Why do large bodies of sand seem to ‘sing’?
Singing sand is a big mystery. Desert sand dunes often make an amazingly loud booming; Marco Polo heard it on his travels while crossing sandy deserts and the locals ascribed it to the spirit of the dunes. But booming is not limited to deserts - singing sand has sometimes been heard on a British beach as the sand is trodden on and disturbed. One scientific walker even got a local violinist to listen and match the dominant note of the sand on his violin.
Those laboratory samples sang well when poured from, or disturbed in, a laboratory mortar. But even so, they did not reveal the secret of singing sand. Must the particles have some uniform size or shape? I can imagine how the action of the sea could stratify the sand grains. It might cluster grains of a similar hydraulic character together, while removing finer or coarser ones to another spot. Those beach samples seemed to sing better after fragments of broken shell were sieved away.
Another clue may be that, in the laboratory, singing sand often loses much of its musical quality in a day or so. Perhaps the high humidity of the laboratory makes it damp. In the desert, sand seems to boom best in the evening. It will have spent the whole day in the hot sun. This suggests to me that singing needs a very dry powder.
Yet other chemicals, however dry, seem not to sing. I have never heard of singing sodium carbonate, or musical manganese dioxide, or operatic alumina. Although one chemist did notice that a sample of Deloxan, an alkylsulfonic acid ion-exchange resin from Degussa, seemed to sing when being poured from its bottle.
From singing to silence
Indeed, there is still no generally accepted theory of singing sand. Two French physicists recently studied the matter. They intended to look at the shape of Moroccan dunes; but their study was soon taken over by the singing of their sand. They reached quite different conclusions, and now work in separate laboratories. Nobody understands why dunes sing. My suspicion that the chemistry of the particles - ignored by physicists - may somehow matter, is just a guess. That vast array of silent chemicals suggests to me that sand is near one end of some sonic spectrum and that at the other end, some unsuspected sound-absorbing chemical may lurk. If so, a powerful new technology is waiting to be developed!
These days, sound is a major nuisance: vehicles, planes, pop music, urban noise, all contribute to the general hubbub that greatly reduces the quality of modern life. A vast amount of technical cunning goes into various sound proofing systems - from sound-absorbing ceiling tiles, acoustic plaster, concert-hall sound damping, noise-cancelling headphones, low-noise fans, and so on - to very little useful effect.
If a good theory of singing sand suggests a counter-substance, maybe one whose high intermolecular friction absorbs the noise, imagine what a vast demand would develop for that chemical!
The only clues I can offer are a text book comment that the burning flame of carbon monoxide ‘gives a curious impression of silence’, and that engineers favour cast iron for the frames of machines, because (among other virtues) it damps the noise of the machine.
The theory of sound was pretty much developed in the 19th century. If 21st century chemistry suggests a counter-theory of silence, how we would welcome it!