunay triangulation and searched via an adjacency walk, making the framework robust with respect to irregularly positioned HRTF measurements and computationally efficient. An objective evaluation of the proposed framework indicates good accordance between measured and interpolated near-field HRTFs.

Auditory discrimination on the distance dependence of near-field head-related transfer function magnitudes

Distance dependence of head-related transfer functions (HRTFs) for nearby sound sources within 1 m is regarded as one of auditory distance perception cues. Accordingly, the near-field HRTFs have been used to synthesize sound sources at different distances in virtual auditory display. The present work analyzes the audibility of variation in near-field HRTF magnitudes with source distance. The calculated near-field HRTFs from KEMAR artificial head with a distance resolution of 0.01 m and a binaural model are used in analysis. The changes in loudness level spectra and interaural level difference are used as audible criteria. The result indicates that, as source distance decreases, the variation in HRTF magnitude with source distance become prominent and thereby audible. A psychoacoustic experiment is also carried out to validate the analysis. This work provides insight into the distance resolution of near-field HRTFs required in binaural virtual source synthesis.

Conventional and spatial principal component analysis on near-field head-related transfer functions

Head-related transfer functions (HRTFs) vary with both frequency and source position. The near-field HRTFs with source distance less than 1.0 m are particularly complicated due to their distance dependence. Principal component analysis (PCA), which is conventionally carried out in the frequency or time domain, has been widely applied to reduce the dimensionality of far-field HRTF data with source distance greater than 1.0 m. The present work first extends the conventional PCA, and then proposes a spatial PCA method in the spatial domain rather than in the frequency or time domain to reduce the dimensionality of near-field HRTF data. An illustrative case indicates that near-field HRTF magnitudes at 9 distances with 493 directions for each distance can be approximately represented by the weighted sum of 15 spectral shape basis functions using the conventional PCA or the weighted sum of 15 spatial basis functions using the spatial PCA. Both representations account for more than 98% energy variation of the original data, and reduce the dimensionality of the original data to about a quarter. The proposed method is also applicable to the head-related impulse responses in the time domain. Furthermore, the spatial PCA scheme is potentially applicable to simplify near-field HRTF measurement.

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.

Calculation and Analysis of Near-Field Head-Related Transfer Functions from a Simplified Head-Neck-Torso Model

To evaluate the overall effect of the neck and torso on the head-related transfer function (HRTF), a simplified head-neck-torso (HNT) model, which consists of a spherical head, spherical torso and cylindrical neck, is proposed and the corresponding HRTFs are calculated using the boundary element method (BEM). The results indicate that the HRTF magnitudes for the HNT model are different from those of the existing spherical head-and-torso model (HAT) above 0.5 kHz, especially in the near-field and contralateral region. The discrepancy in the HRTF magnitudes leads to a discrepancy in the interaural level differences (ILDs) for the HNT and HAT models, which reaches a level of ±10 dB at source distance of 0.2 m. As the source distance increases, the discrepancy in the results of the HNT and HAT models reduces. Measurement on practical HNT and HAT models validates the analysis. Therefore, the neck influences near-field HRTFs and should be included in the near-field HRTF calculation.

Directivity of Spherical Polyhedron Sound Source Used in Near-Field HRTF Measurements

The omnidirectional character is one of important requirements for the sound source used in near-field head-related transfer function (HRTF) measurements. Based on the analysis on the radiation sound pressure and directivity character of various spherical polyhedron sound sources, a spherical dodecahedral sound source with radius of 0.035m is proposed and manufactured. Theoretical and measured results indicate that the sound source is approximately omnidirectional below the frequency of 8 kHz. In addition, the sound source has reasonable magnitude response from 350Hz to 20kHz and linear phase characteristics. Therefore, it is suitable for the near-field HRTF measurements.