What is musical sand ?

Why does musical sand produce sound?

@ In order to help answer that question, I have taken some X-ray images of beach sand during impact/shearing events to monitor the distribution of pressure waves. Normal beach sand is shown on the left, squeaking beach sand is on the right. Here the sand was struck from above with a rod.

Normal beach sand Sqeaking beach sand

Visually, the differences are quite clear. In the case of squeaking beach sand, coherent waves of pressure form periodically along slip lines. This is thought to be a result of the higher intergranular friction that most squeaking sand exhibits.