  Unlike optical waves, acoustic waves in fluids are described by scalar
pressure fields, and therefore are considered spinless. Here, we demonstrate
experimentally the existence of spin in acoustics. In the interference of two
acoustic waves propagating perpendicularly to each other, we observed the spin
angular momentum in free space as a result of the rotation of local particle
velocity. We successfully measured the acoustic spin, and spin induced torque
acting on a lossy acoustic meta-atom that results from absorption of the spin
angular momentum. The acoustic spin is also observed in the evanescent field of
a guided mode traveling along a metamaterial waveguide. We found spin-momentum
locking in acoustic waves whose propagation direction is determined by the sign
of spin. The observed acoustic spin could open a new door in acoustics and
their applications for the control of wave propagation and particle rotation.
