Explain why can sand grains "sing"?

The physical characteristics of singing sand grains

The sand grains capable of "singing" share common physical characteristics: they are generally round, well-sorted in size, and their surface is particularly smooth and polished, as if they have been polished over time. Unlike ordinary sand, these grains have very little dust or fine particles between them, which limits unwanted friction. Another interesting point is that the size of the grains is often quite uniform, typically between 0.1 and 0.5 mm, which helps create synchronized vibrations when they slide or roll. And it is precisely these general vibrations that cause this surprising sound phenomenon.

The physical mechanisms behind the sound phenomenon.

When dry, rounded, and uniformly sized grains of sand start to move, they collide and rub against each other. This friction generates vibrations that resonate between the upper layers of sand and the more compact layers below. When the vibrations align, they enter into resonance, and the sound amplifies. This phenomenon is similar to what happens when a wet finger is rubbed on a glass: if the frequency of the rubbing is stable, the sound becomes loud and clear. The more the grains move together and at a steady speed, the more powerful and clearly audible the produced sound is.

The environmental conditions favorable to the singing dunes

The dunes sing mainly in dry weather, as high humidity prevents the sand grains from vibrating freely. A warm and arid climate significantly promotes these particular sound vibrations. Similarly, a uniform grain size, combined with a perfectly dry layer of sand above a more compact layer, further enhances this phenomenon. Regular but moderate winds, just enough for the grains to slide rather than fly away, are also essential to trigger this natural song. Finally, the dunes that sing the best are often very steep, with precise slopes that facilitate the flow of sand.

Acoustic analysis and frequency of the sound produced by sand

When the dunes sing, the sound emitted often resembles a kind of deep buzzing or powerful hum. Scientists have measured these sounds and observed that the sound frequency generated typically lies between 70 and 120 Hz. What is surprising is that each dune has its own sound signature—that is to say, its own precise almost "musical" frequency. This frequency mainly depends on the diameter of the grains, their shape, and how they collide and slide against each other. Essentially, round and regular grains mean a purer and more stable sound, while irregular grains will produce less clear and more variable sounds. Thus, we have dunes capable of producing a precise and constant note, almost like a giant natural musical instrument.

Famous examples and observations of singing dunes around the world.

Some dunes around the world are particularly known for their astonishing sounds. In Morocco, the Merzouga dune attracts the curious with its deep and mysterious rumble that impresses even the most skeptical. In China, the Dunhuang dunes, often called "singing sands," produce a deep sound resembling a sort of muffled rolling when one slides down their slopes. In the United States, the Kelso dune located in the Mojave Desert is renowned for its muted roars, which can still be heard from a distance. Similarly, in Namibia, the dunes of the Namib desert sometimes release powerful sounds similar to a low humming when the sand moves under the action of the wind or under the feet of visitors. These places with grand landscapes clearly show how some dunes possess this natural talent to offer a truly unique concert.