Head-related Transfer Functions Research Articles

A multiplayer VR showdown game for people with visual impairment

ABSTRACT Researchers have developed virtual reality (VR) systems for people with visual impairment using various audio feedback techniques. However, few studies have investigated collaborative VR systems where people with and without visual impairment can participate together. We developed a VR Showdown, similar to a real showdown game, where players with and without visual impairment can play together in a remote virtual environment. We incorporated auditory distance perception using a head-related transfer function based on the spatial position of the Showdown ball in VR. We developed two modes of VR Showdown. One is the player vs. AI agent (PvA) mode, where visually impaired people can play alone. The other is the player vs. player (PvP) mode, in which visually impaired people can play with another player in a network. The user study results showed that people with and without visual impairment enjoyed playing VR Showdown games that were well-matched.

Effect of sound-induced vibrations of the pinna on head-related transfer functions: Experimental and numerical investigations.

Numerical simulations of head-related transfer functions (HRTFs) conventionally assume a rigid boundary condition for the pinna. The human pinna, however, is an elastic deformable body that can vibrate due to incident acoustic waves. This work investigates how sound-induced vibrations of the pinna can affect simulated HRTF magnitudes. The work will motivate the research question by measuring the sound-induced vibrational patterns of an artificial pinna with a high-speed holographic interferometric system. Then, finite element simulations are used to determine HRTFs for a tabletop model of the B&K 5128 head and torso simulator for a number of directions. Two scenarios are explored: one where the pinna is modeled as perfectly rigid, and another where the pinna is modeled as linear elastic with material properties close to that of auricular cartilage. The findings suggest that pinna vibrations have negligible effects on HRTF magnitudes up to 5 kHz. The same conclusion, albeit with less certainty, is drawn for higher frequencies. Finally, the importance of the elastic domain's material properties is emphasized and possible implications for validation studies on dummy heads 1as well as the limitations of the present work are discussed in detail.

Efficient prediction of individual head-related transfer functions based on 3D meshes

Individual head-related transfer functions (HRTFs) are critical for binaural spatial audio rendering. In contrast to anthropometric parameters and pinnae images, 3D meshes allow for a more direct and comprehensive representation of the anthropometric structure, which provides highly effective inputs for modeling individualized HRTFs. This paper presents a neural network-based method for predicting individualized HRTFs in full space based on 3D meshes. Unlike many previous methods that estimate HRTF spectra at sampling grids or frequencies separately, the proposed model predicts the HRTF spectra of each vertical plane by considering the spectral correlation and continuity across adjacent sampling grids and frequencies. Evaluation results indicate that the proposed method enhances the prominence of peaks and notches in the obtained HRTF spectra and improves the speed and accuracy of HRTF individualization. The log spectral distortion of the proposed method is lower than that of state-of-the-art methods using anthropometric parameters and pinnae images. Further evaluation confirms that the proposed method requires significantly fewer points in 3D meshes when compared to numerical simulation methods. The evaluation based on localization models demonstrates that the HRTFs predicted by the proposed method are perceptually similar to the measured HRTFs.

Spatial release from masking in the median plane with non-native speakers using individual and mannequin head related transfer functions.

Spatial release from masking (SRM) in speech-on-speech tasks has been widely studied in the horizontal plane, where interaural cues play a fundamental role. Several studies have also observed SRM for sources located in the median plane, where (monaural) spectral cues are more important. However, a relatively unexplored research question concerns the impact of head-related transfer function (HRTF) personalisation on SRM, for example, whether using individually-measured HRTFs results in better performance if compared with the use of mannequin HRTFs. This study compares SRM in the median plane in a speech-on-speech virtual task rendered using both individual and mannequin HRTFs. SRM is obtained using English sentences with non-native English speakers. Our participants show lower SRM performances compared to those found by others using native English participants. Furthermore, SRM is significantly larger when the source is spatialised using the individual HRTF, and this effect is more marked for those with lower English proficiency. Further analyses using a spectral distortion metric and the estimation of the better-ear effect, show that the observed SRM can only partially be explained by HRTF-specific factors and that the effect of the familiarity with individual spatial cues is likely to be the most significant element driving these results.

Modeling of Individual Head-Related Transfer Functions (HRTFs) Based on Spatiotemporal and Anthropometric Features Using Deep Neural Networks

Modeling of Individual Head-Related Transfer Functions (HRTFs) Based on Spatiotemporal and Anthropometric Features Using Deep Neural Networks

Spatial Release From Masking With Bilateral Bone Conduction Stimulation at Mastoid for Normal Hearing Subjects.

This study investigates the effect of spatial release from masking (SRM) in bilateral bone conduction (BC) stimulation at the mastoid. Nine adults with normal hearing were tested to determine SRM based on speech recognition thresholds (SRTs) in simulated spatial configurations ranging from 0 to 180 degrees. These configurations were based on nonindividualized head-related transfer functions. The participants were subjected to sound stimulation through either air conduction (AC) via headphones or BC. The results indicated that both the angular separation between the target and the masker, and the modality of sound stimulation, significantly influenced speech recognition performance. As the angular separation between the target and the masker increased up to 150°, both BC and AC SRTs decreased, indicating improved performance. However, performance slightly deteriorated when the angular separation exceeded 150°. For spatial separations less than 75°, BC stimulation provided greater spatial benefits than AC, although this difference was not statistically significant. For separations greater than 75°, AC stimulation offered significantly more spatial benefits than BC. When speech and noise originated from the same side of the head, the "better ear effect" did not significantly contribute to SRM. However, when speech and noise were located on opposite sides of the head, this effect became dominant in SRM.

The scalp time-varying network of auditory spatial attention in “cocktail-party” situations

Sound source localization in "cocktail-party" situations is a remarkable ability of the human auditory system. However, the neural mechanisms underlying auditory spatial attention are still largely unknown. In this study, the "cocktail-party" situations are simulated through multiple sound sources and presented through head-related transfer functions and headphones. Furthermore, the scalp time-varying network of auditory spatial attention is constructed using the high-temporal resolution electroencephalogram, and its network properties are measured quantitatively using graph theory analysis. The results show that the time-varying network of auditory spatial attention in "cocktail-party" situations is more complex and partially different than in simple acoustic situations, especially in the early- and middle-latency periods. The network coupling strength increases continuously over time, and the network hub shifts from the posterior temporal lobe to the parietal lobe and then to the frontal lobe region. In addition, the right hemisphere has a stronger network strength for processing auditory spatial information in "cocktail-party" situations, i.e., the right hemisphere has higher clustering levels, higher transmission efficiency, and more node degrees during the early- and middle-latency periods, while this phenomenon disappears and appears symmetrically during the late-latency period. These findings reveal different network patterns and properties of auditory spatial attention in "cocktail-party" situations during different periods and demonstrate the dominance of the right hemisphere in the dynamic processing of auditory spatial information.

Development of a Smart Game Application for Auditory Training of Children with Spatial Processing Disorder in Iran: A Pilot Study

Background and Aim: Identifying sound localization defects in children and using appropriate rehabilitation methods lead to improve their problem to use binaural processing skills to successfully suppress background noise and selectively attend to a particular auditory source. This pilot study aims to design a game for auditory training of children with Spatial Processing Disorder (SPD) in Iran and compare their performance in engagement time and speech recognition in noise with those of normal peers.
 Methods: The game application was designed based on the spatial separation of target and competing stimuli by applying head-related transfer functions. The child’s task was to identify the picture of the target word from among the displayed options. Participants were 24 children with SPD and 27 normal peers aged 7–9 years, who performed the speech recognition tasks using the designed game.
 Results: The game was fascinating and easy to use for most participants. There was no significant difference in mean engagement time between the two groups. The mean engagement time of participants was significantly different between the two groups in terms of age. The children with SPD obtained lower scores in speech recognition in noise tasks. There was no significant difference in speech recognition scores between girls and boys.
 Conclusion: The designed game application has the potential for speech recognition in- noise training of children with SPD in Iran. It is a fascinating and user-friendly tool for simulating the real-life auditory situations for these children.
 Keywords: Smart game; speech recognition; application; auditory training; design; spatial auditory processing disorder.

Head-related transfer functions of rabbits within the front horizontal plane

The head-related transfer function (HRTF) describes the direction-dependent acoustic filtering by the head that occurs between a source signal in free-field space and the signal at the tympanic membrane. HRTFs contain information on sound source location via interaural differences of their magnitude or phase spectra and via the shapes of their magnitude spectra. The present study characterized HRTFs for source locations in the front horizontal plane for nine rabbits, which are a species commonly used in studies of the central auditory system. HRTF magnitude spectra shared several features across individuals, including a broad spectral peak at 2.6kHz that increased gain by 12 to 23dB depending on source azimuth; and a notch at 7.6kHz and peak at 9.8kHz visible for most azimuths. Overall, frequencies above 4kHz were amplified for sources ipsilateral to the ear and progressively attenuated for frontal and contralateral azimuths. The slope of the magnitude spectrum between 3 and 5kHz was found to be an unambiguous monaural cue for source azimuths ipsilateral to the ear. Average interaural level difference (ILD) between 5 and 16kHz varied monotonically with azimuth over ±31dB despite a relatively small head size. Interaural time differences (ITDs) at 0.5kHz and 1.5kHz also varied monotonically with azimuth over ±358 μs and ±260 μs, respectively. Remeasurement of HRTFs after pinna removal revealed that the large pinnae of rabbits were responsible for all spectral peaks and notches in magnitude spectra and were the main contribution to high-frequency ILDs (5–16kHz), whereas the rest of the head was the main contribution to ITDs and low-frequency ILDs (0.2–1.5kHz). Lastly, inter-individual differences in magnitude spectra were found to be small enough that deviations of individual HRTFs from an average HRTF were comparable in size to measurement error. Therefore, the average HRTF may be acceptable for use in neural or behavioral studies of rabbits implementing virtual acoustic space when measurement of individualized HRTFs is not possible.

Evaluation of the Respective Effects of Microphone Type and Dummy Head Type on Measured Head-Related Transfer Functions

A dummy head is a tool used to generate binaural recordings, i.e. recordings when listened to through headphones that allow for the listener to hear from the dummy's perspective. A dummy head typically includes pinnae and ear canals in which microphones are placed. The global shape of a dummy head replicates an average-sized human head, and variations around this average will depend on the manufacturer (pinnae of different size, presence of a nose or a mouth). This paper describes measurements conducted on two dummy heads in order to evaluate and rank the respective influence of microphone type and dummy head shape on spatialization cues. Four possible cross configurations are tested, that is head 1 or 2 with microphones pair 1 or 2. For each configuration, head-related transfer functions are measured in an anechoic room for various azimuth and elevation angles. The results obtained show that the effect of the dummy head shape is larger than the effect of the microphone type on measured head-related transfer functions. A practical implication of this result is found when microphones have to be replaced on a given dummy head.

Near-field modeling of the head-related transfer function by using the dummy head sound source

Various approaches based on direct measurement have been used to obtain the precise headrelated transfer function (HRTF). However, several problems persist, such as scattering of sound source and distance effect, which are related to the measurement configuration. In this work, the modified dummy head having two sound sources located at the ear positions is applied as the sound source reciprocally to overcome the trouble. By using the near-field acoustic holography, the sound sources located in the ear holes are reconstructed from the measured data in the near-field hologram. One can predict the acoustic transfer function between the ear holes and the field positions at any point using the recovered source information described in spherical harmonics. The obtained transfer function is valid for predicting the binaural response from any sound source position. This configuration is possible to measure the near-field response with the relatively small scattering effect because of the low Helmholtz number of microphones. The far-field reconstruction of HRTF is also possible with the near-field measurement data using acoustical holography. The proposed method is demonstrated with the modified B&K HATS system, in which the microphones in the ear holes are replaced by pipe sources connected to the loudspeaker.

DeepBSL: 3-D Personalized Deep Binaural Sound Localization on Earable Devices

The prevalence of earable devices such as earbuds and headphones allows people to converse or listen to audio recordings on the move but often at the cost of reduced alertness of imminent health and safety threats in their surroundings. 3D Binaural Sound Localization (BSL), which aims to locate sound sources in space, plays a key role in improving one’s situation awareness. BSL on earable devices is inherently challenging due to the limited number of microphones available as well as subject-and location-dependent filtration effects of a person’s pinna, head and torso, described by Head-Related Transfer Functions (HRTFs). In this work, we develop DeepBSL, a deep neural network model to estimate azimuth and elevation angles of sound sources relative to a person’s head. For a new subject, an efficient procedure is developed to collect HRTFs at sparse locations using in-ear microphones and a mobile phone, which are then utilized to synthesize sounds of any type at arbitrary locations to train personalized DeepBSL models. Extensive evaluations using synthetic data from a public dataset and through real-world experiments demonstrate that the personalized DeepBSL models are data-efficient and can achieve better-than-human performances in BSL while significantly outperforming a state-of-art model that can only predict azimuth angles of sound sources. Our best performing model has an average azimuth prediction error of 2.9∘ (4.1∘) and elevation prediction error of 1.4∘ (2.9∘) in an indoor (outdoor) environment.

The effect of motion frequency and sound source frequency on steady-state auditory motion evoked potential

The ability of humans to perceive motion sound sources is important for accurate response to the living environment. Periodic motion sound sources can elicit steady-state motion auditory evoked potential (SSMAEP). The purpose of this study was to investigate the effects of different motion frequencies and different frequencies of sound source on SSMAEP. The stimulation paradigms for simulating periodic motion of sound sources were designed utilizing head-related transfer function (HRTF) techniques in this study. The motion frequencies of the paradigm are set respectively to 1-10 Hz, 15 Hz, 20 Hz, 30 Hz, 40 Hz, 60 Hz, and 80 Hz. In addition, the frequencies of sound source of the paradigms were set to 500 Hz, 1000 Hz, 2000 Hz, 3000 Hz, and 4000 Hz at motion frequencies of 6 Hz and 40 Hz. Fourteen subjects with normal hearing were recruited for the study. SSMAEP was elicited by 500 Hz pure tone at motion frequencies of 1-10 Hz, 15 Hz, 20 Hz, 30 Hz, 40 Hz, 60 Hz, and 80 Hz. SSMAEP was strongest at motion frequencies of 6 Hz. Moreover, at 6 Hz motion frequency, the SSMAEP amplitude was largest at the tone frequency of 500 Hz and smallest at 4000 Hz. Whilst SSMAEP elicited by 4000 Hz pure tone was significantly the strongest at motion frequency of 40 Hz. SSMAEP can be elicited by periodic motion sound sources at motion frequencies up to 80 Hz. SSMAEP also has a strong response at lower frequency. Low-frequency pure tones are beneficial to enhance SSMAEP at low-frequency sound source motion, whilst high-frequency pure tones help to enhance SSMAEP at high-frequency sound source motion. The study provides new insight into the brain's perception of rhythmic auditory motion.

Head-related transfer functions upsampling with physics-informed spherical convolutional neural network

Head-related transfer functions (HRTFs) play a crucial role in virtual acoustics and spatial audio applications. However, obtaining personalized, high-resolution HRTFs remains challenging due to the time-consuming and expensive measurements. Recently, deep learning-based methods have shown promise in predicting high-resolution HRTFs from sparse measurements. Nevertheless, most of these methods often treat HRTF upsampling as an image super-resolution task, which overlooks critical spatial information and acoustic principles, leading to overfitting on datasets. This paper proposes a physics-informed spherical convolutional neural network for HRTF upsampling. First, spherical convolutional layers are used to capture spatial features on the sphere, allowing efficient handling of spherical sampled HRTF data. Second, in the upsampling process, the proposed method incorporates the Helmholtz equation as a constraint, adhering tothe physics of the acoustic system. This method ensures the generation of physically feasible HRTF interpolations, thus promoting better generalization.

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 sound localization and speech recognition with a sound enhancing connection tube for non-linear earplugs

Noise-induced hearing-loss (NIHL) can occur when an individual is exposed to high-intensity acoustic stimuli. To prevent NIHL, individuals may use non-linear-earplugs (NLEPs) that provide some attenuation, substantially within the frequency range of 1–4 kHz. Wearing NLEPs can reduce speech recognition as well as sound localization. As weapon noise is mostly present in the 1–4 kHz range and is therefore partly attenuated by NLEPs, there is a downside to the reduced ability of a soldier using them to localize the source of enemy gunfire. A perforated sound-enhancing connection tube (SECT) was designed to connect to NLEPs for improving the user’s ability to localize sound and speech recognition in noise while retaining the NLEPs's noise attenuation benefits of protecting one's hearing. A previous experiment examined the effect of the SECT with NLEPs on the Head Related Transfer Function (HRTF) of a head-and-torso-simulator (HATS) (manikin). The obtained HRTF demonstrated a 3–10 dB difference in the 1–4 kHz frequency range relative to the HRTF with the NLEPs alone, implying that sound stimuli in that frequency range might be more audible to the user. The presented work will demonstrate the speech recognition in noise and sound localization of participants when using the SECT with NLEPs.

Experimental evaluation of bilateral Ambisonics-based binaural room transfer function synthesis with application to personal sound zones

This study experimentally evaluates the bilateral Ambisonics method for synthesizing binaural room transfer functions (BRTFs) and explores its application in generating personal sound zones (PSZs) around listeners’ ears. Bilateral Ambisonics is proposed for improving the spatial sound reproduction accuracy at a limited Ambisonics order, by shifting the origin of Ambisonics representation from the head center to the two ears. While numerical simulations have demonstrated its superiority over traditional Ambisonics, little attention has been given to validating its usability for reproduction in realistic environments with loudspeakers. Experiments are conducted wherein the BRTFs of a dummy head listener are first synthesized by combining the spatial room transfer functions (SRTFs) measured with a spherical Ambisonics microphone array at the ear position and the corresponding head-related transfer functions (HRTFs), and then compared with the actual in situ measured ones. Furthermore, the quality of the synthesized BRTFs is examined in the context of an in-house PSZ system, by objectively evaluating the performance of PSZ filters designed with such BRTFs in terms of acoustic isolation metrics. The application of synthesized BRTFs to PSZ systems circumvents the need for in-situ BRTF measurements, making it highly feasible to deploy such systems while achieving high acoustic isolation.

Influence of background noise on spatial audio localization for helicopter pilot headsets

A helicopter pilot is subjected to high workloads. A high noise level in the cockpit further increases stress. This stress can be reduced through headsets for hearing protection including active noise control (ANC). An actual research topic is the improvement of the human-machine-interface by 3-D-audio systems. These systems can improve pilot’s situational awareness and, therefore, reduce stress. This paper presents a 3-D-audio system for headphones. This binaural system uses head-tracking to process spatial information about the location of the sound source. The spatial sensation is achieved by convolving the source signal with head-related transfer functions. This sensation is influenced by background noise, which can be reduced by ANC. A first listening test with physical sound sources provides information on the localizability of sound sources at different elevation angles. This is compared to a second listening test with virtual sound sources generated by the 3-D-audio system. In a third listening test, the virtual sound source is located at variable azimuth and elevation angles. The system is evaluated with respect to sound source localization and the influence of background noise and ANC. This study highlights the potential of the 3-D-audio system to improve the helicopter pilot’s situational awareness in spatial acoustic environments.

Effects of vowel coloration on perceived directional offsets between single-syllable speech sounds

The ability of normal hearing listeners to spatially localize single-syllable speech sounds was examined via a two-interval, two alternative forced choice (2AFC) spatial offset discrimination task. To examine directional biases in horizontal sound localization, this task required listeners to observe the direction of an unseen second talker’s voice immediately after hearing a first talker’s voice in an alternative direction. Listeners reported only the relative frontward versus rearward spatial offset between the two voices heard during each trial. Headphone-based virtual acoustic simulation enabled blind listening tests that were completed without feedback to the listener, revealing the ease with which listeners judged the relative spatial offset of the second speech sound, although those judgments were strongly biased by differences in the source spectra. The single-syllable speech-sound stimuli were processed using individually measured head-related transfer functions (HRTFs) of 15 listeners for whom individually measured earphone transfer functions (ETFs) were also employed. While analysis of the 2AFC response data via receiver operating characteristic (ROC) curves showed a generally high level of directional sensitivity, systematic biases were observed that could be explained via the differences in vowel coloration between the first and second source presented during each trial.