A 220 kHz Air-Coupled Spiral Ultrasonic Phased Array Using Waveguides

The freedom of air-coupled phased array design is restricted due to the size constraints of off-the-shelf transducers. Waveguides can be a solution, because they allow to design the element aperture as well as the array aperture independently of transducer sizes. Recent work (Rutsch et al. 2021) has demonstrated the feasibility of air-coupled waveguides at frequencies around 40 kHz for phased arrays. This work explores the use of waveguides at a higher frequency of 220 kHz with commercially available piezoelectric ultrasonic transducers for creating air-coupled phased arrays, since higher frequency allows higher resolution. We employ tapered, cylindrical 3D-printed waveguides of equal lengths in an arc shape, with each 220 kHz transducer individually controllable for 3D beam steering. A spiral arrangement of the transducers is chosen for its lower maximum side lobe level and independence of the λ/2 criterion. The point spread function of the resulting phased array is characterized in an anechoic chamber. Results confirm the feasibility of the waveguided phased array at 220 kHz, with a -3 dB main lobe width of 9° and a maximum side lobe level of -10 dB. Therefore the waveguide approach is feasible even at higher frequencies such as 220 kHz for selecting the aperture independent of the transducer.