Singing sand dunes, an example of the phenomenon of singing sand, produce a sound described as roaring, booming, squeaking, or the "Song of Dunes". This is a natural sound phenomenon of up to 105 decibels, lasting as long as several minutes, that occurs in about 35 desert locations around the world. The sound is similar to a loud low-pitch rumble. It emanates from crescent-shaped dunes, or barchans. The sound emission accompanies a slumping or avalanching movement of sand, usually triggered by wind passing over the dune or by someone walking near the crest.