Sound mechanics from squeaky and booming dune sands

Abstract

It is verified that the source of the acoustic emissions when beach sand or other squeaky grains are stepped on or impacted by a pestle lies in a thin shear band directly under the pestle. The grain layers in this band slide one over another at a slow creepy pace giving rise to energy transfer, via the stick-slip effect, from the impacting pestle to the elastic vibrations in the shear bands at the grain contact areas. In turn, this vibration energy feeds the elastic modes of vibration along the vertical grain columns or equivalently, the elastic modes of vibration in the shear band under the pestle comprising the vibrating columns, with dominant frequency in the range of 1,000 Hz. In search of an explanation of the acoustic emissions when booming dune sand is pushed by a blade or is freely avalanching, we adopt the concept of the collision shear band, where, due to the high degree of fluidity of the sand mass, the grain layers slide one over another at a brisk pace so that the average collision frequency between grains in two adjacent grain layers defines the dominant frequency of the acoustic emission in the range of 100 Hz.