Sound waves traveled steadily despite being confined to irregular spots

Experiments in a disordered acoustic lattice found that strong localization of wave states did not stop wave packets from moving ballistically.

Editorial illustration — not from the study.

The team built an acoustic system based on a disordered Hatano–Nelson chain with imaginary gauge fields, a type of engineered lattice that can have nonstandard gain and loss. They used spectroscopy to map its complex energy spectrum and wave states, then tracked wave-packet motion over time in the same system.

The measurements showed strong, disorder-dependent localization with irregular bulk positions and subexponential decay, alongside ballistic transport. A theory linked the main propagation site to the largest excursion of a random walk within the expanding region reached by the wave, and its predicted Lévy-arcsine statistics agreed quantitatively with the experiment.

What the sound waves did

The researchers realized a regime they call erratic non-Hermitian skin localization in a disordered acoustic lattice. Wave states became strongly localized at disorder-dependent, irregular positions, but wave packets still propagated ballistically. The team’s transport theory connected the dominant propagation site to the largest random-walk excursion within the wave’s expanding light cone and predicted Lévy-arcsine statistics, which matched the experimental measurements quantitatively.

Why the separation matters

The findings show that strong localization of a system’s wave states does not necessarily determine how wave packets move through it. By separating these two aspects of wave dynamics, the work identifies a distinct regime for studying transport in disordered, engineered systems and challenges the usual link between localization and impeded motion.

Evidence and limits

The evidence comes from an experiment in an engineered acoustic lattice, combining Green’s-function-based spectroscopy, which probes the system’s response, with time-resolved measurements of wave packets. The observations were compared with a transport theory and showed quantitative agreement with its predicted statistics. The abstract describes this one acoustic realization and does not establish whether the same behavior occurs in other physical systems.

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