Sculpturing sound fields with real-space topology

Abstract: Artificial structures have been widely used to manipulate sound fields. Most properties of these structures derive from the material and geometry. Few are explicitly related to the structural topology in the real space. Here, we discover a fundamental connection between the real-space topology of acoustic structures and the topological properties of sound fields. We find that the genus of acoustic cavities can protect the birth of singularities in velocity polarization and isopressure line fields. The total topological index of the surface singularities is equal to the cavities' Euler characteristic. This intriguing relationship is rooted in the Poincare-Hopf theorem and is irrelevant to the specific material, geometric details, or excitation properties. The isopressure line singularities lead to acoustic hotspots and quiet zones. The velocity polarization singularities give rise to nontrivial polarization Mobius strips and skyrmion textures. The results enable sound sculpturing with structural topology and can find applications in acoustic communications, acoustic sensing, and noise control.