When plagued by whipping desert winds, sand dunes signal their displeasure with haunting moans that reverberate across the arid landscape. Some emit
single-note songs while others mimic a jumbled chorus—but no one knew why they sang these different songs until now. New research published online this
week in Geophysical Research Letters finds that the size of sand grains shapes a dune's song. Scientists collected sand from a singing dune in
Morocco that moans at around 105 hertz (Hz)—or, to a musician, that's G-sharp two octaves below middle C. They compared those grains to sand
collected from a dune in Oman, which produces notes ranging from 90 Hz to 150 Hz (F-sharp to D). By creating mini-dune avalanches in the lab, scientists
recreated these desert songs, finding that different layers of sand aren't necessary to produce the moans, as previous researchers contended. They also
found that when they sieved the Omani sand so that the grains were similarly sized, the resulting "avalanche" produced a single-note song. The synchronized
movement of sand grains, they conclude, produces the famed moaning, while grain size determines the notes contained in the song.
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