Back to Basics | May 2016 Hearing Review
As clinical audiologists I am sure all of us have received similar training around the world. Early in my audiology program, my colleagues and I were dropped off in the nearest desert with a single bottle of water, a hat, and 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 was part of our introductory hearing aid class (and not a lesson on how to fit hearing aids in the desert, although that would be pretty interesting 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.)
Our task was to measure and explain the strange sounds that people typically hear emanating from the sand dunes, but only during the summertime. Even Marco Polo, on his first trip to China in the 13th century across the Mongolian desert, heard this sound.
So, what do a first-year hearing aid class in graduate school taken during the 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 this humming sounds more like my favorite instrument: the cello. (I don’t actually 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 colleagues1 looked into this and published a nice article called “Linear and nonlinear wave propagation in booming sand dunes” in the 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. There are a number of similarities between clinical acoustics that we deal with every day in our offices and fluid dynamics.
Hunt—like most of my audiology colleagues would do—ventured into the desert during the middle of the summer (hopefully with a hat and more than one bottle of water), and popped the geophones into the sand dunes. Her radar results indicated that the sand was not uniform, but had a 5-foot dry layer on top, with a much denser wet layer of sand beneath that.
As the sand of 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 5-foot dry layer of sand that bounded repeatedly off the moist, dense layer below the dry sand, and the surface. These two impedance discontinuities (moist layer below and dry surface above) set up a standing wave which oscillated at about 80 Hz…the musical note E at two octaves below middle C. (It’s actually about halfway 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 a behind-the-ear hearing aid works, or how a trumpet makes sound-reflections off two parallel discontinuities or structures with a higher impedance.
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: 1) There is a lot of music in nature, and 2) 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 moist sand.
Parts of this column previously appeared at: http://hearinghealthmatters.org/hearthemusic/2015/sand-dunes-are-tuned-to-e-below-middle-c/
Marshall Chasin, AuD, is an audiologist and director of research at the Musicians’ Clinics of Canada, Toronto. He has authored five books, including Hearing Loss in Musicians, The CIC Handbook, and Noise Control—A Primer, and serves on the editorial advisory board of HR. Dr Chasin has guest-edited three special editions of HR on music and hearing loss (August 2014, February 2009, and March 2006), as well as a special edition on hearing conservation (March 2008).
Correspondence to: [email protected]
Vriend NM, Hunt ML, Clayton RW. Linear and nonlinear wave propagation in booming sand dunes. Phys Fluids. 2015;27(10). Available at: http://dx.doi.org/10.1063/1.4931971
Original citation for this article: Chasin M. Back to Basics: Sand Dunes Hum in the Key of E. Hearing Review. 2016;23(5):12.?
Image credits: Marshall Chasin; copyright Dreamstime