A focused two-dimensional air-coupled ultrasonic array for non-contact generation

Air-coupled ultrasonic sources are relatively inefficient because the high impedance mismatch at the air/solid boundary means that most of the input energy (in air) is reflected at this boundary. The objective of this research is to increase efficiency-specifically an increase in ultrasonic signal amplitude-by designing and building a focused, 2D-array of electrostatic transducers (individual diameters of 38 mm). The operating frequency of this array is in the range of 50-100 kHz; this range is selected for civil infrastructure applications. Numerical simulations are used to design an array by modeling the pressure field in air, and then optimizing an array consisting of 20 transducers to create a line-source. An array is then built (following this design) and the emitted pressure field (in air) of the as built array is measured with a microphone and compared to the pressure field predicted by the numerical model. Finally, the as built focused array is used as an ultrasonic source, and its robustness is verified by comparing the numerical simulation of a transient line-load on an elastic half-space with (completely non-contact) experimentally measured values. There is excellent agreement between these two representations, which confirms the possibility of developing a completely non-contact, scanning ultrasonic system in the 50-100 kHz range.