What is Acoustic Resonance
If you've ever heard the ringing of a crystal wine glass then you know something about acoustic resonance. Resonance is the elasticity/vibration phenomenon responsible for the characteristic tone that results when the glass is struck. What's more relevant is that everything has the ability to resonate because all solid matter exhibits elasticity at some level. And just as the physical properties of the glass, its shape, mass, composition, and temperature, influence the unique tone, the same range of properties determine the resonant behavior of any object.
It is also possible to describe resonance using the example of a vibrating string. We can imagine that the straight, thin stem of an engine valve behaves like a string, shown at left, and has the ability to exhibit many different types of resonant waves which vary by frequency. Resonance can also have other dimensions such as wave direction and "mode" that are also important but not shown in this animation.
The resonances of interest to the EARS method and the ARIS systems occur in the ultrasonic spectrum and generally are of the plane-strain or shear wave mode variety. These, high frequency resonant signals are similarly influenced by material features but of a much smaller scale. Such features can include any of the microstructural or process features mentioned on the previous page and their effects on resonant wave formation create the basis for a defect detection test of manufactured metal products.