- Sand dunes only sing in a few areas across the globe, and their songs – always a low, droning sound — have been an object of curiosity for centuries. Marco Polo encountered their haunting drone during his travels and Charles Darwin, in his book “The Voyage of the Beagle,” wrote of testimonials from Chileans about the sound of a sandy hill they called the “bellower.”
The song of the sands is a low hum at a frequency within the bottom half of a cello’s musical range. These dunes only sing when the sand is sliding down their sides. People can set the sand in motion themselves or, more eerily, the wind can create sand avalanches, creating a sudden, booming chorus.
Scientists previously thought the sound arose because avalanching sand created vibrations in the more stable underlayers of the dunes. But evidence that the avalanche of sand itself sings, not the dunes, emerged from experiments in 2009 by researchers who got a shallow pile of sand to sing while spilling down a laboratory incline. Now, the same research team has investigated a deeper mystery of the dunes — how multiple notes can sound simultaneously from one dune.
To study this question, physicist Simon Dagois-Bohy and his fellow researchers at Paris Diderot University in France recorded two different dunes: one near Tarfaya, a port town in southwestern Morocco, and one near Al-Askharah, a coastal town in southeastern Oman. No matter where recordings were made near the Moroccan dune, the sands sang consistently at about 105 hertz, in the neighborhood of G-sharp two octaves below middle C. The Omani sands also sang powerfully, but sometimes unleashed a cacophony of almost every possible frequency from 90 to 150 hertz, or about F-sharp to D, a range of nine notes.
