Guided waves are elastic waves which propagate in a medium where the boundary conditions of the waveguide define the distinct modes which exist.  This is in contrast with traditional bulk wave ultrasound, for which elastic waves propagate in a medium whose two mode types, longitudinal and shear, are not changed by changes in boundary condition.  For guided waves, an infinite number of distinct wave modes exist.  Each wave mode has a distinct wave structure (i.e., displacement, stress, etc.,  distributions through the waveguide's thickness) which varies with mode and frequency.  Dispersion curves illustrate the relationship between phase (or group) velocity and frequency for a particular waveguide.  Each structure and waveguide will have a distinct set of dispersion curves, which identify the modes that exist in that waveguide.  The choice of guided wave mode and frequency is important when performing an inspection. 
Historically, optimal guided wave mode and frequency selection has been mysterious.  A rise in the understanding of guided wave mechanics and the ability to calculate mode solutions using modern computing power has illuminated the subject.  However, there are still situations for which the best mode choice is unknown or the material properties are unknown and proper mode and frequency selection choice still remains a mystery. 
Previous studies considered the use of phased arrays for mode control in both piping and plate structures.  For example, a phased comb array has been used to excite different order axisymmetric modes in pipe.  A simple ring piezoelectric element does not provide control over the relative excited amplitude of L(0,1) and L(0,2) modes.  Manual adjustment of comb array spacers on pipe may be used to produce different sets of modes.  Since different mode points have different characteristics (displacement wave structures), different modes should be sensitive to different defect types.  Phased array transducers have been used in both isotropic and anisotropic plates.  Individual elements may be phased to change the excited guided wave modes.  Circular coil EMATs have been used in a phased array to excite guided wave plate modes.  These transducers were used to excite the A0 and S0 modes for defect detection.  Wedge-shaped guided wave SHM transducers (or CLoVER transducers) have been used.  CLoVER sectors are activated to individually to perform an angular scan.