Reduction of the Hydrodynamic Noise on a Beamforming Array

The design and build process of an 80 microphone beamforming array is reported. The array is designed to be installed on the wall of a large closed circuit wind tunnel, primarily for helicopter rotor noise measurement and source identification studies. The acoustic data collected using the array is prone to contamination due to the presence of a strong hydrodynamic field on the wall of the wind tunnel, caused by the wall boundary layer and also the down-wash flow from the helicopter rig. A study has been carried out for the reduction of the hydrodynamic noise using different stainless steel wire-mesh materials, recessing the microphones behind the screen and adding porous foam material in the space behind the screen and microphones. The effect of these treatments are investigated and evaluated for a number of test cases. In addition, different beamforming algorithms, such as the delay-and-sum and functional beamforming are used and compared in this study for different type of noise sources over the entire frequency range of interest. The designed array can deliver a dynamic range of 13-15 dB and low sidelobe levels.