Challenges in the Creation of Artificial Reverberation for Sound Field Synthesis: Early Reflections and Room Modes

Practical implementations of sound field synthesis evoke considerable artifacts that have to be considered in the creation of artificial reverberation. The most prominent artifact is spatial aliasing, which manifests itself as additional wave fronts that follow the desired synthetic wave front in time. These additional wave fronts propagate into different directions and occur at intervals that are similar to the intervals at which acoustic reflections occur in real rooms. It may be assumed that the human auditory system is not capable of differentiating aliasing artifacts and room reflections so that a synthetic reflection pattern should be designed such that it evokes a plausible pattern together with the aliased wave fronts. Two potential solutions are outlined. Finally, the capability of sound field synthesis of synthesizing room resonances (room modes) is analyzed and the promising results are illustrated based on numerical simulations.DFG, AH 269/2-1, Erarbeitung und Evaluierung von Methoden zur Generierung, Aufnahme und Wiedergabe von Hall für die Schallfeldsynthes

Ambisonic Encoding of Signals From Equatorial Microphone Arrays

The equatorial microphone array presented in (Ahrens et al., 2021) computes a spherical harmonic (SH) representation of a sound field based on pressure sensors along the equator of a rigid spherical baffle. The original formulation uses complex-valued SH basis functions. This is inconvenient if the SH representation of the captured sound field is intended to be stored in time domain by means of real-valued audio signals as it is common in the spatial audio format of ambisonics. The present document summarizes the modifications that need to be applied to the mathematical formulation from (Ahrens et al., 2021) to produce an ambisonic representation of the captured sound field that is compatible with the established ambisonic software tools like SPARTA and the IEM Plugin Suite. An example MATLAB script that implements this formulation is provided

Ambisonic Encoding of Signals From Spherical Microphone Arrays

This document illustrates how to process the signals from the microphones of a rigid-sphere higher-order ambisonic microphone array so that they are encoded with N3D normalization and ACN channel order and thereby can be used with the standard ambisonic software tools such as SPARTA and the IEM Plugin Suite. A MATLAB script is provided

Evaluation of Non-Spherical Scattering Bodies for Ambisonic Microphone Arrays

The XMA was recently presented, which is a higher-order ambisonic microphone array with a non-spherical scattering body. The approach is compatible with the also recently presented equatorial microphone array so that also XMAs can be designed with the microphones distributed solely on a circumferential contour around the scattering body. This greatly reduces the required number of microphones compared to classical spherical microphone arrays that require the microphones to be distributed over the entire surface of the scatterer. The equatorial XMA has so far only been evaluated as a head-mounted array, i.e. with a human head as the baffle. Other form factors of a range of sizes are also of practical relevance, particularly those form factors of 360 cameras as these are capable of capturing a complete panoramic audio-visual experience from a first-person view when combined with an equatorial XMA. We present a set of simulations based on which we identify what spherical harmonic orders can be obtained with what accuracy for a set of convex scattering body geometries that are of relevance in the given context. We demonstrate that the shape of the body is not very critical, and even corners are possible. The main limitation is that small bodies do not allow for extracting higher orders at low frequencies

Evaluation of Non-Spherical Scattering Bodies for Ambisonic Microphone Arrays

The XMA was recently presented, which is a higher-order ambisonic microphone array with a non-spherical scattering body. The approach is compatible with the also recently presented equatorial microphone array so that also XMAs can be designed with the microphones distributed solely on a circumferential contour around the scattering body. This greatly reduces the required number of microphones compared to classical spherical microphone arrays that require the microphones to be distributed over the entire surface of the scatterer. The equatorial XMA has so far only been evaluated as a head-mounted array, i.e. with a human head as the baffle. Other form factors of a range of sizes are also of practical relevance, particularly those form factors of 360 cameras as these are capable of capturing a complete panoramic audio-visual experience from a first-person view when combined with an equatorial XMA. We present a set of simulations based on which we identify what spherical harmonic orders can be obtained with what accuracy for a set of convex scattering body geometries that are of relevance in the given context. We demonstrate that the shape of the body is not very critical, and even corners are possible. The main limitation is that small bodies do not allow for extracting higher orders at low frequencies

Measurement-Based Modeling of Higher-Order Non-Linearities of the Parametric Loudspeaker

The so called ”Parametric Audio” effect is a way to create highly directional audible sound using ultrasonic carrier waves. It stems from an acoustical phenomena where two high frequency waves at high intensity will interfere and generate intermodulation tones due to non-linear effects in the wave-propagation. Most analytical models apply a second order approximation to predict the level of the au- dible sound. With a second order approximation it is not possible to compensate distortions in the audible sound caused by higher order non-linear effects. Higher order distortions in the audible sound were observed anecdo- tally by the authors, raising the question if they as well could be compensated.This paper describes a set of measurements of the am- plitudes of the harmonics of a bifrequency plane wave, and compares the measured results with a simple theo- retical model. The measured results indicate that there is significant nonlinear distortion in the electrical and/or mechanical subsystems. Even if a model is developed to predict and compensate for said distortion the model would be tied to a specific amplifier and transducer com- bination, with very limited real-life applicability

Modeling continuous source distributions in wave-based virtual acoustics

All acoustic sources are of finite spatial extent. In volumetric wave-based simulation approaches (including, e.g., the finite difference time domain method among many others), a direct approach is to represent such continuous source distributions in terms of a collection of point-like sources at grid locations. Such a representation requires interpolation over the grid and leads to common staircasing effects, particularly under rotation or translation of the distribution. In this article, a different representation is shown, based on a spherical harmonic representation of a given distribution. The source itself is decoupled from any particular arrangement of grid points, and is compactly represented as a series of filter responses used to drive a canonical set of source terms, each activating a given spherical harmonic directivity pattern. Such filter responses are derived for a variety of commonly encountered distributions. Simulation results are presented, illustrating various features of such a representation, including convergence, behaviour under rotation, the extension to the time varying case, and differences in computational cost relative to standard grid-based source representations

Computation of spherical harmonic representations of source directivity based on the finite-distance signature

The measurement of directivity for sound sources that are not electroacoustic transducers is fundamentally limited because the source cannot be driven with arbitrary signals. A consequence is that directivity can only be measured at a sparse set of frequencies—for example, at the stable partial oscillations of a steady tone played by a musical instrument or from the human voice. This limitation prevents the data from being used in certain applications such as time-domain room acoustic simulations where the directivity needs to be available at all frequencies in the frequency range of interest. We demonstrate in this article that imposing the signature of the directivity that is obtained at a given distance on a spherical wave allows for all interpolation that is required for obtaining a complete spherical harmonic representation of the source’s directivity, i.e., a representation that is viable at any frequency, in any direction, and at any distance. Our approach is inspired by the far-field signature of exterior sound fields. It is not capable of incorporating the phase of the directivity directly. We argue based on directivity measurement data of musical instruments that the phase of such measurement data is too unreliable or too ambiguous to be useful. We incorporate numerically-derived directivity into the example application of finite difference time domain simulation of the acoustic field, which has not been possible previously