Listening To Dunes
Source: New Scientist
The French national research agency, CNRS, recently shipped in sand from Morocco to find out how it is that desert avalanches can be tuneful. Although they managed to get some notes by pushing some types of sand, it lost its ability to sing after around a month of being pushed around. They reason that tuneful sand has a smooth layer of silicon, iron and manganese that probably formed when the dunes were under an ocean. As the muted grains had this layer worn away, this appears to explain why only some dunes can sing.
They now postulate that during an avalanche, grains jostle each other setting up acoustic emissions within the cascading layer arising from standing waves that make the moving layer vibrate rather like a loudspeaker.
Apparently, the top 20 to 30mm is the depth of the resonating sand that causes other mobile grains to vibrate in step.
By adjusting sand quantity and velocity, CNRS have generated different notes (a CD is being made) and now, although researchers in Chile and Stuttgart are onto the same theme, there is plenty more research left to understand more about sand tune riddles.