The growth of a third harmonic wave is limited by the difference between the phase velocity of the pump and driven waves.
The phase velocity dispersion curves are then used to calculate phase-matched filters to improve detection.
The phase velocity is used as an indicator of the structural soundness of the pile.
British Dictionary definitions for phase velocity
(physics) the speed at which the phase of a wave is propagated, the product of the frequency times the wavelength. This is the quantity that is determined by methods using interference. In a dispersive medium it differs from the group speed Also called wave speed, wave velocity