On the authenticity of individual dynamic binaural synthesis

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.

The Potential of Speech as the Calibration Sound for Level Calibration of Non-Laboratory Listening Test Setups

The pandemic of COVID-19 and the resulting countermeasures have made it difficult or impossible to perform listening tests in controlled laboratory environments. This paper examines the possibility of using speech for level calibration of sound reproduction systems used in listening tests performed in non-laboratory conditions, i.e., when such tests are distributed through the means of electronic communication and performed in a home environment. Moreover, a larger pool of potential test subjects can be reached in this manner. The perception of what the “normal” level of reproduced speech should be was examined through a listening experiment by letting the listeners set the level of reproduced speech samples as they saw fit, depending on the used sound reproduction system, the (non)existence of visual stimulus, and the voice of the speaker. The results show that the perception of normal speech level is highly individual when it comes to setting that level by listening to reproduced speech. The interindividual differences between the subjects are considerably larger than the impact of the three main effects. The understanding of what the “normal” level of read speech should be was examined experimentally as well by asking the subjects to read a paragraph of text, depending on the visual stimulus. The results show that the “normal” level of read speech is reasonably consistent and averages at 55 dBA at a normal conversational distance of 1 m, in a room with room acoustics conditions typical for home environment and low background noise, and with the visual stimulus that mimics the interlocutor put within the personal space of the reader. A preliminary proposal is given of a level calibration method for non-laboratory listening experiments based on these results, and some of its aspects that require further research are discussed.

Perceptual Matching of Room Acoustics for Auditory Augmented Reality in Small Rooms - Literature Review and Theoretical Framework

For the realization of auditory augmented reality (AAR), it is important that the room acoustical properties of the virtual elements are perceived in agreement with the acoustics of the actual environment. This perceptual matching of room acoustics is the subject reviewed in this paper. Realizations of AAR that fulfill the listeners' expectations were achieved based on pre-characterization of the room acoustics, for example, by measuring acoustic impulse responses or creating detailed room models for acoustic simulations. For future applications, the goal is to realize an online adaptation in (close to) real-time. Perfect physical matching is hard to achieve with these practical constraints. For this reason, an understanding of the essential psychoacoustic cues is of interest and will help to explore options for simplifications. This paper reviews a broad selection of previous studies and derives a theoretical framework to examine possibilities for psychoacoustical optimization of room acoustical matching.

The "Missing 6 dB" Revisited: Influence of Room Acoustics and Binaural Parameters on the Loudness Mismatch Between Headphones and Loudspeakers

Generations of researchers observed a mismatch between headphone and loudspeaker presentation: the sound pressure level at the eardrum generated by a headphone has to be about 6 dB higher compared to the level created by a loudspeaker that elicits the same loudness. While it has been shown that this effect vanishes if the same waveforms are generated at the eardrum in a blind comparison, the origin of the mismatch is still unclear. We present new data on the issue that systematically characterize this mismatch under variation of the stimulus frequency, presentation room, and binaural parameters of the headphone presentation. Subjects adjusted the playback level of a headphone presentation to equal loudness as loudspeaker presentation, and the levels at the eardrum were determined through appropriate transfer function measurements. Identical experiments were conducted at Oldenburg and Aachen with 40 normal-hearing subjects including 14 that passed through both sites. Our data verify a mismatch between loudspeaker and binaural headphone presentation, especially at low frequencies. This mismatch depends on the room acoustics, and on the interaural coherence in both presentation modes. It vanishes for high frequencies and broadband signals if individual differences in the sound transfer to the eardrums are accounted for. Moreover, small acoustic and non-acoustic differences in an anechoic reference environment (Oldenburg vs. Aachen) exert a large effect on the recorded loudness mismatch, whereas not such a large effect of the respective room is observed across moderately reverberant rooms at both sites. Hence, the non-conclusive findings from the literature appear to be related to the experienced disparity between headphone and loudspeaker presentation, where even small differences in (anechoic) room acoustics significantly change the response behavior of the subjects. Moreover, individual factors like loudness summation appear to be only loosely connected to the observed mismatch, i.e., no direct prediction is possible from individual binaural loudness summation to the observed mismatch. These findings – even though not completely explainable by the yet limited amount of parameter variations performed in this study – have consequences for the comparability of experiments using loudspeakers with conditions employing headphones or other ear-level hearing devices.

A round robin on room acoustical simulation and auralization

A round robin was conducted to evaluate the state of the art of room acoustic modeling software both in the physical and perceptual realms. The test was based on six acoustic scenes highlighting specific acoustic phenomena and for three complex, "real-world" spatial environments. The results demonstrate that most present simulation algorithms generate obvious model errors once the assumptions of geometrical acoustics are no longer met. As a consequence, they are neither able to provide a reliable pattern of early reflections nor do they provide a reliable prediction of room acoustic parameters outside a medium frequency range. In the perceptual domain, the algorithms under test could generate mostly plausible but not authentic auralizations, i.e., the difference between simulated and measured impulse responses of the same scene was always clearly audible. Most relevant for this perceptual difference are deviations in tone color and source position between measurement and simulation, which to a large extent can be traced back to the simplified use of random incidence absorption and scattering coefficients and shortcomings in the simulation of early reflections due to the missing or insufficient modeling of diffraction.

Interaural Level Difference Optimization of Binaural Ambisonic Rendering

Ambisonics is a spatial audio technique appropriate for dynamic binaural rendering due to its sound field rotation and transformation capabilities, which has made it popular for virtual reality applications. An issue with low-order Ambisonics is that interaural level differences (ILDs) are often reproduced with lower values when compared to head-related impulse responses (HRIRs), which reduces lateralization and spaciousness. This paper introduces a method of Ambisonic ILD Optimization (AIO), a pre-processing technique to bring the ILDs produced by virtual loudspeaker binaural Ambisonic rendering closer to those of HRIRs. AIO is evaluated objectively for Ambisonic orders up to fifth order versus a reference dataset of HRIRs for all locations on the sphere via estimated ILD and spectral difference, and perceptually through listening tests using both simple and complex scenes. Results conclude AIO produces an overall improvement for all tested orders of Ambisonics, though the benefits are greatest at first and second order.

Matched transaural synthesis with probe microphones for psychoacoustical experiments

Transaural synthesis using loudspeaker signals determined through contemporaneous ear canal calibration is proposed as an alternative to headphone presentation for critical psychoacoustical experiments. The proposed technique can afford greater accuracy, improved reproducibility, and continuous signal monitoring. It allows the experimenter to compare listener responses to real and virtual presentations. In this article, the advantages of transaural (three or four loudspeakers) compared to crosstalk cancellation (two loudspeakers) are shown through computer modeling and manikin measurements in a moderately reverberant room. Measurements employ binaurally challenging signals and speech from a distant source. Transaural synthesis is shown to be a better solution to the essential inverse problem resulting in reduced average synthesis amplitudes, fewer large-amplitude outliers, improved amplitude and phase accuracy for real and imagined sources, and improved noise immunity. Immunity to inadvertent listener head rotation depends sensitively on loudspeaker placement and is not an advantage in general. Appendixes review the relevant mathematical foundation and extend it to the relationship between ear canal signals and eardrum signals.