  Acceleration and sound measurements during granular discharge from silos are
used to show that silo music is a sound resonance produced by silo quake. The
latter is produced by stick-slip friction between the wall and the granular
material in tall narrow silos. For the discharge rates studied, the occurrence
and frequency of flow pulsations are determined primarily by the surface
properties of the granular material and the silo wall. The measurements show
that the pulsating motion of the granular material drives the oscillatory
motion of the silo and the occurrence of silo quake does not require a resonant
interaction between the silo and the granular material.
