Bound states in the continuum (BICs) enable elastic-wave resonances with ultralow radiation loss, but in multimode plates, localized modes readily couple to multiple radiation channels. Here we extend a symmetry-mismatch design recently implemented in one-dimensional Lamb-wave structures to a two-dimensional phononic crystal plate and realize elastic BICs and quasi-BICs by exploiting a mismatch between symmetric ($S$) and antisymmetric ($A$) Lamb modes. The plate is engineered such that the $A$ mode forms a passband while the $S$ mode exhibits a band gap; a central defect then hosts localized $S$ modes whose eigenfrequencies lie in this band gap yet overlap the $A$-mode continuum, forming $S$-mode BICs. Finite-element simulations and laser Doppler vibrometry show that the number of BIC modes can be controlled by varying the number of defect unit cells. By weakly breaking the midplane mirror symmetry through tilted hole sidewalls, we induce controlled $S\text{−}A$ hybridization and convert these strictly bound states into quasi-BICs that are efficiently excited by plane waves. Using this mechanism, we implement a planar multicavity structure where spatially separated quasi-BIC resonators are addressed at distinct frequencies, thereby realizing planar space-frequency multiplexing and providing a simple route to multifrequency elastic-wave signal processing and integrated phononic circuits.

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• Acoustic wave phenomena
• Bound states in the continuum
• Defects
• Elastic waves
• Mechanical & acoustical properties
• Metamaterials
• Phononic crystals
• Finite-element method