Lamb Wave Generation
Lamb waves are similar to longitudinal waves, with compression and rarefaction, but they are bounded by the sheet or plate surface causing a wave-guide effect.
Electromagnetic-acoustic transducers (EMAT) designed to generate Lamb waves vibrate the atoms within the material being investigated. When a wire is placed near to the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region. If a static magnetic field is also present, these currents will experience Lorentz forces of the form
F = J x B
F is the body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction.
Pictured above is a cross-sectional view of a meander coil EMAT for exciting Lamb waves in plates.