Multi-Layer Planar Near Field Holography using MEMS microphone arrays in a noisy environment
University of Technology Eindhoven
Wednesday 3 june, 2015, 09:00 - 09:20
0.8 Rome (118)
Near-field acoustic measurements in noisy environments suffer from interference generated by undesired sources, especially when these are located close to the measurement equipment. Planar Near-field Acoustic Holography (PNAH) using a single layer pressure sensor array, is a proven method for determining sound pressure fields at the source of vibrating objects in high detail, however, its accuracy is impacted when an undesired source is present closely behind the measurement array. A few examples are performing acoustic measurements of vehicle interiors or inside of devices such as power generators or computers. This paper presents a method for performing PNAH using multiple pressure sensor arrays. The advantages of this method are twofold. Firstly, it has the ability to measure in environments where undesired sources are present behind the measurement plane, especially when these sources are in close proximity to the array. Secondly, assuming only pressure measurements, a measurement array for the method can be easily constructed using MEMS microphone technology. This paper will first describe the Multi-Layer method, then cover some of the difficulties when implementing and using it such as the half wavelength condition needed for non-aliased measurements and the signal processing techniques used to improve quality of the results and finally show the advantages of this method in noisy environments. The results are demonstrated by measurements using two MEMS microphone arrays which show that Multi-Layer PNAH is a qualitative improvement over single layer PNAH in both noisy but also non-noisy environments and that it can be easily implemented and used in practice by utilizing MEMS technology.
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