Acoustic beams with modulated helicity and a simple helicity-selective acoustic reciever

• Published in: The Journal of the Acoustical Society of America 2009-10, Vol.126 (4_Supplement), p.2187-2187
• Main Authors: Marston, Timothy M., Marston, Philip L.
• Format: Article
• Language: English
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.3248558

Acoustic helicoidal beams have an azimuthal phase gradient and an axial null in amplitude. The sign and magnitude of the azimuthal phase gradient determine the helicity and topological charge of the beam. Beams with a unit-magnitude topological charge have been generated with a four-quadrant sectored array, adjacent quadrants being driven with a 90 deg phase offset [B. T. Hefner and P. L. Marston, J. Acoust. Soc. Am. 106, 3313–3316 (1999)]. The scattering by symmetric targets placed on the axis of such beams preserves the helicity and the axial null [P. L. Marston, J. Acoust. Soc. Am. 124, 2905–2910 (2008); F. G. Mitri, Ann. Phys. 323, 2840–2850 (2008)]. The present work shows how to rapidly alter (or modulate) the helicity with an appropriately timed reversal of the excitation of a single pair of opposite quadrants. A simple four-element helicity-selective detector (using appropriately offset elements) was also demonstrated. These beams may be useful for digital acoustic communication and they have a modulated axial angular momentum flux. They should also generate a modulated acoustic torque on targets. [Work partially supported by ONR.]