Focusing airborne ultrasonic helical wavefronts using single-element transducers

Abstract

The feasibility to easily generate a focused airborne ultrasonic helical wavefront (HW) is demonstrated. Vortex beams of great quality are obtained by gluing an electroactive ferroelectret film on an oblique open helical surface. The resultant radiant surface of the device resembles that obtained by moving a skew line along a circular helix, i.e. a helico-conical surface. The resultant acoustic source combines two main features: a) An in-phase motion of its continuous radiating surface which allow for the HW generation and b) A concave conical structure of constant slope which concentrate the acoustic beam. Experimental results are obtained using a transducer prototype with outer diameter of 40 mm, inner diameter of 4 mm and inclination/ focalization angle of 70' with respect to the principal axis. The lead of the helix is 4.9 mm. Experimental characterization results are contrasted with simulations. Also, the dependency of the structure of the resultant HW on the geometrical parameters of the new device, along with a discussion of its potential of use, are included.