Nondivergent spinning substructures near acoustic field nodes

In this work, we examine the extraordinary behavior of polarization and spin angular momentum density in the vicinity of longitudinal field zeros in three-dimensional monochromatic acoustic fields. We demonstrate that, as governed by the continuity equation, the velocity fields of arbitrary acoustic sources maintain nondiffractive elliptical polarization structures that enclose longitudinal field zeros, despite having divergent transverse spatial profiles of intensity. Furthermore, embedded in these nonparaxial field contours, for infinite distance, are threads of circular polarization singularities. We illuminate these inherent properties in acoustic vortex fields, dipole arrays, and the famous Young's double-slit experiment. Our results reveal spin characteristics of sound waves that provide a platform for future studies and applications of structured acoustic waves and chiral acoustic phenomena.