A measure of integrity of a material determined by sound emission when a material is stressed. Sometimes these sounds are audible such as wood cracking or rock particles abrading against one another. More often the acoustic emissions are not audible, due to their low amplitude or high frequency, or both.
For rolling element bearing analysis, it is the periodic energy generated by the over rolling of particles or flaws and detected by the display of the bearing flaw frequencies.
See also: Acoustics.