Optimization of thin film protection for waveguided ultrasonic phased arrays

Acoustic waveguides offer grating lobe-free beam-forming for air-coupled phased arrays. Due to the separation of the transducers aperture and the radiating aperture, the output of the phased arrays can be freely designed independent of the transducers geometry. However, since the output ports of the waveguide are open they can be easily clogged due to liquids, dirt or dust. In previous work, we investigated multiple solutions for this issue and concluded that there is a compromise between water resistance and minimal acoustic losses. In this work we reduced the losses of thin films to combine low insertion losses (IL) with high protection classes of up to IP 68. By introducing thermo-viscous effects to the waveguide, the resonance frequencies of the transducer and the film can be combined. In addition, the position of the film inside the waveguide was optimized and we switched from low density polyethylene (LDPE) to polylactic acid (PLA) as a film material. As a result, we were able to reduce the IL to 0.4 dB±1 dB while remaining the same water resistance from previous work.