How Does it Work?
All parts, when excited, will make sound produced by the vibrations of the part. Each "mode" of vibration can be attributed to a specific resonant frequency. An example of this is shown in the video below of a tuning fork vibrating after being struck.
Even though those vibrations may not be visible, they are there and cause patterns of deformation in the structure.
Likewise, individual modes can be attributed to the movement of different geometries of the structure.
Any change that affects the part structurally (such as a defect) will shift the peak values (modes) on the frequency spectrum. Acoustic Resonant Testing (ART) uses these concepts to look at a part and inspect it for structural changes and/or abnormalities.