GENERATING VIRTUAL MICROPHONE SIGNALS USING GEOMETRICAL INFORMATION GATHERED BY DISTRIBUTED ARRAYS Giovanni Del Galdo 1 , Oliver Thiergart 2 , Tobias Weller 1 , and Emanu¨ el A.P. Habets 2 1 Fraunhofer Institute for Integrated Circuits IIS, Erlangen, Germany 2 International Audio Laboratories Erlangen, Germany Email: ame-info@iis.fraunhofer.de ABSTRACT Conventional recording techniques for spatial audio are limited to the fact that the spatial image obtained is always relative to the po- sition in which the microphones have been physically placed. In many applications, however, it is desired to place the microphones outside the sound scene and yet be able to capture the sound from an arbitrary perspective. This contribution proposes a method to place a virtual microphone at an arbitrary point in space, by computing a signal perceptually similar to the one which would have been picked up if the microphone had been physically placed in the sound scene. The method relies on a parametric model of the sound field based on point-like isotropic sound sources. The required geometrical in- formation is gathered by two or more distributed microphone arrays. Measurement results demonstrate the applicability of the proposed method and reveal its limitations. Index Terms— Spatial sound, Sound localization, Audio recording, Parameter estimation 1. INTRODUCTION Spatial sound acquisition aims at capturing either an entire sound scene or just certain desired components, depending on the applica- tion at hand. Several recording techniques providing different advan- tages and drawbacks are available for these purposes. For instance, close talking microphones are often used for recording individual sound sources with high SNR and low reverberation, while more distant configurations such as XY stereophony represent a way for capturing the spatial image of an entire sound scene. More flexibility in terms of directivity can be achieved with beamforming, where a microphone array can be used to realize steerable pick-up patterns. Even more flexibility is provided by parametric methods, such as directional audio coding (DirAC) [1], in which it is possible to real- ize spatial filters with arbitrary pick-up patterns [2] as well as other signal processing manipulations of the sound scene [3, 4]. All these methods have in common that they are limited to a rep- resentation of the sound field with respect to only one point, namely the measurement location. Thus, the required microphones must be placed at very specific, carefully selected positions, e. g., close to the sources or such that the spatial image can be captured optimally. In many applications however, this is not feasible and therefore it would be beneficial to place several microphones further away from the sound sources and still be able to capture the sound as desired. There exist several field reconstruction methods for estimating the sound field in a point in space other than where it was measured. One method is acoustic holography [5], which allows to compute the sound field at any point within an arbitrary volume given that the sound pressure and particle velocity is known on its entire surface. Therefore, when the volume is large, an unpractically large num- ber of sensors is required. Moreover, the method assumes that no sound sources are present inside the volume, making the algorithm unfeasible for our needs. The related wave field extrapolation [5] aims at extrapolating the known sound field on the surface of a vol- ume to outer regions. The extrapolation accuracy however degrades rapidly for larger extrapolation distances as well as for extrapola- tions towards directions orthogonal to the direction of propagation of the sound [6]. In [7] a plane wave model is assumed, such that the field extrapolation is possible only in points far from the actual sound sources, i. e., close to the measurement point. To overcome the drawbacks of these field reconstructing meth- ods, this contribution proposes a parametric method capable of es- timating the sound signal of a virtual microphone placed at an arbi- trary location. In contrast to the methods previously described, the proposed method does not aim directly at reconstructing the sound field, but rather at providing sound that is perceptually similar to the one which would be picked up by a microphone physically placed at this location. This is possible thanks to a parametric model of the sound field based on isotropic point-like sound sources (IPLS). The required geometrical information, namely the instantaneous po- sition of all IPLS, is gathered via triangulation of the directions of arrival (DOA) estimated with two or more distributed microphone ar- rays. Therefore, knowledge on the relative position and orientation of the arrays is required. Notwithstanding, no a priori knowledge on the number and position of the actual sound sources is neces- sary. Given the parametric nature of the method, the virtual micro- phone can possess an arbitrary directivity pattern as well as physical or non-physical behaviors, e. g., with respect to the pressure decay with distance. The presented approach is verified by studying the pa- rameter estimation accuracy based on measurements in a reverberant environment. The paper is structured as follows: In Section 2 the sound field model is introduced and the geometric parameter estimation algo- rithm is derived. In Section 3 the virtual microphone approach is presented and discussed in detail. The algorithm is verified with measurement results in Section 4. Section 5 concludes the paper. 2. GEOMETRIC PARAMETER ESTIMATION 2.1. Sound Field Model The sound field is analyzed in the time-frequency domain, for in- stance obtained via a short-time Fourier transform (STFT), in which k and n denote the frequency and time indices, respectively. The complex pressure P v (k, n) at an arbitrary position pv for a certain 2011 Joint Workshop on Hands-free Speech Communication and Microphone Arrays May 30 - June 1, 2011 978-1-4577-0999-9/11/$26.00 ©2011 IEEE 185