Two-dimensional acoustic particle velocity sensors based on a crossing wires topology

This thesis describes the design and realization of two-dimensional acoustic particle velocity sensors based on thermal convection. The sensors are of the order of 1 mm×1 mm and consist of two crossing wires with each wire sensing the acoustic particle velocity in the direction parallel to it. Their small size allows the measurement of particle velocity with high spacial resolution and minimal disturbance of the sound field. Due to the inherent geometry of the design, the sensors have equal sensitivity in both directions and polar patterns exactly orthogonal to each other. Therefore, no calibration is required. Sensors with varying electrical and mechanical properties were designed, fabricated and characterized. The results give insight in the optimum sensor configuration as well as the influence of each aspect on the quality of the sensor in terms of sensitivity, bandwidth and power consumption. The influence of oscillatory boundary layer effects on the performance of the sensor was studied in more detail and lead to a sensor design optimized for low frequency measurements. The impact of metal films on the performance of the crossed-wires particle velocity was studied. This new insight resulted in the fabrication of sensors with higher sensitivity across the entire bandwidth, better reproducibility and long term stability. Experiments in liquid environment show that this design is also suitable for the measurement of particle velocity in such environments. While further investigation is needed to optimize the design for underwater measurements, the current results are very promising. The devices are smaller and mechanically more robust than existing parallel-wires particle velocity sensors. This allows for higher resistance against drag forces during high intensity measurements, harsh fabrication steps and, consequently, higher fabrication yield. Contrary to the fabrication method used for previous versions of particle velocity sensors which relied on dicing, the crossed-wires design allows a break-out method to be used for separating the chips from the wafers. This technique is faster, cheaper and less harsh on the sensors hence further increasing yield. These sensors require only four electrical connections to the external electrical circuit reducing assembly time and thus also costs. In conclusion, the design proves to be versatile and highly customizable allowing it to be tailored to applications with specific requirements in regard to sensitivity, mechanical robustness, power consumption and frequency region of interest.