Traditional Bulk wave ultrasonic sensors send sound energy into an area directly below a transducer. They can do thickness measurements, defect detection, and material characterization. The main disadvantage to this technique is that to cover a large region the probe must mechanically scan the entire area. Additionally, it is difficult to inspect hidden structures with bulk wave ultrasonics.

A guided wave travels along or between two physical boundaries. The simplest example of a wave guide is a plate. Other examples include pipes rail, beams, composite materials, and any other structure that has physical boundaries. Guided waves take advantage of these boundaries to inspect larger regions. It is possible to generate guided waves with different types of displacements and pressure distributions. Exploiting these characteristics gives a skilled designer the ability to create waves that are more or less sensitive to different types of defects and loading conditions. Perhaps you only want to see sub surface cracks, or you may want to generate a wave that is insensitive to liquid loading.

The generation of certain modes at particular frequencies, to accomplish special tasks is scientifically and physically based. Advanced understanding and utilization is possible today because of the tremendous advances in computational power and analysis that is now available. For a given structure a dispersion curve can be generated and from that wave structure profiles can be produced. A dispersion curve will show all of the possible modes that can be excited in a given structure. From the dispersion curve, wave structure profiles can be created. The wave structure profiles show how different types of energy are distributed throughout the thickness of the structure. For example, is all of the energy concentrated at the surface or is it evenly distributed throughout the thickness, or is the particle displacement predominantly in or out of plane.

Taking these concepts further, guided waves can be used for structural health monitoring, as well as defect detection. Furthermore, Guided waves can be used with tomographic algorithms to monitor very large areas with great accuracy. It is also possible to have multiple phased array exciters to focus guided wave energy within structures.