Two to Tango

Many explanations have been put forth as to why some sand produces sounds. However, scientists are still trying to find out exactly how this phenomenon happens. Stephane Douady, a scientist from the French National Research Council, and his colleagues theorize that sound comes from the synchronization of the motion of the sand grains as they pass each other. Using sand from Morocco, Douday and his colleagues found that by simply pushing sand grains together with their hands, they could produce musical notes. That led Douday to declare that sounds produced from sand have nothing to do with the dunes themselves, but rather are produced by the motions of the sand grains. Douday went on to state that when sand avalanches, grains bump over each other at different frequencies, setting up standing waves in flowing stand layers. The waves then reinforce one another and make the layers vibrate.

While Douday proposed that sounds are produced by sand dunes when they avalanche, Bruno Andreotti, a former student of Douday, proposed that sounds come from vibrations in the sand bed that have been excited by collisions between grains of sand. By measuring vibrations in a sand bed and acoustic emissions in the air at the same time, Andreotti was able to gather information about frequency, amplitude and phases of these signals. Andreotti later said, "The sounds result from avalanches in which the grains drum on one another, exciting elastic waves on the dune surface, with the vibration of the sand bed tending to synchronize the collisions. In many ways, the surface of the sand bed acts like the membrane in a loudspeaker."     

As a team, Douday and Andreotti agreed that the frequency of the sound produced is the same as that of the collision rate of grains in the "sheer" layer of sand moving down the dune surface. Douday proposed that singing dunes resulted from air being pushed in and out between synchronized grains. And though Andreotti agreed with Douday that synchronization of sand grains is responsible for the song of the dunes, he reasoned that the sound was due to the vibration of the surface of the avalanche rather than the result of squeezed air. Eventually realizing that they were forming different hypotheses, Douday and Andreotti began to research the mystery of the singing sands individually. Following different patterns of research, the two scientists ended up in complete disagreement and now, despite working in the same research field, they completely avoid each other. To learn more about the hypothesis and research of Douday and Andreotti, please click on the related links section of this SuperGraphic write-up.