  We examine acoustic radiation force and torque on a small (subwavelength)
absorbing isotropic particle immersed in a monochromatic (but generally
inhomogeneous) sound-wave field. We show that by introducing the monopole and
dipole polarizabilities of the particle, the problem can be treated in a way
similar to the well-studied optical forces and torques on dipole Rayleigh
particles. We derive simple analytical expressions for the acoustic force
(including both the gradient and scattering forces) and torque. Importantly,
these expressions reveal intimate relations to the fundamental field properties
introduced recently for acoustic fields: the canonical momentum and spin
angular momentum densities. We compare our analytical results with previous
calculations and exact numerical simulations. We also consider an important
example of a particle in an evanescent acoustic wave, which exhibits the
mutually-orthogonal scattering (radiation-pressure) force, gradient force, and
torque from the transverse spin of the field.
