Guided waves are a mode of ultrasound propagation that is confined in long structures with distinct boundaries. Lamb waves are the guided waves propagating in plates.
In the schematic, a tilted longitudinal wave is guided into an aluminum plate using a wedge. Part of the incident wave is refracted into shear waves due to mode conversions. The shear and longitudinal waves propagate further in the plate, and are reflected and refracted at boundaries. At some distance away, superpositions of the waves that have same phases and velocities form a static wave progression envelope, which is usually refered as Lamb waves.
Due to boundary effects, Lamb wave propagation is more complex than bulk waves. Single mode excitation of Lamb waves maybe achieved by tuning frequency and incident angle (phase velocity). In this example, the transmit frequency was 2.1 MHz and the incident angle was 45 degrees.