Measurement-Based Modal Beamforming Using Planar Circular Microphone Arrays

This paper describes how to use a planar circular pressure-zone table-top microphone array for modal beamforming. Its goals are similar as for spherical arrays: higher-order resolution and a more-or-less steering-invariant beampattern design in the three-dimensional half space. As conventional circular arrays lack control of the beampattern in the vertical array plane, the proposed arrangement tries to fix this shortcoming to allow both horizontal and vertical control of beamforming. To provide a fully calibrated decomposition into the directional modes, the proposed beamforming approach is based on measurement data. From a MIMO (multiple-input-multiple-output) system description of the measurement data in the spherical harmonics domain, an inverse MIMO system of filters is designed for decomposing the microphone array signals into those spherical components eligible for modal beamforming. For an efficient measurement and robust set of decomposition filters, a reduced set of measurement positions and a regularisation strategy is suggested

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

The PerspectiveLiberator -- an upmixing 6DoF rendering plugin for single-perspective Ambisonic room impulse responses

Nowadays, virtual reality interfaces allow the user to change perspectives in six degrees of freedom (6DoF) virtually, and consistently with the visual part, the acoustic perspective needs to be updated interactively. Single-perspective rendering with dynamic head rotation already works quite reliably with upmixed first-order Ambisonic room impulse responses (ASDM, SIRR, etc.). This contribution presents a plugin to free the virtual perspective from the measured one by real-time perspective extrapolation: The PerspectiveLiberator. The plugin permits selecting between two different algorithms for directional resolution enhancement (ASDM, 4DE). And for its main task of convolution-based 6DoF rendering, the plugin detects and localizes prominent directional sound events in the early Ambisonic room impulse response and re-encodes them with direction, time of arrival, and level adapted to the variable perspective of the virtual listener. The diffuse residual is enhanced in directional resolution but remains unaffected by translatory movement to preserve as much of the original room impression as possible.Comment: 4 pages, submitted to conference: DAGA 2021, Vienna, Austria, 202

Transverse impedance of a resistive cylinder of finite length

In this report we calculate the transverse impedance of a charged particle beam, displaced from the axis of a circular cylindrical beam pipe which has finite resistivity only over a finite length. For this purpose we replace it with a cavity filled with the resistive material and connected to the region surrounding the beam by a gap of the same length. Matching of the electro-magnetic fields on both sides of the gap leads to an infinite set of linear equations for the field expansion coefficients with terms given by infinite sums over integrals which are evaluated by summing over their residues. By truncating these sums one can determine the transverse impedance and evaluate it numerically. Keeping only the lowest terms gives simple approximate expressions which are often sufficient to estimate the values and parameter dependence of the impedance