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Acoustic or stress wave tomography is a non-destructive measurement method for the visualization of the structural integrity of a solid object. It is being used to test the preservation of wood or concrete, for example. The term acoustic tomography refers to the perceptible sounds that are caused by the mechanic impulses that are used for measuring. Instead, the term stress wave tomography describes the measurement method more accurately.
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The method is based on multiple measurements of the propagation speed of stress waves which are connected to a two- or three-dimesional sampling grid. In case of the acoustic stress wave tomography of trees (see also: tree diagnosis), concussion sensors are attached in one or several planes around a trunk or a branch and their positions are measured. Impulses are induced through strokes of a hammer and the propagation speeds amongst the sensors are recorded.
The propagation speed of impulses in solid objects correlate with the density and the elastic modulus of the material (see also: speed of sound). Internal damages, like rot or cracks, slow down the impulses or form boundaries that render transition of impulses impossible. This leads to longer propagation times and gets interpreted as reduced speeds. Special mathematical algorithms turn the matrix of samples into a color or greyscale image (tomogram) and thereby enables for an assessment of the amount of damage. The precision of the method is limited by the amount of sensors used. But it is distinctly inferior to X-ray computed tomography.
Devices of this kind are the Arbotom, the PiCUS acoustic tomograph and the Arborsonic 3D.