Spherical microphone arrays (SMAs) are widely used to capture spatial sound fields that can then be rendered in various ways as a virtual acoustic environment (VAE) including headphone-based binaural synthesis. Several practical limitations have a significant impact on the fidelity of the rendered VAE. The finite number of microphones of SMAs leads to spatial undersampling of the captured sound field, which, on the one hand, induces spatial aliasing artifacts and, on the other hand, limits the order of the spherical harmonics (SH) representation. Several approaches have been presented in the literature that aim to mitigate the perceptual impairments due to these limitations. In this article, we present a listening experiment evaluating the perceptual improvements of binaural rendering of undersampled SMA data that can be achieved using state-of-the-art mitigation approaches. In particular, we examined the Magnitude Least-Squares algorithm, the Bandwidth Extraction Algorithm for Microphone Arrays, Spherical Head Filters, SH Tapering, and a newly proposed equalization filter. In the experiment, subjects rated the perceived differences between a dummy head and the corresponding SMA auralization. We found that most mitigation approaches lead to significant perceptual improvements, even though audible differences to the reference remain.
We present a method for computing a spherical harmonic representation of a sound field based on observations of the sound pressure along the equator of a rigid spherical scatterer. Our proposed solution assumes that the captured sound field is height invariant so that it can be represented by a two-dimensional (2D) plane wave decomposition (PWD). The 2D PWD is embedded in a three-dimensional representation of the sound field, which allows for perfectly undoing the effect of the spherical scattering object. If the assumption of height invariance is fulfilled, then the proposed solution is at least as accurate as a conventional spherical microphone array of the same spherical harmonic order, which requires a multiple of the number of sensors. Our targeted application is binaural rendering of the captured sound field. We demonstrate by analyzing the binaural output signals that violations of the assumptions that the solution is based on—particularly height invariance and consequently also horizontal propagation—lead to errors of moderate magnitude.
Spherical microphone array (SMA) recordings are particularly suited for dynamic binaural rendering as the microphone signals can be decomposed into a spherical harmonic (SH) representation that can be freely rotated to match the head orientation of the listener. The rendering of such SMA recordings is a non-trivial task as the SH signals are impaired due to truncation of the SH decomposition order, spatial aliasing and the gain limitation of the employed radial filters. The perceptually most relevant consequence of this is an alteration of the magnitude transfer function at high frequencies. Previously, the magnitude least squares (MagLS) renderer for binaural rendering of SH signals was proposed to mitigate these effects under the assumption of ideal order-truncated plane waves, i.e., disregarding the influence of spatial aliasing as well as of non-ideal radial filters. Based on the MagLS renderer, we present a binaural rendering method for SMA recordings that integrates a comprehensive SMA model into the magnitude least squares objective. We evaluate the proposed end-to-end renderer by analyzing the reproduced binaural magnitude response. Our results suggest that the method significantly improves the highfrequency rendering mainly due to the inherent binaural diffusefield equalization, while it achieves a slight improvement in the low and mid frequency range, where the error of the conventional method is already small. A reference implementation of the method accompanies this paper.
The current emergency response concept for the International Space Station (ISS) includes the support of the Flight Control Team. Therefore, the team members need to be trained in emergencies and the corresponding crew procedures to ensure a smooth collaboration between crew and ground. In the case where the astronaut and ground personnel training is not collocated it is a challenging endeavor to ensure and maintain proper knowledge and skills for the Flight Control Team. Therefore, a virtual 3D simulator at the Columbus Control Center (Col-CC) is presented, which is used for ground personnel training in the on-board emergency response. The paper briefly introduces the main ISS emergency scenarios and the corresponding response strategy, details the resulting learning objectives for the Flight Controllers and elaborates on the new simulation method, which will be used in the future. The status of the 3D simulator, first experiences and further plans are discussed.
We present a method for decomposing a sound field into spherical harmonics (SH) based on observations of the sound field around the circumference of a human head. The method is based on the analytical solution for observations of the sound field along the equator of a rigid sphere that we presented recently. The present method incorporates a calibration stage in which the microphone signals for sound sources at a suitable set of calibration positions are projected onto the SH decomposition of the same sound field on the surface of a notional rigid sphere by means of a linear filtering operation. The filter coefficients are computed from the calibration data via a least-squares fit. We present an evaluation of the method based on binaural rendering of numerically simulated signals for an array of 18 microphones providing 8th SH order to demonstrate its effectiveness.
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