The Su-Schrieffer-Heeger (SSH) model is an important cornerstone in modern condensed-matter topology, yet it is the simplest one-dimensional (1D) tight binding approach to dwell into the characteristics of spinless electrons in chains of staggered bonds. Moreover, the chiral symmetry assures that its surface-confining states pin to zero energy, i.e., they reside midgap in the energy dispersion. Symmetry is also an attribute related to artificial media that are subject to parity $\mathcal{P}$ and time-reversal $\mathcal{T}$ operations. This non-Hermitian family has been thoroughly nourished in a wave-based context, where anti-$\mathcal{P}\mathcal{T}$ ($\mathcal{A}\mathcal{P}\mathcal{T}$) symmetric systems are the youngest belonging members, permitting refractionless optics, inverse $\mathcal{P}\mathcal{T}$-symmetry breaking transition, and asymmetric mode switching. Here, we report the first extension of $\mathcal{A}\mathcal{P}\mathcal{T}$ symmetry in an acoustic setting by endowing a SSH lattice with gain and loss components. We show that the in-gap topological defect state hinges on the non-Hermitian phase, in that the broken symmetry suppresses it, yet when $\mathcal{P}\mathcal{T}$ or $\mathcal{A}\mathcal{P}\mathcal{T}$ symmetry is intact, it is observed with either damped or evanescent decay, respectively. Our experiments showcase how the non-Hermitian SSH lattice serves as a utile platform to investigate topological properties across various $\mathcal{P}\mathcal{T}$ symmetric phases using sound.

Recent progress in minimizing sound absorbers is driven by their great scientific significance and engineering value; however, compact devices for low-frequency sound are still challenging. Here, we construct an ultra-thin metasurface by parallel connecting resonators with high dissipation loss to a non-resonant reactance-dominated boundary (RDB) with high radiation loss, which realizes extreme absorption asymmetry at the exceptional point of scattering eigenvalue. We develop a parallel transfer matrix method to design the system, and a deep-subwavelength absorber (the operating wavelength is 120 times of its thickness) with 99.2% and 0.5% absorption for sound incident from opposite ports is achieved. The extreme absorption asymmetry is ascribed to the distinct coupling between the RDB and resonant meta-atom in an unbalanced dissipating state with excellent robustness against geometrical reconfigurations originated from the broadband near-unity reflection characteristics of the RDB. In particular, the proposed strategy brings the design of a degree of freedom rather than typical multiple resonant modes, and an extensible prototype showing >90% (<1%) absorptance for left- (right-) incidence within a wavelength from 23.3 to 18 times its thickness is demonstrated.

The unexpected connections between classical wave phenomena and higher-order topological phases have attracted the interest of researchers across various fields. Although attaining lower-dimensional localized states, such as corner states, has been a common achievement in the realm of sonic crystals, a notable limitation arises where wave energy primarily localizes at the outermost site with the degenerate eigenfrequencies, thereby restricting further applications in acoustics. In this paper, we address this limitation by introducing previously overlooked extra next-nearest-neighbor interactions into the system. Remarkably, the inclusion of these extra nonlocal interactions generates an array of unconventional corner-localized states, each with distinct off-site localization positions. Experimental validation of these theoretical predictions is meticulously conducted using fabricated sonic crystals. Notably, in contrast to natural crystals, the advantageous characteristics of artificial crystals provide a promising platform for probing the effect of long-range interactions in topological phases and materials, which is facilitated by distance-independent coupling strengths. Our findings not only expand the boundaries of sound confinement but also provide momentum for exploring long-range interactions in the field of artificial crystals.

Demulitplexing is used to connect data or signals from one single channel stream to multiple destinations. The streams may be additionally divided through their frequency, polarizations, angular momentum, etc. Here, we show how one is able to demultiplex sound waves at different physical positions and frequency components by employing stacked valley-Hall topological insulators made of merged triangular rigid scatterers. By tuning their two relative angles, we are able to break or restore the underlying lattice symmetry within two topologically independent band gaps. Thus, we experimentally stacked various individually tuned valley-Hall insulators to enable spectrally specific radiated sound to be routed to a particular destination via scattering-immune valley-projected edge states. We foresee that these findings will advance further studies in condensed matter physics, classical wave physics, but also in technological areas of communication by capitalizing on unconventional valley-Hall physics.

The pursuit of artificial structures exhibiting unusual acoustic properties is a major scientific endeavor, in which anti-parity-time ($\mathcal{PT}$) symmetry has been coming into view recent years. At the same time, with the emergence of new acoustic metamaterials, the classical Fabry–Perot resonance mode also exhibits fascinating scattering features similar to those of the anti-$\mathcal{PT}$-symmetric system. We derive the generalized relation for the scattering parameters of an acoustic anti-$\mathcal{PT}$-symmetric system with a transfer matrix, including conjugate bidirectional reflection coefficients and pure real feature of transmission coefficient. In the absence of the real or the imaginary (representing gain/loss) part of the refractive index, the anti-$\mathcal{PT}$-symmetric system degrades into a pair of complementary media, resulting in the bidirectional total transmission. Moreover, a Fabry–Perot resonance mode exhibiting conjugate bidirectional reflection coefficients and a pure imaginary transmission coefficient has been demonstrated. Our results are meaningful for guiding the experimental test of an acoustic anti-$\mathcal{PT}$-symmetric system and the design of associated bidirectional response prototype devices.