Multichannel equalisation for high-order spherical microphone arrays using beamformed channels

High-order spherical microphone arrays offer many practical benefits including relatively fine spatial resolution in all directions and rotation invariant processing using eigenbeams. Spatial filtering can reduce interference from noise and reverberation but in even moderately reverberant environments the beam pattern fails to suppress reverberation to a level adequate for typical applications. In this paper we investigate the feasibility of applying dereverberation by considering multiple beamformer outputs as channels to be dereverberated. In one realisation we process directly in the spherical harmonic domain where the beampatterns are mutually orthogonal. In a second realisation, which is not limited to spherical microphone arrays, beams are pointed in the direction of dominant reflections. Simulations demonstrate that in both cases reverberation is significantly reduced and, in the best case, clarity index is improved by 15 dB

Sampling and Reconstruction of Spatial Fields using Mobile Sensors

Spatial sampling is traditionally studied in a static setting where static sensors scattered around space take measurements of the spatial field at their locations. In this paper we study the emerging paradigm of sampling and reconstructing spatial fields using sensors that move through space. We show that mobile sensing offers some unique advantages over static sensing in sensing time-invariant bandlimited spatial fields. Since a moving sensor encounters such a spatial field along its path as a time-domain signal, a time-domain anti-aliasing filter can be employed prior to sampling the signal received at the sensor. Such a filtering procedure, when used by a configuration of sensors moving at constant speeds along equispaced parallel lines, leads to a complete suppression of spatial aliasing in the direction of motion of the sensors. We analytically quantify the advantage of using such a sampling scheme over a static sampling scheme by computing the reduction in sampling noise due to the filter. We also analyze the effects of non-uniform sensor speeds on the reconstruction accuracy. Using simulation examples we demonstrate the advantages of mobile sampling over static sampling in practical problems. We extend our analysis to sampling and reconstruction schemes for monitoring time-varying bandlimited fields using mobile sensors. We demonstrate that in some situations we require a lower density of sensors when using a mobile sensing scheme instead of the conventional static sensing scheme. The exact advantage is quantified for a problem of sampling and reconstructing an audio field.Comment: Submitted to IEEE Transactions on Signal Processing May 2012; revised Oct 201

Diffuseness quantification of a reverberation chamber and its uncertainty with fine-resolution measurements

Insufficient diffuseness is the major cause of poor inter-laboratory reproducibility of acoustic measurements conducted in a reverberation chamber. Many previous studies have proposed new methods to quantify the diffuseness of a reverberation chamber more accurately, but there is no general agreement among researchers on the most reliable method. The number of measurement samples required for these diffuseness metrics is also unclear, even though it significantly impacts the robustness of the methods. This study, therefore, aims to quantify the diffuseness of a reverberation chamber by using the three widely used diffuseness metrics of spatial variation of sound pressure levels, the relative standard deviation of decay rates, and the degree of time-series fluctuations. The measurements were also carried out with fine resolution microphone positions and varied configurations of acoustic diffusers. With the measurement data, the minimum number of measurement samples to obtain an accurate diffuseness quantification was determined. It is shown that nine independent microphone positions are sufficient to provide the acceptable confidence interval for frequencies above 315 Hz for all three metrics. However, twenty or more microphone positions are needed for the same accuracy if lower frequencies are considered for the reverberation chamber under investigation

Perzeptive Bewertung von Fehlereinflüssen bei der binauralen Auralisation von Kugelarraydaten unter Verwendung des Spatial Audio Quality Inventory - SAQI

When recording a sound field with a spherical microphone array, there always occur measurement errors. These errors can be spatial aliasing, microphone noise and positioning errors. These errors are noticeable as low frequency noise as well as distortion in the high frequency range. During playback of spatial sound fields, which can be recorded by spherical microphone arrays, these errors also have a negative effect on the auralisation quality. In this work these errors will be perceptually examined for binaural auralisation via headphones. For this purpose a listening test is designed to determine the perceptual thresholds of these errors. A second listening test, which is based on the repertory grid technique, is used to assign characteristics to the individual errors. To describe these errors, the recently published Spatial Audio Quality Inventory (SAQI) will be used, which is an up to date collection of quality descriptors. The analysis is used to determine whether the descriptive characteristics can be grouped so that recording errors and perceptual features can be linked. The features used in SAQI describe spatial properties and artifacts such as an increase in the low frequency range or a metallic timbre. A principal component analysis was conducted to organize and evaluate the data. Thus it is possible to identify important feature groups and to assess the perceptual characteristics of the error influences.Bei der Aufnahme von Schallfelder durch Kugelmikrofonarrays treten Messfehler auf. Diese Fehler können zum Beispiel räumliches Aliasing, Mikrofonrauschen und Positionierungsfehler sein. Bemerkbar machen sich diese Fehler als tieffrequentes Rauschen sowie Verzerrungen im hochfrequenten Bereich. Bei der Wiedergabe von räumlichen Schallfeldern, welche mittels Kugelmikrofonarrays aufgenommen werden können, wirken sich diese Fehler ebenso negativ auf die Auralisationsqualität aus. In dieser Arbeit sollen diese Fehler bei der binauralen Wiedergabe über Kopfhörer perzeptiv untersucht werden. Dazu werden zwei Hörtests durchgeführt. Ein Hörtest wurde konzipiert, um den Schwellwert der Wahrnehmung dieser Fehler zu bestimmen. Der zweite Hörtest, welcher auf der Repertory Grid Technik basiert, dient der Zuordnung von einzelnen Fehlern zu akustischen Merkmalen. Zur Beschreibung der Fehler wird das Spatial Audio Quality Inventory (SAQI) herangezogen, welches eine aktuelle Sammlung qualitätsbeschreibender Merkmale darstellt. Die verwendeten Merkmale aus SAQI beschreiben räumliche Eigenschaften und Artefakte wie beispielsweise eine Anhebung des tieffrequenten Bereichs oder eine metallische Klangfarbe. Eine Hauptkomponentenanalyse wurde durchgeführt um die Daten zu ordnen und zu bewerten. Damit ist es möglich, wichtige Merkmalsgruppen zu identifizieren und die perzeptiven Merkmale der Fehlereinflüsse zu bewerten.Ilmenau, Techn. Univ., Masterarbeit, 201

Binaural Reproduction of Higher Order Ambisonics - A Real-Time Implementation and Perceptual Improvements

During the last decade, Higher Order Ambisonics has become a popular way of capturing and reproducing sound fields. It can be combined with the theory of spherical microphone arrays to record sound fields, and this three-dimensional audio format can be reproduced with loudspeakers or headphones and even rotated around the listener. A drawback is that near perfect reproduction is only possible inside a sphere of radius r given by kr < N, where N is the Ambisonics order and k is the wavenumber. In this thesis, the theory of spherical harmonics and Higher Order Ambisonics has been reviewed and expanded, which serves as a foundation for a real-time system that was implemented. This system can record signals from a commercial spherical microphone array, convert them to the Higher Order Ambisonics format, and reproduce the sound field through headphones. To compensate for head motion, a head-tracking device is used. The real-time system operates with a latency of around 95 milliseconds between head motion and consequent sound field rotation. Further, two new methods for improving the headphone reproduction were assessed. These methods do not need to be applied in real-time, so no further system resources are used. Simulations of headphone reproduction with Higher Order Ambisonics show that both methods yield quantitative improvements in binaural cues such as the Interaural Level Difference, spectral cues and spectral coloration of the sound field. Median error values are reduced as much as 50 % between 4 and 7 kHz. The findings indicate that Higher Order Ambisonics reproduction over headphones can be improved at frequencies above limit frequency given by kr < N, but these findings need to be confirmed by subjective assessments, such as listening tests. The work conducted in this thesis has also resulted in a comprehensive basis for further development of a real-time three-dimensional audio reproduction system

Ambisonics

This open access book provides a concise explanation of the fundamentals and background of the surround sound recording and playback technology Ambisonics. It equips readers with the psychoacoustical, signal processing, acoustical, and mathematical knowledge needed to understand the inner workings of modern processing utilities, special equipment for recording, manipulation, and reproduction in the higher-order Ambisonic format. The book comes with various practical examples based on free software tools and open scientific data for reproducible research. The book’s introductory section offers a perspective on Ambisonics spanning from the origins of coincident recordings in the 1930s to the Ambisonic concepts of the 1970s, as well as classical ways of applying Ambisonics in first-order coincident sound scene recording and reproduction that have been practiced since the 1980s. As, from time to time, the underlying mathematics become quite involved, but should be comprehensive without sacrificing readability, the book includes an extensive mathematical appendix. The book offers readers a deeper understanding of Ambisonic technologies, and will especially benefit scientists, audio-system and audio-recording engineers. In the advanced sections of the book, fundamentals and modern techniques as higher-order Ambisonic decoding, 3D audio effects, and higher-order recording are explained. Those techniques are shown to be suitable to supply audience areas ranging from studio-sized to hundreds of listeners, or headphone-based playback, regardless whether it is live, interactive, or studio-produced 3D audio material