Beamforming in a reverberating environment with the use of measured steering vectors

The paper tackles the problem of aerodynamic noise measurements in wind tunnels with an acoustically hard closed test section. Data processing based on phased array techniques like the Classical Beamformer often fails due to the reflections caused by the test section walls. In a previous paper the authors proposed a modification of the Classical Beamformer called Reflection Canceller. This algorithm incorporates the reflections caused by rectangular test section walls by expanding the steering vectors with mirror sources. This algorithm performed well in simulations and measurements in a reverberating chamber and showed even an increased spatial resolution compared to the anechoic environment. Before the Reflection Canceller is used in real wind tunnel measurements the algorithm, foremost its basic quantity, the steering vector, has to be validated with some basic experiments. For this the steering vector was measured using a monopole point source. This was done in two steps, (1) in a closed test section wind tunnel without flow to confirm the effect of increased spatial resolution and (2) in a nonreflecting environment wind tunnel with flow to compare the measured steering vectors with the modeled ones. This paper shows results from both experiments. The first one, performed in the Laminar Wind Tunnel at the University of Stuttgart, shows the predicted increased spatial resolution. The second one, performed in the Acoustic Wind Tunnel at the Technical University of Dresden, shows good agreement between measured and modeled steering vectors. Finally an algorithm is presented which allows to increase the performance of multiplexing DAQ-Cards for steady state sound fields.