Spatial Room Impulse Responses Research Articles

Simulation of acoustical parameters of churches in a virtual acoustics laboratory

Current ISO standards for acoustical assessment limit the use of traditional mono-dimensional microphones for measuring equipment. However, microphone arrays offer more accurate spatial information compared to traditional microphones. This presents an opportunity to enhance research on architectural acoustics and preserve the acoustics of cultural heritage more effectively. Furthermore, modern recording studios equipped with virtual acoustics systems allow for the integration of spatial room impulse responses. This enables real-time auralization in controlled environments and enhances the overall immersive audio experience for users and musicians. This paper analyzes the main acoustical parameters obtained from spatial impulse responses captured in churches using various ambisonic microphones and inserted into a controlled, interactive, and immersive virtual acoustics system. The captured spatial acoustic measurements are then reproduced and evaluated in a virtual acoustics laboratory to identify any discrepancies between real and virtual spaces.

Directional distribution of the pseudo intensity vector in anisotropic late reverberation.

The pseudo intensity vector (PIV) is often used to analyze the directional properties of spatial room impulse responses. In the early part of the response, it is capable of estimating the directions of individual reflections. However, thus far, its behaviour in the late field is unclear. Specifically, it is unknown whether anisotropy, i.e., a direction-dependent energy distribution, is captured by the directional estimates. In this study, a closed-form expression of the directional distribution of the pressure-normalized pseudo intensity vector contingent on a general stochastical model of anisotropic fields was analytically derived. This paper shows that the probability density function of this PIV is a multivariate Cauchy distribution, which does indeed depend on the energy distribution of the field, yet the directional distribution has very limited degrees of freedom. The derived distribution is compared to the results of Monte Carlo simulations and fields captured with a microphone array in a real room. These results facilitate better understanding of the behaviour of parametric spatial room impulse response methods and may enable improved directional estimators for anisotropic fields.

Co-immersion in Audio Augmented Virtuality: the Case Study of a Static and Approximated Late Reverberation Algorithm.

In immersive Audio Augmented Reality, a virtual sound source should be indistinguishable from the existing real ones. This property can be evaluated with the co-immersion criterion, which encompasses scenes constituted by arbitrary configurations of real and virtual objects. Thus, we introduce the term Audio Augmented Virtuality (AAV) to describe a fully virtual environment consisting of auditory content captured from the real world, augmented by synthetic sound generation. We propose an experimental design in AAV investigating how simplified late reverberation (LR) affects the co-immersion of a sound source. Participants listened to simultaneous virtual speakers dynamically rendered through spatial Room Impulse Responses, and were asked to detect the presence of an impostor, i.e., a speaker rendered with one of two simplified LR conditions. Detection rates were found to be close to chance level, especially for one condition, suggesting a limited influence on co-immersion of the simplified LR in the evaluated AAV scenes. This methodology can be straightforwardly extended and applied to different acoustics scenes, complexities, i.e., the number of simultaneous speakers, and rendering parameters in order to further investigate the requirements for immersive audio technologies in AAR and AAV applications.

Single-value frequency-average measures of early decay time and clarity index to predict reverberance and clarity

The most accepted acoustical parameters for the assessment of concert hall acoustics are summarized in the Annex A of the ISO 3382-1 standard, along with recommendations that are important for the practical application of these parameters. These recommendations have their origin in the practical experience of acoustical consultants and a few scientific studies, and further research is needed to properly validate them. In particular, more information is needed on how to average the octave-band values of acoustical parameters to obtain single-value frequency-average measures (SVFA) that are better correlated with the associated perceptual aspects. In this study, SVFA measures of early decay time (EDT) and clarity index (C80) were obtained by means of a listening test in which 24 subjects rated a set of stimuli in terms of reverberance and clarity. The participants listened to auralizations of orchestral music convolved with measured spatial room impulse responses in an anechoic chamber using third-order Ambisonic reproduction. The results of the listening test indicate that the mid-frequency, 500 and 1000 Hz, octave-band average of EDT and the average of the 4000and the 8000 Hz octave bands of C80, have the highest sensitivity in the prediction of reverberance and musical clarity, respectively.

Generative models for sound field reconstruction.

This work examines the use of generative adversarial networks for reconstructing sound fields from experimental data. It is investigated whether generative models, which learn the underlying statistics of a given signal or process, can improve the spatio-temporal reconstruction of a sound field by extending its bandwidth. The problem is significant as acoustic array processing is naturally band limited by the spatial sampling of the sound field (due to the difficulty to satisfy the Nyquist criterion in space domain at high frequencies). In this study, the reconstruction of spatial room impulse responses in a conventional room is tested based on three different generative adversarial models. The results indicate that the models can improve the reconstruction, mostly by recovering some of the sound field energy that would otherwise be lost at high frequencies. There is an encouraging outlook in the use of statistical learning models to overcome the bandwidth limitations of acoustic sensor arrays. The approach can be of interest in other areas, such as computational acoustics, to alleviate the classical computational burden at high frequencies.

Direct and Residual Subspace Decomposition of Spatial Room Impulse Responses

Psychoacoustic experiments have shown that directional properties of the direct sound, salient reflections, and the late reverberation of an acoustic room response can have a distinct influence on the auditory perception of a given room. Spatial room impulse responses (SRIRs) capture those properties and thus are used for direction-dependent room acoustic analysis and virtual acoustic rendering. This work proposes a subspace method that decomposes SRIRs into a direct part, which comprises the direct sound and the salient reflections, and a residual, to facilitate enhanced analysis and rendering methods by providing individual access to these components. The proposed method is based on the generalized singular value decomposition and interprets the residual as noise that is to be separated from the other components of the reverberation. Large generalized singular values are attributed to the direct part, which is then obtained as a low-rank approximation of the SRIR. By advancing from the end of the SRIR toward the beginning while iteratively updating the residual estimate, the method adapts to spatio-temporal variations of the residual. The method is evaluated using a spatio-spectral error measure and simulated SRIRs of different rooms, microphone arrays, and ratios of direct sound to residual energy. The proposed method creates lower errors than existing approaches in all tested scenarios, including a scenario with two simultaneous reflections. A case study with measured SRIRs shows the applicability of the method under real-world acoustic conditions. A reference implementation is provided.

Measuring the Acoustical Properties of the BBC Maida Vale Recording Studios for Virtual Reality

In this paper we present a complete acoustic survey of the British Broadcasting Corporation Maida Vale recording studios. The paper outlines a fast room acoustic measurement framework for capture of spatial impulse response measurements for use in three or six degrees of freedom Virtual Reality rendering. Binaural recordings from a KEMAR dummy head as well as higher order Ambisonic spatial room impulse response measurements taken using a higher order Ambisonic microphone are presented. An acoustic comparison of the studios is discussed, highlighting remarkable similarities across three of the recording spaces despite significant differences in geometry. Finally, a database of the measurements, housing the raw impulse response captures as well as processed spatial room impulse responses is presented.

Spatial room impulse response measurement and post-processing for virtual reproduction of a stereo sound system

Spatial room impulse responses (SRIRs) characterize the spatial, temporal, and spectral properties of the sound field received at a particular location in a room for a given source-receiver configuration. SRIRs can be captured using spherical microphone arrays and are useful both for evaluating the acoustic performance of a space and in applications where it is desired to auralize the sound field being characterized with varying signals being played from the source. In this work, SRIRs were measured in a living room (mid-frequency RT approximately 0.4 s) using an mh acoustics Eigenmike em32 as the receiver and both a studio monitor loudspeaker and a subwoofer as the sources. Studio monitor placements replicated a left-right-center stereo monitoring configuration, with the subwoofer on the floor below the center loudspeaker, while attempting to avoid source and receiver placement on estimated modal planes in the room. Details of the post-processing used to finalize the SRIRs for use as a virtual stereo sound system will be presented, including cross-over filtering, tuning, and correction of the overall frequency response to a curve that is favorable for in-room loudspeaker listening. Measurement system details, SRIR data visualizations, and RT estimates will also be discussed.

A study of the just noticeable difference of early decay time for symphonic halls.

The just noticeable differences (JNDs) of room acoustic parameters are important for the design of concert halls and, in general, research of room acoustics. Precise knowledge of JNDs helps the concert hall designer in assessing the impact that changes in the geometry or materials of the hall will have on its perceived acoustics. When designing a concert hall, creating an appropriate feeling of reverberance for the audience is of prime importance. The early decay time (EDT) parameter has proved to be a better predictor of the perception of reverberance than the classical reverberation time (T30), but no studies have been conducted to specifically determine the EDT JND. In the present study, the EDT JND was investigated for broadband conditions and assessed for individual frequency ranges. A subjective study was conducted with 26 subjects with musical training, in which 21 were considered reliable. The participants listened to orchestral music convolved with measured spatial room impulse responses from three concert halls. The stimuli were auralized in an anechoic chamber using third-order Ambisonic reproduction. The obtained values show that the JNDs for the broadband conditions are lower than those for the individual frequency ranges. The EDT JND for the broadband conditions was found to be approximately 18% of the EDT value.

Effect of Environment-Related Cues on Auditory Distance Perception in the Context of Audio-Only Augmented Reality

Audio-only augmented reality consists of enhancing a real environment with virtual sound events. A seamless integration of the virtual events within the environment requires processing them with artificial spatialization and reverberation effects that simulate the acoustic properties of the room. However, in augmented reality, the visual and acoustic environment of the listener may not be fully mastered. This study aims to gain some insight into the acoustic cues (intensity and reverberation) that are used by the listener to form an auditory distance judgment, and to observe if these strategies can be influenced by the listener’s environment. To do so, we present a perceptual evaluation of two distance-rendering models informed by a measured Spatial Room Impulse Response. The choice of the rendering methods was made to design stimuli categories in which the availability and reproduction quality of acoustic cues are different. The proposed models have been evaluated in an online experiment gathering 108 participants who were asked to provide judgments of auditory distance about a stationary source. To evaluate the importance of environmental cues, participants had to describe the environment in which they were running the experiment, and more specifically the volume of the room and the distance to the wall they were facing. It could be shown that these context cues had a limited, but significant, influence on the perceived auditory distance.

Perceptual roughness of spatially assigned sparse noise for rendering reverberation.

Multichannel auralizations based on spatial room impulse responses often employ sample-wise assignment of an omnidirectional response to form loudspeaker responses. This leads to sparse impulse responses in each reproduction loudspeaker and the auralization of transient signals can sound rough. Based on this observation, we conducted a listening test to examine the general phenomenon of roughness due to spatial assignment. First, participants assessed the roughness of both Gaussian noise and velvet noise, assigned sample-wise to up to 36 loudspeakers by two algorithms. The first algorithm assigns channels merely by selecting random indices, while the second one constrains the time between two peaks on each channel. The results show that roughness already occurs when few channels are used and that the assignment algorithm influences it. In a second experiment, virtualizations of the test were used to examine the factors contributing to increased roughness. We systematically show the effect of spatial assignment on noise and conclude that besides time-differences, level-differences caused by head-shadowing are the principal cause for the perceived roughness. The results have significance in spatial room impulse response rendering and spatial reverberator design.

Virtual sound source distance evaluation in acoustically and visually incongruent contexts

Visual and acoustic environment may influence the perception of auditory distance. In the context of Audio-only augmented reality (AAR), the coherence of the perceived virtual sound sources with the apparent room geometry and acoustics cannot always be guaranteed. The perceptual consequences of these incoherences are not well known. We conducted two online perceptual studies with a sound distance rendering model based on measured spatial room impulse responses (SRIR). A first study evaluated the perceptual performances of the model in incongruent visual contexts. The incongruent environment-related visual cues (spatial visual boundary and room volume) demonstrated a significant effect on the auditory distance perception (ADP) of virtual sound sources, through a calibration effect. A second study evaluated the impact of acoustical incongruence. Virtual sound sources distances were judged after the participant listened to distracting sound sources conveying distance cues relative to a different acoustical environment. When this distracting sound sources corresponded to a larger room than the one reproduced by the model, a higher compression effect was observed on the ADP of virtual sound sources. However, when the intensity cue conveyed by the distracting sound sources were coherent with the acoustical environment simulated by the model, their distracting effect were negligible.

Synthesis of acoustics of coupled volumes—Perceptual approach

Connected, acoustically coupled spaces are typically characterized by non-exponential decay with double or multiple slopes. In some cases, two volumes of different absorption characteristics are connected through an aperture that may be opened to vary the engagement between the two acoustics, e.g., the auditorium and the reverb chamber. In a large complex room, acoustic energy emanating from the source will simultaneously and sequentially engage a number of different volumes (regions), depending on how the space is physically laid out, and where the source and receiver are. The regions shape and exchange energy collectively. The listener’s overall room impression will be due to an overlayed mix of spatially and temporally distributed ambient responses reaching the listener or receiver position. In synthesis of complex acoustic spaces, we take a perceptual approach where we combine elements of measured spatial room impulse responses, divide them into meaningful temporal segments, shape their envelopes, and recombine these varied characteristics for multichannel convolution with a source. Different variants of processing the impulse responses create different perceptual outcomes, and a variety of aurally diverse spaces can be created for applications in virtual acoustics and immersive audio production. These perceptual approaches, balancing clarity and reverberation, will be discussed in this presentation.

Locating Image Sources from Multiple Spatial Room Impulse Responses

Measured spatial room impulse responses have been used to compare acoustic spaces. One way to analyze and render such responses is to apply parametric methods, yet those methods have been bound to single measurement locations. This paper introduces a method that locates image sources from spatial room impulse responses measured at multiple source and receiver positions. The method aligns the measurements to a common coordinate frame and groups stable direction-of-arrival estimates to find image source positions. The performance of the method is validated with three case studies—one small room and two concert halls. The studies show that the method is able to locate the most prominent image sources even in complex spaces, providing new insights into available Spatial Room Impulse Response (SRIR) data and a starting point for six degrees of freedom (6DoF) acoustic rendering.

The just noticeable difference of early decay time (EDT)

The purpose of this study was to determine the just noticeable difference (JND) of early decay time (EDT) for broadband conditions and individual frequency bands. JNDs of room acoustic parameters are important for assessing if changes in geometry or materials in a room will have a discernible impact in the listener’s perception. The EDT JND has been reported to be 5% in the ISO 3382-1:2009 standard, which is the same value usually quoted for the JND of classical reverberation time (T30). No formal studies have been undertaken so far to validate if this JND value is appropriate for EDT, or to determine its dependency on frequency. Measured spatial room impulse responses (SRIRs) from three concert halls were used as base cases, which were then modified with a computer algorithm to attain specified values of EDT. All stimuli were reproduced in an anechoic chamber with an array of 30 loudspeakers using third-order ambisonics. The method of constant stimuli, in which a psychometric function is fitted to the measured data, was used to determine the JND. Thirty subjects with musical backgrounds participated in the study. The obtained broadband JND was 26%, while individual frequency band JNDs were higher.

Using a linear microphone array to evaluate inhomogeneous sound-fields of recreated enclosures

Auditory Virtual Environments (AVE) typically recreate the sound at a specific listener position. Extended loudspeaker-based systems typically use the rich capabilities to enlarge the sweet spot, so a broader audience can experience the sound from the same virtual listener position. An alternative approach is taken with Rensselaer's Collaborative-Research Augmented Immersive Virtual Environment Laboratory (CRAIVE-Lab). Using 128 horizontally arranged loudspeakers, inhomogeneous sound fields are produced over a user area of 10 × 12 sqm, recreating the sound of acoustical enclosures using a ray-tracing method. A linear microphone array, consisting of 40 omnidirectional microphones, spaced 15 cm apart, is used to measure the fidelity of the reproduced inhomogeneous sound fields. This is accomplished by comparing the spatial room impulse responses measured in the CRAIVE-Lab to those generated synthetically (or computationally). The arrival times of direct sound and reflections are compared for both simulation methods. The analysis focuses on the spatial properties of the arriving wave fronts, including the direction of arrival and curvature, to determine the degree of accuracy to which the inhomogeneous properties of the sound field are reproduced. [Work supported by NSF #1909229.]

A robust denoising process for spatial room impulse responses with diffuse reverberation tails.

Spatial room impulse responses (SRIRs) measured using spherical microphone arrays are seeing increasingly widespread use in reproducing room reverberation effects on three-dimensional surround sound systems (e.g., higher-order ambisonics) through multi-channel SRIR convolution. However, such measured impulse responses inevitably present a non-negligible noise floor, which may lead to a perceptible "infinite reverberation effect" when convolved with an input sound. Furthermore, individual sensor noise and momentary measurement artefacts may additionally corrupt the resulting impulse response. This paper presents a robust SRIR denoising procedure applicable to impulse responses with diffuse late reverberation tails, which can be modeled by a stochastic process. In such cases, the non-decaying frequency-dependent noise floor may be replaced by a synthesized incoherent tail parameterized by the SRIR's energy decay envelope. It is shown that performing such tail re-synthesis in the spherical harmonic domain, using an independent zero-mean Gaussian noise for each component, preserves both the reverberation tail's frequency-dependent decay as well as its spatial coherence properties. The proposed process is then evaluated through its application to SRIRs measured in real-world conditions, and finally some aspects of performance and consistency verification are discussed.

Boundary absorption approximation in the spatial high-frequency extrapolation method for parametric room impulse response synthesis.

The spatial high-frequency extrapolation method extrapolates low-frequency band-limited spatial room impulse responses (SRIRs) to higher frequencies based on a frame-by-frame time/frequency analysis that determines directional reflected components within the SRIR. Such extrapolation can be used to extend finite-difference time domain (FDTD) wave propagation simulations, limited to only relatively low frequencies, to the full audio band. For this bandwidth extrapolation, a boundary absorption weighting function is proposed based on a parametric approximation of the energy decay relief of the SRIR used as the input to the algorithm. Results using examples of both measured and FDTD simulated impulse responses demonstrate that this approach can be applied successfully to a range of acoustic spaces. Objective measures show a close approximation to reverberation time and acceptable early decay time values. Results are verified through accompanying auralizations that demonstrate the plausibility of this approach when compared to the original reference case.

An investigation of listener envelopment utilizing a spherical microphone array and third-order ambisonics reproduction.

Listener envelopment (LEV), the sense of being surrounded by the sound field, is a perception that has been found to be related to the overall impression of a concert hall. The purpose of this study was to investigate the relationship between the perception of LEV and the direction and arrival time of energy from spatial room impulse responses (IRs). IRs were obtained in a 2000-seat concert hall using a 32-channel spherical microphone array and analyzed using a third-order plane wave decomposition. Additionally, the IRs were convolved with anechoic music and processed for third-order Ambisonic reproductions and presented to subjects over a 30-loudspeaker array. Instances were found in which the energy in the late sound field did not correlate with LEV ratings as well as energy in a 70-100 ms time window. Follow-up listening tests were conducted with hybrid IRs containing portions of an enveloping IR and an unenveloping IR with crossover times ranging from 40 to 140 ms. Additional hybrid IRs were studied wherein portions of the spatial IRs were collapsed into all frontal energy with crossover times ranging from 40 to 120 ms. The tests confirmed that much of the important LEV information exists in the early portion of these IRs.

Achieving realism and repeatability of an orchestra simulated within a concert hall

Virtual acoustic techniques can allow for direct comparison between concert halls, but care must be taken to ensure the virtual reconstruction connects back to the subtle perceptions of the real environment. To achieve better realism with repeatability between measured halls, a compact spherical loudspeaker array is being used to simulate an orchestra. This sound source is placed at twenty different source locations around the stage. For each instrument measurement position, a swept-sine signal is processed to radiate from the source with a particular instrument’s directivity, and the spatial room impulse response (RIR) is measured. This processing is accomplished through filters designed from an instrument directivity database measured with 32 microphones. For each instrument, a set of full-frequency filters, one for each loudspeaker driver, were designed from one-third octave band spherical harmonic weights. These processing techniques are directly analogous to higher order Ambisonics (HOA). In the audience, a 32-element spherical microphone array is used to capture the spatial RIR, and each measurement is convolved with single-instrument anechoic recordings and auralized using HOA. Separate auralizations of each instrument are combined together into a full-orchestral auralization. The measurement setup, filter design, and equalization techniques will be presented. [Work supported by NSF Award 1302741.]