Lamb waves are similar to longitudinal waves, with compression
but they are bounded by the sheet or plate surface causing a wave-guide
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.