Design of a microphone array technique for a wind tunnel application
Examensarbete för masterexamen
Until now most improvements in noise reduction in passenger cars involves tyres and wheel suspension, improved materials and designs in the engine compartment. These are the dominating sources experienced in the passenger compartment at low speeds, but for higher speeds the exterior flow is the dominating source. Therefore it is important to be able to predict and reduce this source early in the car design. One method to measure and evaluate car geometries in favour of wind noise in an ordinary wind tunnel, is the use of a microphone array combined with signal processing. The objective of this work is to design a microphone array technique for measuring and localizing sound sources in a wind tunnel. Particularly, the situation with a side mirror model placed in the wind tunnel at Chalmers University of Technology, and to sort out if the technique is appropriate for that situation. The sound propagation in a wind tunnel is influenced by the wind flow. To describe the sound propagation in windy conditions the Fast Field Program (FFP) method is chosen, and modified for a wind tunnel application. Standard array techniques for localizing sources are studied for conditions without wind. The selected techniques are verified by simulations and experiments. The standard technique is adapted to a wind tunnel application, including a wind correction calculated with the modified FFP method. Finally the microphone array technique is tested with simulations. Simulations with the microphone array technique show that sources are localized at the correct positions, even at high wind speeds. The simulations also show that various wind profiles have different influence on the source localization. Sources with different frequency content can be separated by the use of filtering, while sources with the same frequency content are hard to separate. The microphone array technique is not completely appropriate for the side mirror model placed in the Chalmers' wind tunnel, because of limited performance in the interesting frequency range, due to the small dimensions. The microphone array technique needs to be further developled and tested with experiments, before implementing it in the wind tunnel.
Samhällsbyggnadsteknik , Civil Engineering