Lamb waves

Normal waves propagate only in plates with a thickness comparable to the wavelength. In this case, complex resonance phenomena occur in the plate, leading to the formation of standing waves.

A normal wave consists of various modes moving at different speeds. Distinguish symmetrical (denoted as${\displaystyle {\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">s</span></span>}}_{\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">i</span></span>}}}$ {\ displaystyle s_ {i}}   ) and asymmetric modes (denoted by${\displaystyle {\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">a</span></span>}}_{\mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">i</span></span>}}}$ {\ displaystyle a_ {i}}   ) normal wave. When the symmetric mode moves, the plate surfaces move in opposite directions, and when the asymmetric mode moves, in the same direction.

A characteristic feature of a normal wave is dispersion — the dependence of the propagation velocity on the wavelength. The speed of the wave also depends on the thickness of the plate.

Thus, for a fixed wavelength and plate thickness, various wave modes propagate in the latter, the speed of each of which can be determined by calculation or by special dispersion curves.

http://rspa.royalsocietypublishing.org/content/93/648/114.full.pdf+html "On Waves in an Elastic Plate" March 1, 1917, published, doi: 10.1098 / rspa.1917.0008 93 1917 Proc. R. Soc. Lond. Horace lamb