Spherical double-layer microphone array for 3D recordings and directional metrics measurements: Design and fabrication Pierre Grandjean, Philippe-Aubert Gauthier and Alain Berry
5. Tests, optimization and design
1. Context
The first investigation for SMA conception is the choice of microphone capsules. In order to occupy less space, MEMS microphones are investigated. Frequency response and directivity are measured for different capsules to chose the suitable solution.
In music, cinema, and/or video games, to enlarge listening experiments, sound field can be reproduced in 2D or 3D. For that, and thanks to loudspeaker array, methods such as Wave Field Synthesis (WFS) and High Order Ambisonic (HOA) were developed. Swept sine
2. Problem and objectives Sound field reproduction devices for 3D audio requires high mastering efforts and/or high computational resources. To circumvent this problem, recording directly sound in 3D is an appropriate solution. But 3D Spherical Microphones Array (SMA) have a limited reproduction area for a limited frequency range [1]. The main objective is to build a microphone array in order: • to have the largest possible frequency range • to accurately reproduce acoustic pressure and sound localization • to allow 3D reproduction on the widest possible area.
For the selection of the two radii of the double-layer array, the works from Jin are used [4]. A maximization of the bandwidth where SNR is above 20dB provides the optimal ratio ρ between the two radius. For our design, ρ = 1,557. From the optimal ρ, from the size of MEMS microphones, and for a 5-cm-radius inner rigid sphere, 3D models are realized with Solidworks. It validates the mechanical fabrication feasibility.
3. Evaluation of directional metrics Spherical harmonic (SH) theory allows for acoustic pressure extrapolation from spherical microphone recordings into a radius sphere [1-3]: is the wave number is the SH order
Direction Of Arrival (DOA) is a metrics which represents the acoustic intensity flow direction of propagation. It is a relevant predictor of sound sources localization. Acoustic pressure and particle velocity SH extrapolation from microphone array recordings allow for the DOA extrapolation into a radius sphere [3]:
Finally, acoustic comparative measures of MEMS flushed-mounted on a rigid sphere and on a rod, are carried out. Acoustics is not affect by rods.
6. Conclusion This poster present our design of a double-layer SMA to increase the size of the reproduction area in HOA and to increase the usability bandwidth. In the first step, spherical harmonic domain was studied and the possibility to record localization cues, in addition to acoustic pressure was considered. Then, to enlarge the 3D reproduction area, microphone characteristics are chosen:
4% error contour
• Double-layer with inner rigid sphere and outer open spheres • 50 microphones per layer, positioned on 50-node Lebedev grid Finally, MEMS caps are tested, ratio of rigid and open sphere radius is optimized to enlarge the frequency bandwidth and 3D models are achieved.
7. Future Works
4. Array geometry design To design a HOA SMA, microphones have to be positioned on a spherical grid which respects the orthonormality of SH. The SMA designed by P.Lecomte in [2] is composed of 50 microphones positioned on a 50-node Lebedev grid. The SMA go up to the SH order 5. B. Rafaely described the theory of spherical array processing [1]. Several SMA processing are presented as for microphones flush-mounted on a rigid sphere or arranged on an open sphere. Combination of rigid-sphere SMA and opensphere SMA as to form a spherical double layer microphone array is also investigated and allows for an extended frequency range.
Questions Questions should be addressed to Pierre Grandjean at:
[email protected]
Fabrication and assembly of the double-layer SMA based on the reported study. Design of the pre-amplification system for a 100-microphones SMA. Test the 3D sound field microphone array.
References [1] B. Rafaely (2015): Fundamentals of Spherical Array Processing. Chapter 4. Springer-Verlah Berlin Heidelberg , vol. 8, p 79-99. [2] P. Lecomte (2016): Ambisonie d’ordre élevé en 3D : Captation, transformation et décodage adaptatif de champs sonores. Thèse de doctorat, Conservatoire des Arts et Métiers, Paris, Université de Sherbrooke, Sherbrooke. . [3] P. Grandjean, P.-A. Gauthier and A. Berry (2018): Size of the accurate-reproduction area as function of directional metrics in spherical harmonic domain. AES Conference on Spatial Reproduction, Tokyo, Japan. [4] C.T. Jin, N. Epain and A. Parthy (2014): Design, Optimization and Evaluation of a Dual-Radius Spherical Microphone Array. IEEE Transaction on Audio, Speech, and Language Processing, Vol. 22, No. 1, p 193 - 204
