Hanchi Chen

Theory and design of compact hybrid microphone arrays on two-dimensional planes for three-dimensional soundfield analysis

Soundfield analysis based on spherical harmonic decomposition has been widely used in various applications; however, a drawback is the three-dimensional geometry of the microphone arrays. In this paper, a method to design two-dimensional planar microphone arrays that are capable of capturing three-dimensional (3D) spatial soundfields is proposed. Through the utilization of both omni-directional and first order microphones, the proposed microphone array is capable of measuring soundfield components that are undetectable to conventional planar omni-directional microphone arrays, thus providing the same functionality as 3D arrays designed for the same purpose. Simulations show that the accuracy of the planar microphone array is comparable to traditional spherical microphone arrays. Due to its compact shape, the proposed microphone array greatly increases the feasibility of 3D soundfield analysis techniques in real-world applications.

In-car noise field analysis and multi-zone noise cancellation quality estimation

The loudspeaker array plays a key role in an active noise cancellation (ANC) system. In most in car ANC systems, the cars pre-installed multimedia loudspeakers are employed as the secondary sources of the ANC system. In this paper, we evaluate the in-car loudspeaker systems capability in multi-zone noise field cancellation by analyzing the simultaneous noise field at multiple control regions inside a car cabin. We show that the average noise power in multi-zone spatial configurations can be expressed using a series of coefficients, and that the noise field can be decomposed into several basis noise patterns. Based on this model, we also estimate the integrated loudspeaker systems maximum noise cancellation capability, which can be used to assist design optimization. Through analyzing the noise field measurements in a car, we show that the cars integrated stereo loudspeaker system can attenuate the in-car noise by approximately 20 dB for the head position of two seats simultaneously, and up to 200 Hz.

Estimating the Direct-to-Reverberant Energy Ratio Using a Spherical Harmonics-Based Spatial Correlation Model

The direct-to-reverberant ratio (DRR), which describes the energy ratio between the direct and reverberant component of a soundfield, is an important parameter in many audio applications. In this paper, we present a multichannel algorithm, which utilizes the blind recordings of a spherical microphone array to estimate the DRR of interest. The algorithm is developed based on a spatial correlation model formulated in the spherical harmonics domain. This model expresses the cross correlation matrix of the recorded soundfield coefficients in terms of two spatial correlation matrices, one for direct sound and the other for reverberation. While the direct path arrives from the source, the reverberant path is considered to be a nondiffuse soundfield with varying directional gains. The direct and reverberant sound energies are estimated from the aforementioned spatial correlation model, which then leads to the DRR estimation. The practical feasibility of the proposed algorithm was evaluated using the speech corpus of the acoustic characterization of environments challenge. The experimental results revealed that the proposed method was able to effectively estimate the DRR of a large collection of reverberant speech recordings including various environmental noise types, room types and speakers.

Evaluation of spatial active noise cancellation performance using spherical harmonic analysis

This paper presents a novel metric to evaluate the performance of spatial active noise cancellation (ANC) systems. We show that the acoustic potential energy within a spherical region can be expressed by a weighed squared sum of spherical harmonic coefficients. The proposed metric allows convenient evaluation of spatial ANC performance using a spherical microphone array. In order to evaluate the effectiveness of this metric, we set up a experimental ANC system and conducted a series narrow band and wide band ANC experiments, the results show that the proposed potential energy method provides a reliable characterization of the performance of spatial ANC systems.

3D sound field analysis using circular higher-order microphone array

This paper proposes the theory and design of circular higher-order microphone arrays for 3D sound field analysis using spherical harmonics. Through employing the spherical harmonic translation theorem, the local spatial sound fields recorded by each higher-order microphone placed in the circular arrays are combined to form the sound field information of a large global spherical region. The proposed design reduces the number of the required sampling points and the geometrical complexity of microphone arrays. We develop a two-step method to calculate sound field coefficients using the proposed array structure, i) analytically combine local sound field coefficients on each circular array and ii) solve for global sound field coefficients using data from the first step. Simulation and experimental results show that the proposed array is capable of acquiring the full 3D sound field information over a relatively large spherical region with decent accuracy and computational simplicity.

A planar array of differential microphones for 3D sound capture

Three dimensional soundfield decomposition based on spherical harmonic analysis is becoming an integral part of 3D audio for virtual and augmented reality systems. Spherical harmonic analysis reveals the underlying characteristics of the soundfield, thus allowing high accuracy manipulation and analysis of the soundfield. This requires a microphone array with 3D pick up capability to detect the soundfield. A well-studied type of such array configuration is the spherical array. Since its geometry coincides with the spherical harmonics, the sound signal captured by a spherical microphone array is well-suited for the spherical harmonic transform. The placement of microphones on a spherical array has to follow a strict rule of orthogonality of the spherical harmonics which limits the flexibility of the array configuration. The spherical shape of the array also pose difficulties on implementation as well as practical usage. Recently, the authors presented the theory and design of a compact hybrid microphone arr...

Spherical harmonics based generalized image source method for simulating room acoustics

Allen and Berkleys image source method (ISM) is proven to be a very useful and popular technique for simulating the acoustic room transfer function (RTF) in reverberant rooms. It is based on the assumption that the source and receiver of interest are both omnidirectional. With the inherent directional nature of practical loudspeakers and the increasing use of directional microphones, the above assumption is often invalid. The main objective of this paper is to generalize the frequency domain ISM in the spherical harmonics domain such that it could simulate the RTF between practical transducers with higher-order directivity. This is achieved by decomposing transducer directivity patterns in terms of spherical harmonics and by applying the concept of image sources in spherical harmonics based propagation patterns. Therefore, from now on, any transducer can be modeled in the spherical harmonics domain with a realistic directivity pattern and incorporated with the proposed method to simulate room acoustics more accurately. We show that the proposed generalization also has an alternate use in terms of enabling RTF simulations for moving point-transducers inside pre-defined source and receiver regions.

The spatial coherence of noise fields evoked by continuous source distributions

In this work, analytic expressions for the spatial coherence of noise fields are derived in the modal domain with the aim of providing a sparse representation. For this purpose, the sound field in a region of interest is expressed in terms of a given pressure distribution on a virtual surrounding cylindrical or spherical surface. According to the Huygens-Fresnel principle, the sound pressure on this surface is represented by a continuous distribution of elementary line or point sources, where orthogonal basis functions characterize the spatial properties. To describe spatially windowed pressure distributions with arbitrary angular extensions, orthogonal basis functions of limited angular support are proposed. As special cases, circular and spherical pressure distributions with uncorrelated source modes of equal power are investigated. It is shown that these distributions result, respectively, in cylindrically isotropic and spherically isotropic, i.e., diffuse noise fields. The analytic expressions derived in this work allow for a prediction of the spatial coherence between arbitrary positions within the region of interest, such that no microphones need to be placed at the actual points of interest. Simulation results are presented to validate the derived relations.