Wave structure describes the distribution of particle motion through the thickness of the layer. Some modes have large particle motion amplitudes near the surface while others have more intense motion near the middle of the layer. These motions are combinations of displacements perpendicular and also parallel to the surface of the layer. When multiple layers exist, the wave structure can become more complex. The wave structure determines the sensitivity of the particular mode to a particular flaw type.
Different modes have different regions of sensitivity through the thickness of the material.
Selection of the appropriate mode is very important to inspection optimization. It may be possible to perform an inspection using only a single mode. Therefore, the use of narrowband transducers to limit the number of generated modes is appropriate. If more information is required in the reflector classification process, then a multimode test may be more desirable. Multimode guided wave testing requires broad bandwidth transducers but yields additional information in more difficult test situations.