Two transparent sheets with circles drawn on them. If you stack them exactly, the circles align. But give one sheet a slight twist, and a new pattern emerges: in some places the circles bunch together, in others they spread apart. That's roughly how moiré works — an unexpected pattern born from rotating two periodic structures. In this work, scientists applied the same idea to sound, specifically to the velocity field of acoustic particles above a perforated surface.

On a metasurface (an artificial material that manipulates waves), they created two lattices of acoustic skyrmions. A skyrmion is a vortex-like formation, akin to a tiny tornado, where the field direction smoothly twists from the center outward. By superimposing the lattices with a rotation, the authors obtained 'acoustic skyrmion bags'—groups of vortices enclosed in a common shell. By adjusting the twist angle and the pivot point, they could reshape these bags, altering their size and form.

Next, they tested how robust these composite vortices are against disorder. They deliberately introduced defects into the lattice — random disruptions of periodicity. It turned out that skyrmion bags preserve their topology (the global 'twist') at moderate defect densities. And with a high number of defects, the inner cluster of skyrmions within the bag held up even better than a single lattice without rotation. The platform also allowed switching between lattices of skyrmions and merons (vortices with half the charge) and assembling rotation-controlled clusters from merons. Thus, acoustic moiré becomes a tool for sculpting robust spatial patterns from acoustic fields.