Simulation of protection layers for air-coupled waveguided ultrasonic phased-arrays

Waveguided air-coupled ultrasonic phased arrays offer grating-lobe-free beam forming for many applications such as obstacle detection, non-destructive testing, flow metering or tactile feedback. However, for industrial applications, the open output ports of the waveguide can be clogged due to dust, liquids or dirt leading to additional acoustic attenuation. In previous work, we presented the effectiveness of hydrophobic fabrics as a protection layer for acoustic waveguides. In this work, we created a numerical model of the waveguide including the hydrophobic fabric allowing the prediction of the insertion loss (IL). The numerical model uses the boundary element method (BEM) and the finite element method (FEM) in the frequency domain including the waveguide, the hydrophobic fabric and the finite-sized rigid baffle used in the measurements. All walls are assumed as ideal sound hard and the transducers are ideal piston transducers. The specific flow resistivity of the hydrophobic fabric, which is required for the simulation, is analyzed using a 3D-printed flow pipe. The simulations are validated with a calibrated microphone in an anechoic chamber. The IL of the simulations are within the uncertainties of the measurements. In addition, both the measurements and the simulations have no significant influence on the beamforming capabilities.