Patch holography using a double layer microphone array

In contrast to several techniques in Near-field Acoustical Holography, the Inverse Boundary Element Method (IBEM) can be used on arbitrary source geometries. In IBEM one must define a closed surface which confines/excludes all sources. This surface is subsequently discretized by an element mesh. Normally, the mesh coincides with the boundary of the source, which means that the entire structure must be modeled even if only a small patch reconstruction is of interest. The long computation time when dealing with large sources is one of the main disadvantages with this approach. However, by defining a closed local element mesh that surrounds the microphone array, and with a part of the mesh coinciding with a patch, the entire source is not needed in the model. Since the array has two layers, sources/reflections behind the array are also allowed. The Equivalent Source Method (ESM) is another technique in which the sound field is represented by a set of monopoles placed inside the source. In this paper these monopoles are distributed so that they surround the array, and the reconstruction is compared with the IBEM-based approach. The comparisons are based on computer simulations with a planar double layer array and sources with different shapes.