Abstract Spin and orbital angular momenta (AM) are of fundamental importance in physics. Acoustic waves, as typical longitudinal waves, have been well studied in terms of orbital AM but not for spin AM, as they are often perceived as spin-0 waves. Although spin AM density has been found in acoustics, the total spin AM is, however, often vanishing. At material boundaries, finite total spin AM and spin-orbit interaction can arise for evanescent waves but only for transverse spin AM not for longitudinal spin AM. Here, from a self-consistent theoretical frame, we establish the spin, orbital, and total AM of acoustic vortex beams, and discover that a non-zero integral longitudinal spin AM is carried by the propagating acoustic field. With the longitudinal acoustic spin, we unveil a new mechanism of spin-orbit interaction emerging when a vortex beam is compressed or expanded. Moreover, we reveal the connection and distinction between the acoustic canonical-Minkowski and kinetic-Abraham AM, and prove that only the former is conserved under the corresponding symmetry. Based on these findings, we propose new strategies for manipulating acoustic spin and orbital AM. Our discovery elucidates new fundamental aspects of spin and orbital AM as well as their interplay in acoustics, which can be extended to other classical waves and may open up new ways for AM-based applications in these systems.
Wang et al. (2025) studied this question.