Compressional waves generated by thickness shear resonators in a surrounding gas

Compressional waves are generated by thickness shear mode resonators in a surrounding gas. They are not generated by a longitudinal movement of the quartz resonator, but originate in the shear velocity gradient on the surface of the resonator. Compressional waves are revealed by producing standing waves in a gap between the surface of the quartz resonator and a linearly moved parallel reflector. Whenever the gap length is an integer of half-wavelengths of the compressional waves, at the shear frequency of the resonator, periodic perturbations in motional resistance and frequency are recorded. The effect is important for precision resonators.