https://phys.org/news/2018-05-failure-granular-materials.html

Vibrational modes are the ways in which something can oscillate, or move internally. A small molecule can only oscillate in a few ways, for example, but larger objects will have more modes, which are affected by both the locations and the masses of the components. In a disordered or amorphous system of granular materials, like dirt or gravel, the number of modes quickly becomes too large to either predict or measure directly.

However, each mode has a particular acoustic frequency associated with it. Brzinski and Daniels' approach measures the frequencies of the active vibrational modes in the material, giving them an acoustic snapshot of the material's overall "health."