| Summary: | The origin of the acoustic and seismic emissions from impacted singing
grains and from avalanching dune sand grains is sought in modes of vibration in
discreet grain columns. It is postulated that when the grains in a column are pressed
together, elastic shear bands are formed at the contact areas with distinct elastic
properties. The central part of such contact shear bands, where the stress level is
maximum, is more in a liquid-like rather than in a solid-like state, resulting in very
low elastic moduli. In a given column, the elastic moduli would assume the lowest
values just below the impacting pestle and higher values further below. The transfer
of energy from the pestle to the modes of vibration of such columns is effected by the
stick-slip effect. The concept of grain flowability is used to justify the great disparity
between the acoustic emissions from impacted singing grains and from avalanching
dune sand grains. The concept of grain columns is assumed to apply in the
avalanching sand band, but with larger length to justify the lower frequencies. The
concept of contact shear bands can be used to justify the variation of the emission
frequency with blade speed and pile height when a grain pile is pushed by a blade.
Finally, this approach can provide explanations as to why ordinary sands do not sing,
and why singing sands do not boom and booming sands do not sing.
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