A piezoelectric element is a resonator using the mechanical resonance of piezoelectric ceramics. The vibration behaviors (modes) vary depending on the resonant frequency.
The relationship between resonant frequency and vibration mode can be summarized in the following table.
Vibration in bending directions.
Length vibration that expands or contracts a thin plate.
Area expansion vibration over a surface of a thin plate or disk.
Thickness shear vibration over a surface of a thin plate (when the electric field is perpendicular to the direction of polarization).
Vibration in the thickness direction of a thin plate.
Elastic wave that conveys energy over the board surface and exponentially reduces energy in the board thickness direction.
In principle, the energy conveying behavior is the same as above. The most significant difference between surface acoustic waves and BGS waves is that surface waves convey energy with both longitudinal and transverse waves, while BGS waves convey energy only with transverse waves.
" BGS wave" was named after the initial letters of Bleustein, Gulyaev and Shimizu who discovered this wave at around the same time (around 1968).