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DETERMINATION OF ABSOLUTE LEVELS FROM PHASED ARRAY MEASUREMENTS USING SPATIAL SOURCE COHERENCE

¶The phased array technique is a valuable tool in acoustic testing for its capability to distinguish between different source locations. However, the interpretation of phased array measurements is still difficult due to the simultaneous occurrence of several effects: the size and level of a spot in a conventional acoustic 'source plot' may be affected by a combination of (1) the limited resolution of the array (2) coherence loss during propagation to the array (3) the spatial extent of the source region. This ambiguity complicates the determination of absolute source levels from phased array measurements. The current paper addresses this problem for a noise source that is extended mainly in one direction, i.e. trailing-edge noise. Simulations are done for a line source, and the influence of array resolution and source coherence length on the array output is investigated. Furthermore, an array processing technique is presented which determines the coherence level between different sources in the scan plane. As a first application, the technique is used to identify mirror sources in a closed wind tunnel. The new method is then applied to trailing-edge noise measurements in NLR's Small Anechoic Wind Tunnel, in order to estimate the spanwise coherence length. In conjunction with the simulations this enables an improved determination of absolute trailing-edge noise from phased array measurements.