When a dune falls, it sometimes emits a special sound, a kind of singing. According to some researchers of the French National Research Council, the physical explanation of these sounds in the desert is that the grains of the falling residual layer are touched and create resonant waves that make the residual layer vibrate, being the sound the resonance of that vibration.
For resonance to occur, a residual layer of two or three centimeters is considered sufficient, while as for speed, the grains of sand must move 0.45 meters per second or faster, and the wet sand does not produce resonance.