The song of the dunes as a self-synchronized instrument

TL;DR

This paper investigates the mysterious singing of certain sand dunes, demonstrating that their sound frequency results from synchronized grain motion driven by acoustic resonance within the flowing layer.

Contribution

It provides experimental evidence that dune sounds are caused by grain synchronization due to acoustic resonance, a novel insight into the phenomenon.

Findings

Sound frequency matches the relative motion of grains.

Synchronization occurs above a velocity threshold.

Resonance cavity enables grain self-synchronization.

Abstract

Since Marco Polo (1) it has been known that some sand dunes have the peculiar ability of emitting a loud sound with a well defined frequency, sometimes for several minutes. The origin of this sustained sound has remained mysterious, partly because of its rarity in nature (2). It has been recognized that the sound is not due to the air flow around the dunes but to the motion of an avalanche (3), and not to an acoustic excitation of the grains but to their relative motion (4-7). By comparing several singing dunes and two controlled experiments, one in the laboratory and one in the field, we here demonstrate that the frequency of the sound is the frequency of the relative motion of the sand grains. The sound is produced because some moving grains synchronize their motions. The existence of a velocity threshold in both experiments further shows that this synchronization comes from an acoustic resonance within the flowing layer: if the layer is large enough it creates a resonance cavity in which grains self-synchronize.