Acoustic waves are usually thought of as purely longitudinal, moving back and forth in the direction the wave is travelling and having no intrinsic rotation, therefore no spin (spin‑0). Recent work has shown that acoustic waves can in fact carry local spin‑like behaviour. However, until now, the total spin angular momentum of an acoustic field was believed to vanish, with the local positive and negative spin contributions cancelling each other to give an overall global spin‑0. In this work, the researchers show that acoustic vortex beams can carry a non‑zero longitudinal spin angular momentum when the beam is guided by certain boundary conditions. This overturns the long‑held assumption that longitudinal waves cannot possess a global spin degree of freedom.

Using a self‑consistent theoretical framework, the researchers derive the full spin, orbital and total angular momentum of these beams and reveal a new kind of spin–orbit interaction that appears when the beam is compressed or expanded. They also uncover a detailed relationship between the two competing descriptions of angular momentum in acoustics which are canonical‑Minkowski and kinetic‑Abraham. They demonstrate that only the canonical‑Minkowski form is truly conserved and directly tied to the beam’s azimuthal quantum number, which describes how the wave twists as it travels.

The team further demonstrates this mechanism experimentally using a waveguide with a slowly varying cross‑section. They show that the effect is not limited to this setup, it can also arise in evanescent acoustic fields and even in other wave systems such as electromagnetism. These results introduce a missing fundamental degree of freedom in longitudinal waves, offer new strategies for manipulating acoustic spin and orbital angular momentum, and open the door to future applications in wave‑based devices, underwater communication and particle manipulation.

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Acoustic manipulation of multi-body structures and dynamics by Melody X Lim, Bryan VanSaders and Heinrich M Jaeger (2024)