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About Acoustic Resonance

Resonance and Material Elasticity

Resonance is the mechanical vibration phenomenon responsible for the range of tones that result when a wine glass is struck or a guitar string plucked. It occurs because all materials are elastic and can stretch on some scale and in some direction. Everything has the ability to resonate and the determinants of an individal objects' elastic behaviors can always be linked to the same set of physical properties: geometry and material composition.

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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 EMAR method occur in the ultrasonic spectrum and generally are plane-strain or shear wave modes. 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.  

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