Protection layer for air-coupled waveguide ultrasonic phased arrays

Air-coupled ultrasonic phased arrays with acoustic waveguides offer grating-lobe-free beamforming for different applications such as obstacle detection, range finding, flow metering, acoustic levitation or tactile feedback. However, for real world applications the open outputs of the waveguide must be protected against fluids and dust in order to prevent clogging in the waveguide openings resulting in additional acoustic attenuation. In this work, we compare a hydrophobic fabric and a low density polyethylene (LDPE) foil regarding the insertion loss (IL), the influence on the beamforming, the blind zone in pulse-echo mode and the watertightness. Measurements are conducted in an anechoic chamber in transmit, receive and pulse-echo mode. The watertightness is determined with a submersion experiment in a water column of up to 2 m. The hydrophobic fabric has an IL of 1.8 dB±1 dB and does not effect the blind zone, whereas the LDPE foil has a higher IL of 7.5 dB±1 dB and increases the blind zone by 22%. The LDPE foil reaches IPX8, whereas the hydrophobic fabric reaches IPX7. However, in pulse-echo mode it is possible with both protection layers to detect a 10 mm-hollow steel sphere at a distance of 1 m. Their influence on the beamforming is within the measurement uncertainty.