Spatial Knowledge via Auditory Information for Blind Individuals: Spatial Cognition Studies and the Use of Audio-VR.

Spatial cognition is a daily life ability, developed in order to be able to understand and interact with our environment. Even if all the senses are involved in mental representation of space elaboration, the lack of vision makes it more difficult, especially because of the importance of peripheral information in updating the relative positions of surrounding landmarks when one is moving. Spatial audio technology has long been used for studies of human perception, particularly in the area of auditory source localisation. The ability to reproduce individual sounds at desired positions, or complex spatial audio scenes, without the need to manipulate physical devices has provided researchers with many benefits. We present a review of several studies employing the power of spatial audio virtual reality for research in spatial cognition with blind individuals. These include studies investigating simple spatial configurations, architectural navigation, reaching to sounds, and sound design for improved acceptability. Prospects for future research, including those currently underway, are also discussed.

Auditory perception stability evaluation comparing binaural and loudspeaker Ambisonic presentations of dynamic virtual concert auralizations.

Auralizations can be computed in a variety of ways as well as be rendered over different sound reproduction systems. They are used as a design tool in architectural projects and for fundamental studies on spatial perception and cognition, hence requiring reliability and confidence in the obtained results. This study assessed this reliability through auditory perception stability by comparing the perceived differences between two rendering systems for a given set of second-order Ambisonic auralizations: virtual loudspeaker binaural rendering over head-tracked headphones versus 32-loudspeaker rendering. Anechoic extracts of jazz pieces have been recorded and presented in various acoustic conditions over these two systems, evaluated on the following criteria: Readability, distance, listener envelopment (LEV), apparent source width (ASW), reverberance, and loudness. Results show that consistent significant differences between scene conditions are comparably perceived across the two systems. However, significant effects of the sound reproduction system were observed for ASW, LEV, and reverberance in some configurations.

Sensitivity analysis of pinna morphology on head-related transfer functions simulated via a parametric pinna model.

The head-related transfer function (HRTF) defines the acoustic path from a source to the two ears of a listener in a manner that is highly dependent on direction. This directional dependence arises from the highly individual morphology of the pinna, which results in complex reflections and resonances. While this notion is generally accepted, there has been little research on the importance of different structural elements of the pinna on the HRTF. A parametric three-dimensional ear model was used to investigate the changes in shape of the pinna in a systematic manner with a view to determining important contributing morphological parameters that can be used for HRTF individualization. HRTFs were simulated using the boundary element method. The analysis comprised objective comparisons between the directional transfer function and diffuse field component. The mean spectral distortion was used for global evaluation of HRTF similarity across all simulated positions. A perceptual localization model was used to determine correspondences between perceptual cues and objective parameters. A reasonable match was found between the modelled perceptual results and the mean spectral distortion. Modifications to the shape of the concha were found to have an important impact on the HRTF, as did those in proximity to the triangular fossa. Furthermore, parameters that control the relief of the pinna were found to be at least as important as more frequently cited side-facing parameters, highlighting limitations in previous morphological/HRTF studies.

Forum-Pre-Sabine room acoustic assumptions on reverberation and their influence on room acoustic design.

With his work on the Fogg Art Museum and Boston Symphony Hall between 1895 and 1900, Wallace C. Sabine laid a foundation for the field of architectural acoustics as a science. Prior to that, architects employed various quantifiable notions in acoustic design. Previous studies have reviewed metric guidelines based on the directivity of the human voice, which was utilized in at least 11 rooms in pre-Sabine times. Others studies have reviewed pre-Sabine design guidelines that were based on the quantification of the perception threshold between direct sound and first order reflections and which were followed in several rooms with acoustical performance needs. As the first studies concerned the direct sound and the second set concerned first order reflections, this study reviews opinions and knowledge regarding the later part of the acoustic response, also known as reverberation, during the 19th century. This effort brings to light a room acoustic design evolution showing why concert halls at the end of the 19th century mainly had surface finishes of wood and plaster as well as limited ceiling heights. While not equal to Sabine's thoroughness and completeness, numerous early experimental and theoretical reverberation approaches were found to hint at similar notions with both qualitative and quantitative efforts.

Comparison of direct and indirect perceptual head-related transfer function selection methods.

When a personalized set of head-related transfer functions (HRTFs) is not available, a common solution is identifying a perceptually appropriate substitute from a database. There are various approaches to this selection process whether based on localization cues, subjective evaluations, or anthropomorphic similarities. This study investigates whether HRTF rankings that stem from different selection methods yield comparable results. A perceptual study was carried out using a basic source localization method and a subjective quality judgment method for a common set of eight HRTFs. HRTF rankings were determined according to different metrics from each method for each subject and the respective results were compared. Results indicate a significant and positive mean correlation between certain metrics. The best HRTFs selected according to one method had significant above-average rating scores according to metrics in the second method.

Auralization uses in acoustical design: A survey study of acoustical consultants.

While auralization technology is used in a variety of fields, particularly in architectural acoustics, there is a lack of data on the auralization tools used and actual practices. In this perspective, this work presents the results of a survey study on auralization uses in the acoustical design and consulting community, targeting acoustical consultants. The objectives are (1) to identify the tools and methods used by acousticians to create auralizations as well as effective uses so as to understand the benefits and changes provided by this technology, and (2) to highlight the difficulties and limitations linked to the use of auralizations in concrete projects. Based on the theory of acceptability and use of technology, the study was conducted from a mix of quantitative and qualitative data collection approaches, combining a questionnaire answered by 74 respondents with semi-directed interviews with nine practitioners. Results highlight the main uses of auralizations, the diversity of projects in which auralizations are applied, and how auralizations are currently used in real-world situations. The benefits of using this technology, inherent weaknesses in the tools, and practical difficulties are also discussed.

An archeoacoustic study of the history of the Palais du Trocadero (1878-1937).

During the 17th, 18th, and 19th centuries, a room acoustic theory arose with design goal to avoid echoes from occurring by quantifying the perception threshold between direct sound and 1st-order reflections. The room acoustic design of the large 19th century concert hall Palais du Trocadero (1878-1937) was based on this theory. As the resulting acoustics were notoriously poor, an attempt was undertaken in 1909 to correct the acoustics by installing additional absorbing material following the same "echo theory" design approach. In order to study the consequences of this design theory, four configurations of this concert hall have been numerically reconstructed. Through calibrated simulations, it was shown that in the case of the Palais du Trocadero, design decisions based on "echo theory" had a positive effect on the acoustics; however, due to the many curved surfaces, echoes, and sound focusing, the acoustic conditions remained poor.

Pre-Sabine room acoustic design guidelines based on human voice directivity.

With the work of Wallace C. Sabine on the lecture hall of the Fogg Art Museum and concert hall of Boston Symphony Hall, a foundation for the field of architectural acoustics as a science was laid between 1895 and 1900. Prior to that, architects employed various notions in acoustic design. Previous studies by the authors have reviewed 18th and 19th century design guidelines that were based on the quantification of the perception threshold between direct sound and first order reflections, with these guidelines being followed in the design of several rooms with acoustical demands. This study reviews an alternate metric guideline, based on the directivity and propagation distance of the human voice, which was utilized in several halls also during the 18th and 19th centuries. The related acoustic experiments tested how far sound was perceivable towards the front, sides, and rear of a speaking person. These ratios were used in the acoustical design of at least five lecture halls, four theater halls, one opera hall, and one concert hall, constructed in Germany, England, and the USA. These historic designs, and comparisons to modern measures and guidelines, are reviewed.

Identification of perceptually relevant methods of inter-aural time difference estimation.

The inter-aural time difference (ITD) is a fundamental cue for human sound localization. Over the past decades several methods have been proposed for its estimation from measured head-related impulse response (HRIR) data. Nevertheless, inter-method variations in ITD calculation have been found to exceed the known just noticeable differences (JNDs), leading to possible perceptible artifacts in virtual binaural auditory scenes, when personalized HRIRs are being used. In the absence of an objective means for validating ITD estimations, this paper examines which methods lead to the most perceptually relevant results. A subjective lateralization study compared objective ITDs to perceptually evaluated inter-aural pure delay offsets. Results clearly indicate the first-onset threshold detection method, using a low relative threshold of -30 dB, applied on 3 kHz low-pass filtered HRIRs as consistently the most perceptually relevant procedure across various metrics. Several alternative threshold values and methods based on the maximum or centroid of the inter-aural cross correlation of similarly filtered HRIR or HRIR envelopes also provided reasonable results. On the contrary, phase-based methods employing the integrated relative group delay or auditory model were not found to perform as well.

The influence of visual distance on the room-acoustic experience of auralizations.

Auralizations have become more prevalent in architectural acoustics and virtual reality. Studies have shown that by employing a methodical calibration procedure, ecologically/perceptually valid auralizations can be obtained. Another study demonstrated a manner to include dynamic voice directivity with results indicating these auralizations were judged significantly more plausible than auralizations with static source orientations. With the increased plausibility of auralizations, it is possible to study room-acoustic experience employing virtual reality, having confidence that the results also apply to real-life situations. Limited studies have examined the influence of visuals on room-acoustic experience. Using a virtual reality framework, this study investigated the influence of visuals on the room-acoustic experience of auralizations. Evaluations compared dynamic voice auralizations coherently matched with visualization positions to incoherently matched audio-visual pairs. Based on the results, the test population could be divided into three subgroups: (1) those who judged auralizations more acoustically distant with increased visual distance, (2) those who judged auralizations louder with increased visual distance, and (3) those whose audio judgment was uninfluenced by the visual stimulus.

Correction method for averaging slowly time-variant room impulse response measurements.

Various methods exist for room acoustic measurements. To increase the signal-to-noise-ratio (SNR), averaging of repeated room impulse responses (RIR) can be performed under the assumption of time-invariant systems. Associated with a study of the Paris Notre Dame cathedral, time-variances due to minute temperature changes were observed which confounded results of averaged RIRs, producing significantly shorter reverberation times. A correction method for such time-variance of the acoustic system is proposed, based on time-stretching. Following correction, reverberation times of averaged RIRs were comparable to expected tolerance ranges for measurements, well within one just noticeable difference, while also exhibiting a mid-frequency SNR gain of 8 dB.

Perceptual attributes for the comparison of head-related transfer functions.

The benefit of using individual head-related transfer functions (HRTFs) in binaural audio is well documented with regards to improving localization precision. However, with the increased use of binaural audio in more complex scene renderings, cognitive studies, and virtual and augmented reality simulations, the perceptual impact of HRTF selection may go beyond simple localization. In this study, the authors develop a list of attributes which qualify the perceived differences between HRTFs, providing a qualitative understanding of the perceptual variance of non-individual binaural renderings. The list of attributes was designed using a Consensus Vocabulary Protocol elicitation method. Participants followed an Individual Vocabulary Protocol elicitation procedure, describing the perceived differences between binaural stimuli based on binauralized extracts of multichannel productions. This was followed by an automated lexical reduction and a series of consensus group meetings during which participants agreed on a list of relevant attributes. Finally, the proposed list of attributes was then evaluated through a listening test, leading to eight valid perceptual attributes for describing the perceptual dimensions affected by HRTF set variations.

Perceptive and objective evaluation of calibrated room acoustic simulation auralizations.

Recently, auralizations have become more prevalent in architectural acoustics and virtual reality. However, there have been few studies examining the perceptual quality achievable by room acoustic simulations and auralizations. Such studies have highlighted potential problems in creating perceptually equivalent simulations when compared to measured auralizations in terms of parameter estimation. In order to accomplish realistic auralizations, calibration of the geometrical acoustics model can be considered a necessary step. In situations where the studied space exists, well-calibrated auralizations can be employed for multiple purposes, such as multi-modal virtual reality explorations, studies of the acoustical influence of renovations, and historic research. Using this case type as a base, a perceptual study evaluating state-of-the-art binaural auralizations has been carried out. Three test sites of different complexity and acoustics were selected: the abbey church Saint-Germain-des-Prés, the cathedral Notre-Dame de Paris, and the Théâtre de l'Athénée. Models were calibrated according to omni-directional source-receiver measurements for reverberation and clarity parameters. In the subjective listening test, measured and simulated binaural auralizations were compared according to eight acoustic perceptual attributes. Results showed that the methodical calibration procedure employed in combination with attention to control factors led to ecologically/perceptually valid auralizations.

In situ calibration of the sound strength parameter G.

The room acoustic parameter "strength of sound G" is a measure of room amplification relative to a 10 m free-field reference. Due to this reference requirement, G is often considered excessively difficult to measure. Standards require reference measurements using reverberation or anechoic chambers. While possible for well-equipped laboratories, this is impractical for most practitioners. Considering the entire measurement chain, stability of amplifier and converter gains must be identical between on-site and calibration measurements, which cannot always be assured. An in situ calibration method is proposed, taking advantage of the full hall dataset. Results show significant advantages compared to previous methods.

Perceptual thresholds for realistic double-slope decay reverberation in large coupled spaces.

Reverberation highly influences sound perception in enclosed spaces. The reverberation time (RT) metric, used to quantify reverberation in single volumes, is inappropriate for coupled spaces characterized by non-exponential double-slope energy decays. Previous research on reverberation perception of double-slope decays has been predominantly based on varying basic impulse response characteristics such as decay times corresponding to reverberation times of individual volumes presented as independent variables. Alternatively, several studies have employed geometrical room acoustic software simulations to generate collections of responses while varying architectural parameters such as coupling area and room volumes. To avoid issues related to geometrical acoustics simulations, such as position dependence and limitations of some software to properly simulate coupled volume behavior, this study examines perception of the variability of reverberation typical of a physical coupled volume system. Employing an established statistical model, the control parameter of coupling area aperture which acoustically connects the volumes serves as the independent variable. Two listening tests were conducted to determine perceptual thresholds using an ABX discrimination task. The range of tested values corresponded to physically realizable variations. Just noticeable differences (JNDs) were derived with an average JND of ≈ 10% variation of the coupling aperture. No significant differences were found between different musical excerpts.

A comparative study of Interaural Time Delay estimation methods.

The Interaural Time Delay (ITD) is an important binaural cue for sound source localization. Calculations of ITD values are obtained either from measured time domain Head-Related Impulse Responses (HRIRs) or from their frequency transform Head-Related Transfer Functions (HRTFs). Numerous methods exist in current literature, based on a variety of definitions and assumptions of the nature of the ITD as an acoustic cue. This work presents a thorough comparative study of the degree of variability between some of the most common methods for calculating the ITD from measured data. Thirty-two different calculations or variations are compared for positions on the horizontal plane for the HRTF measured on both a KEMAR mannequin and a rigid sphere. Specifically, the spatial variations of the methods are investigated. Included is a discussion of the primary potential causes of these differences, such as the existence of multiple peaks in the HRIR of the contra-lateral ear for azimuths near the inter-aural axis due to multipath propagation and head/pinnae shadowing.

Investigation of the effective aperture area of sliding and hinged doors between coupled spaces.

Acoustical coupling between architectural spaces can be implemented by sliding or hinged doors. This study compares the effects of these variable coupling area designs on the sound field using temporal energy decay curve analysis. Varying the aperture size alters the multi-slope decay curve properties such as the decay rate of each slope and their point of intersection (time and level). A predictive model is proposed, based on a geometrical approach and statistical theory for coupled volumes. Differences between scale model measurements and analytical predictions are quantified by means of deviations of acoustical parameters; reasonable agreement is found.

Sound energy decay in coupled spaces using a parametric analytical solution of a diffusion equation.

Sound field behavior in performance spaces is a complex phenomenon. Issues regarding coupled spaces present additional concerns due to sound energy exchanges. Coupled volume concert halls have been of increasing interest in recent decades because this architectural principle offers the possibility to modify the hall's acoustical environment in a passive way by modifying the coupling area. Under specific conditions, the use of coupled reverberation chambers can provide non-exponential sound energy decay in the main room, resulting in both high clarity and long reverberation which are antagonistic parameters in a single volume room. Previous studies have proposed various sound energy decay models based on statistical acoustics and diffusion theory. Statistical acoustics assumes a perfectly uniform sound field within a given room whereas measurements show an attenuation of energy with increasing source-receiver distance. While previously proposed models based on diffusion theory use numerical solvers, the present study proposes a heuristic model of sound energy behavior based on an analytical solution of the commonly used diffusion equation and physically justified approximations. This model is validated by means of comparisons to scale model measurements and numerical geometrical acoustics simulations, both applied to the same simple concert hall geometry.

EXPRESS: On prior visual experience in mental map navigation using allocentric and egocentric perspectives in the visually impaired.

Using new developments in the mental comparison task paradigm, this study addresses the question of the influence of prior visual experience in the natural use of mental perspective to achieve mental spatial tasks without any protocol-imposed perspective. During the experiment, 39 participants (11 early blind, 13 late blind, and 15 blindfolded-sighted) explored two corridor maps to memorise the spatial arrangement of 10 objects disposed along corridors. After the learning phase, several tasks addressing spatial memory and reasoning used in the mental spatial representation were performed. Blindfolded-sighted participants preferred an egocentric perspective, while the two visually impaired groups showed no overriding preference between egocentric and allocentric perspectives. Results showed a performance advantage for egocentric over allocentric perspectives, regardless of visual experience. Our results shed light on previous assumptions regarding cognitive mental map construction, suggesting the need to reflect on previous results and their dependence on imposed mental perspectives.

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.