Jonathan Sheaffer

Physical and numerical constraints in source modeling for finite difference simulation of room acoustics.

In finite difference time domain simulation of room acoustics, source functions are subject to various constraints. These depend on the way sources are injected into the grid and on the chosen parameters of the numerical scheme being used. This paper addresses the issue of selecting and designing sources for finite difference simulation, by first reviewing associated aims and constraints, and evaluating existing source models against these criteria. The process of exciting a model is generalized by introducing a system of three cascaded filters, respectively, characterizing the driving pulse, the source mechanics, and the injection of the resulting source function into the grid. It is shown that hard, soft, and transparent sources can be seen as special cases within this unified approach. Starting from the mechanics of a small pulsating sphere, a parametric source model is formulated by specifying suitable filters. This physically constrained source model is numerically consistent, does not scatter incoming waves, and is free from zero- and low-frequency artifacts. Simulation results are employed for comparison with existing source formulations in terms of meeting the spectral and temporal requirements on the outward propagating wave.

Bearing-only acoustic tracking of moving speakers for robot audition

This paper focuses on speaker tracking in robot audition for human-robot interaction. Using only acoustic signals, speaker tracking in enclosed spaces is subject to missing detections and spurious clutter measurements due to speech inactivity, reverberation and interference. Furthermore, many acoustic localization approaches estimate speaker direction, hence providing bearing-only measurements without range information. This paper presents a probability hypothesis density (PHD) tracker that augments the bearing-only speaker directions of arrival with a cloud of range hypotheses at speaker initiation and propagates the random variates through time. Furthermore, due to their formulation PHD filters explicitly model, and hence provide robustness against, clutter and missing detections. The approach is verified using experimental results.

Binaural reproduction of finite difference simulations using spherical array processing

Due to its efficiency and simplicity, the finite-difference time-domain method is becoming a popular choice for solving wideband, transient problems in various fields of acoustics. So far, the issue of extracting a binaural response from finite difference simulations has only been discussed in the context of embedding a listener geometry in the grid. In this paper, we propose and study a method for binaural response rendering based on a spatial decomposition of the sound field. The finite difference grid is locally sampled using a volumetric array of receivers, from which a plane wave density function is computed and integrated with free-field head related transfer functions, in the spherical harmonics domain. The volumetric array is studied in terms of numerical robustness and spatial aliasing. Analytic formulas that predict the performance of the array are developed, facilitating spatial resolution analysis and numerical binaural response analysis for a number of finite difference schemes. Particular emphasis is placed on the effects of numerical dispersion on array processing and on the resulting binaural responses. Our method is compared to a binaural simulation based on the image method. Results indicate good spatial and temporal agreement between the two methods.

Spectral equalization in binaural signals represented by order-truncated spherical harmonics

The synthesis of binaural signals from spherical microphone array recordings has been recently proposed. The limited spatial resolution of the reproduced signal due to order-limited reproduction has been previously investigated perceptually, showing spatial perception ramifications, such as poor source localization and limited externalization. Furthermore, this spatial order limitation also has a detrimental effect on the frequency content of the signal and its perceived timbre, due to the rapid roll-off at high frequencies. In this paper, the underlying causes of this spectral roll-off are described mathematically and investigated numerically. A digital filter that equalizes the frequency spectrum of a low spatial order signal is introduced and evaluated. A comprehensive listening test was conducted to study the influence of the filter on the perception of the reproduced sound. Results indicate that the suggested filter is beneficial for restoring the timbral composition of order-truncated binaural signals, while conserving, and even improving, some spatial properties of the signal.

Robust plane-wave decomposition of spherical microphone array recordings for binaural sound reproduction

Rendering binaural signals from spherical microphone recordings is becoming an increasingly popular approach, with applications in telecommunications, virtual acoustics, hearing science, and entertainment. Such binaural signals can be generated from a plane-wave decomposition of a sound field measured by a spherical microphone array. This process may exhibit ill-conditioned transformations when performed at low frequencies and using high spherical-harmonics orders, thus resulting in a poor robustness to measurement inaccuracies and noise. Previous studies have addressed this issue by employing standard regularization techniques, such as diagonal loading and radial filter limiting. In this paper, we propose an optimal frequency-dependent regularization method that balances system robustness to measurement noise against accuracy of plane-wave decomposition. Unlike previously suggested approaches, the proposed method analytically relates the measured signal-to-noise ratio to the corresponding regularization ...

Equivalence of plane wave and spherical harmonics rendering of binaural room impulse response

Binaural technology has various applications in virtual acoustics, architectural acoustics, tele-communications, and auditory science. One key element in binaural technology is the binaural room impulse response (BRIR), which represents a continuum of plane waves spatially filtered by head related transfer functions (HRTFs). Such BRIRs can be rendered from spherical microphone array recordings and free-field HRTFs, either in the space domain using plane-wave composition or in the spherical-harmonics domain using order-limited spherical harmonics representation of the sound field. While these approaches have been individually employed in a number of recent studies, it appears that the literature does not offer a comprehensive analysis or a theoretical framework relating the two representations with respect to binaural reproduction and perception. In this paper, we provide a mathematical analysis showing that when certain sampling conditions are maintained, the plane-wave and spherical-harmonics representat...

Equalization strategies for binaural room impulse response rendering using spherical arrays

Reconstruction of binaural room impulse responses (BRIRs) from spherical microphone array measurements in a room and a given head-related transfer function set, is beneficial for binaural reproduction with listener individualization, and for applying head rotations without needing to make numerous measurements of the sound field. Such algorithms often entail the truncation of BRIRs at low spherical harmonic orders, producing reduced reconstruction accuracy at high frequencies and accordingly affecting the perceived timbre. With the aim of restoring high-frequency energy and spatial cues, recent studies have proposed the use of an efficient equalization filter based on a spherical head model, or by direct numerical integration in the space domain. This paper extends the equalization approach by introducing a filter that takes into account the full geometric features of the human head. The proposed filter, as well as previously suggested algorithms, are reviewed and then subjectively evaluated in both measured and simulated environments. A MUSHRA listening experiment is employed to investigate the perceptual effects of equalizing order-truncated BRIRs. Results indicate a clear improvement in timbre, and a positive impact on spatial localization in low order cases.

Modelling binaural receivers in finite difference simulation of room acoustics

Binaural room impulse responses are important for auralisation as well as for objective research in room acoustics. In geometrical room simulation methods, obtaining such responses is easily achieved by convolving each computed reflection tap with a corresponding pre-measured angle-dependent head-related impulse response. Unfortunately, employing such an approach in wave based methods is challenging due to temporal overlap of room reflections in the calculated response. One alternative is to physically embed a listener geometry in the grid. Whilst this method is straightforward, it requires voxelization of a geometrically-complex object. Furthermore, with non-conformal boundary conditions, the voxelized geometry is sample-rate dependent, meaning that numerical consistency may become compromised. In this paper we discuss the merits and drawbacks of embedding different listener geometries in the grid, ranging from a simple rigid sphere to a fully featured laser-scan of a Kemar mannikin. Results are analysed...

Binaural perception of direct and reverberant sound fields rendered with mixed-order spherical harmonics

Binaural responses can be rendered from a plane-wave decomposition of a measured or a modeled sound field, spatially integrated with free-field head-related transfer functions. When represented in the spherical-harmonics domain, the decomposition order reflects the maximum spatial resolution, which is limited by the number of microphones in the spherical array. Recent studies suggest a direct relationship between decomposition order and perceptual attributes such as localization blur, timbre, and the sense of externalization. Insofar, studies have employed plane wave density functions in which the different components of the sound field were uniformly decomposed at a single spherical-harmonics order. This work is concerned with binaural signals in which the direct and reverberant parts of the sound field are decomposed at different spherical-harmonics orders. The direct component of the sound field carries significant directional information utilized by the auditory system. Therefore, changing the spheric...