Fiete Winter

On analytic methods for 2.5-d local sound field synthesis using circular distributions of secondary sources

Local sound field synthesis allows for synthesizing a given desired sound field inside a limited target region such that the field is free of considerable spatial aliasing artifacts. Spatial aliasing artifacts are a consequence of overlaps due to unavoidable repetitions of the space-spectral coefficients of the secondary source driving function. We analyze various conceivable analytic ways of restricting the bandwidth of the spatial spectrum of the driving function such that considerable overlapping is prevented: local spatial bandlimitation (A), spectral windowing (B), and local spatial band-limitation plus spectral windowing (C). While solution B is computationally significantly more efficient than A and C, it provides only limited control over the spatial location around which the aliasing-free region evolves. Solutions A and C provide more flexibility and higher accuracy whereby both achieve largely identical results so that the spectral windowing after the local spatial bandlimitation may be skipped. We present a detailed analysis of the properties of the spatial aliasing artifacts arising in the synthesis of a virtual plane wave. We establish a procedure for predicting the maximum possible size of the aliasing-free target region depending on its location and on the propagation direction of the desired sound field. The results can help reducing regularization in numerical solutions as they represent physical limitations that can be considered in the choice of parameters.

A comparison of modal and spatially matched-filter beamforming for rigid spherical microphone arrays in the context of data-based binaural synthesis

Several approaches to data-based binaural synthesis have been published that capture a sound field by means of a spherical microphone array. The captured sound field is decomposed into plane waves, which are auralized using (far-field) head-related impulse responses (HRIRs). In practice, the decomposition into plane waves is performed by beamforming. A well-known technique for spherical microphone arrays is modal beamforming where the sound field is represented with respect to surface spherical harmonics. A numerically stable approximation is the spatio-temporal matched-filter technique. Here the acoustic reciprocity theorem is used in conjunction with the matched filter technique for beamforming. This paper reviews the matched filter beamformer and compares its properties to the modal beamformer in a realistic scenario. The performance of the two beamforming techniques is compared in the context of data-based binaural synthesis. Time-domain properties of the resulting ear-signals, as well as suitable binaural measures, are investigated for simulated and captured scenarios.Several approaches to data-based binaural synthesis have been published that capture a sound field by means of a spherical microphone array. The captured sound field is decomposed into plane waves, which are auralized using (far-field) head-related impulse responses (HRIRs). In practice, the decomposition into plane waves is performed by beamforming. A well-known technique for spherical microphone arrays is modal beamforming where the sound field is represented with respect to surface spherical harmonics. A numerically stable approximation is the spatio-temporal matched-filter technique. Here the acoustic reciprocity theorem is used in conjunction with the matched filter technique for beamforming. This paper reviews the matched filter beamformer and compares its properties to the modal beamformer in a realistic scenario. The performance of the two beamforming techniques is compared in the context of data-based binaural synthesis. Time-domain properties of the resulting ear-signals, as well as suitable bin...

Synthesis of a spatially band-limited plane wave in the time-domain using wave field synthesis

Wave Field Synthesis (WFS) is a spatial sound reproduction technique aiming at a physically accurate reconstruction of a desired sound field within an extended listening area. It was shown in a recent study that the accuracy of the synthesized sound field can be improved in a local area by applying a spatial band-limitation to the driving function. However, the computational complexity of the frequency-domain driving function is demanding because of the involved Bessel functions. In this paper, a time-domain WFS driving function is introduced for the synthesis of a spatially band-limited plane wave. The driving function is obtained based on a time-domain representation of the sound field which is given as a superposition of plane waves with time-varying direction and amplitude. The performance of the proposed approach is evaluated by numerical simulations. Practical issues regarding the discretization of the analytic driving function and dynamic range control are discussed.

Open science in the Two!Ears project—Experiences and best practices

Two!Ears was an EU funded project for binaural auditory modeling with ten international partners involved. One of the project goals was to follow an Open Science approach in all stages. This turned out to be a challenging task as the project involved huge amounts of software, acoustical measurements, and data from listening tests. On the other hand, it was obvious from the positive experience with the Auditory Modelling Toolbox that an Open Science approach would have a positive impact and foster progression afterwards. As there existed no ready solution to achieve this goal at the beginning of the project, different paths for data management were tested. It was especially challenging to provide a solution for data storage. Here, the goal was not only the long term accessibility of the data, but also the revision control of public and private data for the development inside the project. In the end, the project was able to make most of its software and data publicly available, but struggled to apply the re...

Azimuthal localization in 2.5D near-field-compensated higher order ambisonics

Sound Field Synthesis approaches aim at the reconstruction of a desired sound field in a defined target region using a distribution of loudspeakers. Near-Field Compensated Higher Order Ambisonics (NFCHOA) is a prominent example of such techniques. In practical implementations different artifacts are introduced to the synthesized sound field: spatial aliasing is caused by the non-zero distance between the loudspeakers. Modal bandwidth limitation is a well-established approach to reduce spatial aliasing in 2.5D NFCHOA, but introduces temporal and spectral impairments to the reproduced sound field which strongly depend on the relative position to the center of modal expansion. Also, the dimensionality mismatch in a 2.5D synthesis scenario results in a different amplitude decay compared to the desired sound field. Listening experiments already investigated the azimuthal localization in 2.5D NFCHOA. It is however unclear, in how far individual artifacts caused by spatial sampling, modal bandwidth limitation, a...