Beamforming accuracy in closed-section wind tunnels

The ability to perform aeroacoustic measurements in closed-section wind tunnels is important in vehicle design, but presents significant problems compared with anechoic open-jet tunnel testing. The use of microphone arrays and beamforming processing can yield useful source localisation information, but there are concerns as to the consistency of quantitative results from such methods. Most of the limitations are due to the assumption of uncorrelated monopoles having uniform spherical directivity patterns. However there are other effects, such as the acoustic properties of the measuring environment and the position of the source with respect to nearby rigid boundaries. These effects are investigated in this paper using a calibration source with a known sound power level. It was found that the errors in the beamformer levels are frequency dependent, and also depend on the position of the source with respect to nearby boundaries. An image source model can give improvements in the beamformer accuracy, however the frequency-dependence of the errors could only be minimised by using measured transfer functions.