Lamb waves are characterized by which feature?

• A. They travel at a single velocity
• B. They do not occur in plate structures
• C. They exhibit multiple modes with different velocities
• D. They only propagate at the surface

Answer: (C)

Lamb waves are guided waves in solid plates that are dispersive and support multiple modes. The two free surfaces of a plate impose boundary conditions that produce a family of standing-wave patterns across the thickness, each with its own phase velocity. At a given frequency, several modes can exist, so the wave can travel with different speeds depending on which mode is excited. This dispersion and the presence of multiple propagating modes (symmetric and antisymmetric, like S0 and A0 and higher orders) are the defining features. They are not restricted to the surface (that would be Rayleigh waves), and they do occur in plate structures, not as a single uniform velocity as with bulk waves.