Deconstructing nature’s droning dunes

If whistling winds are nature’s high notes, waves a crashing percussion, then desert dunes may provide some nice, smooth bass. Well, maybe not as low as a true bass, but sand dunes can produce a tone around 80 Hertz, comparable to the low hum of a cello. This sound has been casually observed for hundreds of years, but scientists from Caltech have now confirmed the phenomenon and explained what driving this mysterious solo performance.

The researchers mapped a number of different sand dunes in California, plus recorded their performances. It quickly became clear that only a particular kind of dune is able to hum, as some sand is as silent as you’d ordinarily expect. The key requirement is an even layer of dry sand at least five feet thick. This can then be sitting on wetter sand, but the dry layer’s size can’t be any smaller, or the sound won’t build up to an audible level.

Sand sounds off

The sound originates when sand avalanches down the side of a dune. Some of that energy vibrates the remaining sand in the dune, penetrating down the the wet layer five feet below. That boundary between materials reflects some of the vibrations back up, where the process is repeated where the sand meets the air at the top. This vibration ping-pong continues, squelching all frequencies but that critical 80 Hertz tone. That frequency of vibration is the sand dune’s resonant frequency, and rather than dissipate in the to mass of the dune, it builds to a sound audible by human ears.

If you’re not by a dry dune to hear it for yourself, here’s a sample from the Great Sand Dunes National Park.

My first grader asked: What exactly is a natural resonant frequency? What’s that mean.

Well, that’s enough for it’s own post, so take a look.

Source: What Makes Sand Dunes Sing by Christopher Intagliata, 60 Second Science