They Put Sponges in a Plastic Cube to Kill Sound. The Sound Refused to Die.
A team at Wuhan University built a plastic cube, stuffed it with acoustic-absorbing foam, and pumped sound through it. The sound, at certain frequencies, refused to decay. Not through any clever design to compensate for the loss. Through topology.
The result, published May 21 in National Science Review, is the first experimental realization of a higher-order Weyl exceptional ring semimetal in a three-dimensional lossy acoustic metamaterial. The work, led by Prof. Zhengyou Liu at Wuhan University's School of Physics and Technology, combined two traditions of topological physics that do not normally sit together: the protected states of topological semimetals and the gain-loss regime of non-Hermitian systems. From the beginning, the team set out to engineer loss as a design parameter rather than treat it as an impairment to be minimized. The intersection produced something counterintuitive and, the researchers argue, practically useful.
The practical bet is this: because the protection comes from geometry embedded in the bulk structure rather than from pristine material properties at the surface, topological acoustic devices could work in conditions that would kill ordinary metamaterials. Conventional acoustic metamaterials lose their wave-guiding capability the moment absorption or structural imperfections enter the picture — the same properties that make them useful in theory make them fragile in practice. If the topological charge is baked into the shape of the lattice itself, the argument runs, you don't need the material to be perfect. You need the geometry to be right.
The physical structure is a 13-by-13-by-13 grid of unit cells, approximately 2,200 of them, assembled from 3D-printed plastic in a breathing Kagome lattice. According to the EurekAlert press release, the researchers designed the loss by punching small rectangular apertures into specific coupling tubes and filling them with acoustic-absorbing foam. The placement was not uniform — loss was concentrated in the inter-cell couplings rather than applied across the whole structure, which the researchers say drives the Weyl points into exceptional rings rather than simply smearing them out. The team could tune the loss level by adjusting the foam-filled apertures, controlling the degree of non-Hermiticity in the system.
Weyl semimetals are a class of topological material in which electronic or acoustic bands cross at isolated points in three-dimensional momentum space. These Weyl points carry topological charges and give rise to exotic surface states. Add loss, and each Weyl point blooms outward into a ring. Not a trivial smearing — a ring of exceptional points where two bands don't merely cross but have identical eigenstates. A Weyl exceptional ring carries both the original Chern number and an additional spectral winding number that exists only in dissipative systems. Dual charges. Two separate topological protections.
Higher-order topology adds another layer. In first-order topological materials, non-trivial physics lives on two-dimensional surfaces. In higher-order materials, it retreats further: to one-dimensional hinges, where surfaces meet. The topological hinge states the Wuhan team observed are localized at specific edges of their rhombic prism — channelled along the crystal's length rather than spread across its faces.
The team mapped the bulk band structure using a broadband acoustic source and a microphone threaded through the cavities, then Fourier-transformed the resulting 3D field. Two Weyl exceptional rings appeared at around 7.74 kilohertz, matching theoretical predictions with what the paper describes as high concordance between theory, simulation, and experiment.
The counterintuitive part is what happened to the hinge states. In a dissipative system, you expect frequencies to become complex — the imaginary component represents decay rate. The trivial hinge states at around 7.24 and 8.90 kilohertz behaved exactly as expected: substantial imaginary components, decay under loss, correspondingly hard to observe. The topological hinge states near 8.34 kilohertz had essentially zero imaginary frequency component, as ScienceBlog reported. Real frequencies, despite the loss. The researchers argue this is a consequence of the bulk polarization protecting these states — that the non-trivial bulk polarization exists outside the Weyl exceptional ring positions in momentum space and cannot be altered by increasing the loss. The protection, they argue, is mathematical rather than material-quality-dependent.
The broader implication the researchers draw is that loss can be load-bearing. You can design it in deliberately, use it to generate topological structure that would not exist in a conservative system, and end up with edge states that are more robust for the addition rather than less.
Noise control, structural health monitoring, and underwater communications are markets historically blocked by the loss gap — conventional acoustic metamaterials work only in controlled conditions because absorption destroys the very wave propagation that makes them useful. The applications the researchers point toward — topological acoustic waveguides and sensors that work in open air rather than a lab bench — are projections from this result, not demonstrated devices. Whether the same physics scales to practical device sizes, or transfers to photonic or electronic domains, remains unproven. But the principle, if it holds, applies across wave physics.
The paper appeared in National Science Review [DOI: 10.1093/nsr/nwag221].