A compact acoustic spanner to rotate macroscopic objects

Abstract

Waves can carry both linear and angular momentum. When the wave is transverse (e.g. light), the angular momentum can be characterised by the “spin” angular momentum associated with circular polarisation, and the “orbital” angular momentum (OAM) arising from the phase cross-section of the beam. When the wave is longitudinal (e.g. sound) there is no polarization and hence no spin angular momentum. However, a suitably phase-structured sound beam can still carry OAM. Observing the transfer of OAM from sound to a macroscopic object provides an excellent opportunity to study the exchange of energy between waves and matter. In this paper we show how to build a compact free-space acoustic spanner based on a 3D-printed sound-guiding structure and common electronic components. We first characterise the sound fields by measuring both phase and amplitude maps, and then show a video of our free-space acoustic spanner in action, in which macroscopic objects spin in a circular motion and change direction of rotation according to the handedness of the OAM acoustic field.

Real-time visualisation and optimisation of acoustic waves carrying orbital angular momentum

“…Our experimental setup, as shown in figure 1, consists of four parts: (i) a circular array of eight speakers with a funnel-shaped acoustic concentrator, (ii) a rubber sheet, (iii) a fringe illumination system, (iv) and a digital camera. The acoustic OAM generator is the same as that in [40] and is created using the circular array of speakers with programmed phase delays controlled by an Arduino microcontroller. The beam is focused using the acoustic concentrator and directed toward a thin silicon rubber sheet (0.1 mm in thickness).…”

“…Following the method of Toninelli et al [40], the acoustic OAM is created by a circular array of eight speakers driven at a frequency of 650 Hz. The speakers are controlled using the digital outputs of an Arduino Due microcontroller.…”

“…Since the first demonstration of OAM and vortices, several fundamental features, like amplitude and phase distributions [19], OAM transfer to matter [10,11,[36][37][38][39][40], and the rotational Doppler effect [11,[41][42][43][44][45], have been well studied in both optical and acoustic regimes. However, the instantaneous waveform for directly showing the helical structure of the beam hasn't been demonstrated due to the difficulties in recording the large-area fast-oscillating waveform.…”