  The properties of small clusters can differ dramatically from the bulk phases
of the same constituents. In equilibrium, cluster assembly has been recently
explored, whereas out of equilibrium, the physical principles of clustering
remain elusive. These principles underlie phenomena from molecular assembly to
the formation of planets from granular matter. Here, we introduce acoustic
levitation as a platform to experimentally probe the formation of
nonequilibrium small structures in a controlled environment. We focus on the
minimal models of cluster formation: six and seven millimetre-scale particles
in two dimensions. Experiments and modelling reveal that, in contrast to
thermal colloids, in non-equilibrium granular ensembles the magnitude of active
fluctuations controls not only the assembly rates but also their assembly
pathways and ground-state statistics. These results open up new possibilities
for non-invasively manipulating macroscopic particles, tuning their
interactions, and directing their assembly.
