Dispersion of Ultrasonic Waves (Lamb Waves)

Investigation of the phenomenon of the generation and propagation of guided ultrasonic waves (Lamb waves)

The experiment investigates the phenomenon of the formation and propagation of guided ultrasonic waves (Lamb waves). The frequency-dependent speed (dispersion) of Lamb waves in thin glass plates is measured, which are excited and recorded using angle probes.

Keywords: Longitudinal waves, transverse waves, Lamb waves, wave modes, phase velocity, group velocity, dispersion, law of refraction

The phenomenon of Lamb waves is caused by the superposition of longitudinal and transverse sound waves in thin plates whose thickness is smaller than the wavelength. It is interesting that Lamb waves show a frequency-dependent change in the speed of propagation (dispersion). Lamb waves also appear in the form of symmetrical and antisymmetrical modes that propagate independently of one another in the material. In the experiment, different Lamb wave modes are excited in thin glass plates using special angle attachments and the frequency and group velocity are determined in each case. The formation of the various vibration modes of a plate, the influence of the plate thickness and the connection of Lamb waves with longitudinal and transverse waves in connection with the elastic constants of the material are discussed.

Speed-frequency diagram

By combining different angles of incidence and sound frequencies, the S0, A1 and S2 modes were excited in the glass plates of the Lamb wave set. The determined group velocities are plotted in the dispersion diagram as a function of the frequency-thickness product. The diagram also shows the theoretical curve (numerical solution) of the dispersion behavior of the excited Lamb wave modes.

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