Physics
Scientific paper
Jan 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008njph...10a3018s&link_type=abstract
New Journal of Physics, Volume 10, Issue 1, pp. 013018 (2008).
Physics
9
Scientific paper
Light carries a spin angular momentum associated with its polarization and an orbital angular momentum arising from its phase cross-section. Sound, being a longitudinal wave, carries no spin component but can carry an orbital component of angular momentum when endowed with an appropriate phase structure. Here, we use a circular array of loudspeakers driven at a common angular frequency ωs but with an azimuthally changing phase delay to create a sound wave with helical phase fronts described by exp (iellθ). Such waves are predicted to have an orbital angular momentum to energy ratio of ell/ωs. We confirm this angular momentum content by measuring its transfer to a suspended 60 cm diameter acoustic absorbing tile. The resulting torque on the tile (~6.1×10-6 Nm) is measured from observation of the motion for various torsional pendulums. Furthermore, we confirm the helical nature of the acoustic beam by observing the rotational Doppler shift, which results from a rotation between source and observer of angular velocity ωr. We measure Doppler shifted frequencies of ωs±ellωr depending on the direction of relative rotation.
Edgar M.
Padgett Miles J.
Skeldon Kenneth D.
Wilson Christine
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