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.

Capture4.PNG
2542 Hz
4433 Hz
6879 Hz
13,049 Hz
9796 Hz
16,532 Hz
Capture.PNG
20,177 Hz
23,942Hz

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.