The investigation of eXMA method with non-spherical scatters
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Examensarbete för masterexamen
Master's Thesis
Master's Thesis
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Model builders
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Abstract
The XMA was a recently presented higher-order ambisonic microphone array which
is based on the spherical microphone array (SMA) and equatorial microphone array
(EMA) but without a traditional spherical scattering body. Since it is compatible
with the EMA, the XMAs can also be designed with the microphones placed on a
circumferential contour around the scattering body, which is called the equatorial
XMA (eXMA). Compared with the classical SMAs, the eXMA method reduced the
required number of microphones significantly since it did not need the microphones
to be distributed over the whole surface of the scatterer. The eXMA shows a good
application prospect in spatial sound field recording especially when combined with
the VR camera to produce a complete panoramic audio-visual experience from a
first-person view. However, the eXMA has so far only been evaluated as a headmounted
array, i.e. with a human head as the baffle. The performance of eXMA
with other shapes of scatterers are unknown.
In this work, we used the mesh2hrtf implementation of the boundary element method
(BEM) to simulate eXMA calibration measurements for a variety of candidate scatterers
including cylinders, cubics and some shapes that are inspired from real VR
360 cameras. We also deformed those shapes and moved up the microphone array
to see the influence. Based on those simulations, we identify what spherical harmonic
orders can be obtained with what accuracy for a set of convex scattering body
geometries that are of relevance in the given context.
We demonstrate that the shape of the body is not very critical. The eXMA shows
very robust performances with the different shapes of scatterers, some of them even
have corners. Reducing the height of the scatterers or moving up the microphone
array to the edge will increase the error but the accuracy is still acceptable. The
main limitation is the size of the scatters that small bodies do not allow for extracting
higher orders at low frequencies. Limitations of the simulation are discussed and at
the end we also generate some spatial audio recordings based on the cuboid and the
squashed cylinder scatterers.
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Keywords
Spatial Audio, SMA, EMA, XMA, Ambisonic, Spherical Harmonics