Sand Dunes are Tuned to E Below Middle C

As clinical audiologists I am sure that we have all had similar training around the world.  Like most, and certainly in my program of study, we were dropped off in the nearest desert with a single bottle of water, a hat, and of course a series of geophones and radar equipment.  A geophone is a microphone that picks up vibrations in the earth, much like a hydrophone picks up vibrations in water.

This desert trip would have been part of our introductory hearing aid class and how to fit hearing aids in the desert.  Actually it was pretty neat because the speed of sound varies with latitude, altitude, humidity, and temperature which would explain why the traditional “1000 Hz” tubing resonance in behind the ear hearing aids was not always at 1000 Hz. (Actually, now that I think back on it, I was the only one to go, and perhaps they were just dumping me in the desert to get rid of me, since I was such a cantankerous student?  They had tried once before by dumping me in the east end of the city but I found my way back…).

My task (as it was explained to me just before they drove away at a high speed, laughing with glee) was to measure, and explain, the strange sounds that people would hear emanating from the sand dunes, and I was given a clue:  “this sound would only occur during the summer time”.  Even Marco Polo, in his first trip to China in the 13^th century across the Mongolian and other Altaic deserts, heard this sound.

So, what does a first year grad school hearing aid class taken during the hot summer, Marco Polo, and sand dunes, have to do with music?  Well, Marco Polo thought the humming of the dunes sounded like the voice of supernatural spirits talking, but I think that it sounds more like my favorite instrument; the cello. (I don’t play the cello but I love the sound of it).  The sound was a constant hum at about 80 Hz (that’s about an E at two octaves below middle C).

So, let’s back up a bit. Melany Hunt and her colleagues (N. M. Vriend, M. L. Hunt, and R. W. Clayton) looked into this and recently published a nice article called “Linear and nonlinear wave propagation in booming sand dunes” in the Journal of Physics of Fluids. This is one of the American Institute of Physics journals; a sister journal to the Journal of the Acoustical Society of America.

Melany (a mechanical engineer at Caltech), like most of my audiology colleagues,  went out into the desert in the middle of the summer (hopefully with a hat and more than one bottle of water), and popped the geophones in the sand dunes.  Her radar results indicated that the sand was not uniform, but had a 5-foot dry layer on top, and then a much denser wet layer of sand beneath that.

As the sand on the surface started to roll down the sides of the dunes, it was like a bow being drawn over the strings of a cello (or any other stringed instrument).  This sand avalanche generated vibrations in the dry 5-foot layer of sand that rebounded repeatedly off the moist, dense boundary layer below the dry sand, and the surface.  These two impedance discontinuities (moist layer below and surface above) set up a standing wave which oscillated at about 80 Hz… the musical note E, at two octaves below middle C. (Actually it’s about half way between D# and E but this would vary depending on the thickness of the dry layer of sand.)

This standing wave explanation is really no different than how our vocal tracts work, or how a trumpet makes a sound- reflect off two parallel discontinuities or higher impedance structures.

And sand dunes can have various tunes.  During the winter, there is more moisture (even in a desert) and the dry surface layer is much less than 5 feet.  I would presume (but never actually learned this in my first year audiology class) that during the winter, the frequency of oscillation would be higher, and most likely of a lower amplitude.

This teaches us two things- there is a lot of music in nature, and if you are ever stuck in a hot desert during the summer months, dig down about 5 feet, and you may be able to find some water…. et n’oblions jamais!