  We consider spin angular momentum density in inhomogeneous acoustic fields:
evanescent waves and surface waves at interfaces with negative-density
metamaterials. Despite being purely longitudinal (curl-free), acoustic waves
possess intrinsic vector properties described by the velocity field. Motivated
by the recent description and observation of the spin properties in elastic and
acoustic waves, we compare these properties with their well-known
electromagnetic counterparts. Surprisingly, both the transverse spin of
evanescent waves and the parameters of surface waves are very similar in
electromagnetism and acoustics. We also briefly analyze the important role of
dispersion in the description of the energy and spin densities in acoustic
metamaterials.
