Dune tunes

World travelers, including Marco Polo and Charles Darwin, have occasionally come across sand dunes that issue loud sounds, sometimes of great tonal quality. Now, a team of scientists has proven that the sounds come not from some musical resonance such as vibrations of the dune as a whole, but rather from the relative motions of sand grains in avalanches larger than a critical size. Using field studies and controlled experiments, the scientists—from the University of Paris VII (France), Harvard (US), the CNRS (France), and the University Ibn Zohr (Morocco)—also found that the grains couple into synchronized layers that vibrate like a musical instrument’s soundboard, creating a pressure wave. It takes only a few layers to generate the observed acoustic power of about 110 dB. The mechanical coupling depends crucially on the grains’ surfaces, which in singing dunes were found to have a silica gel coating known as desert glaze. The online version of this update item has links to sound files. (S. Douady et al., Phys. Rev. Lett. 97 , 018002, 2006 http://dx.doi.org/10.1103/PhysRevLett.97.018002 .)