The characteristics of QSH (quasi shear horizontal) acoustic waves propagating in thin plates of Y-cut, X-propagation lithium niobate are investigated theoretically and experimentally. The fractional velocity change (/spl Delta//spl nu///spl nu/) produced by electrical shorting of the surface is calculated as a function of the normalized plate thickness h//spl lambda/ (h=plate thickness, /spl lambda/=acoustic wavelength). It was found that values of /spl Delta//spl nu///spl nu/ as high as 0.18 could be obtained. Experimental measurements show good agreement with theory. The properties of QSH waves propagating in the presence of a perfectly conducting electrode separated from the piezoelectric plate by a small air gap have been studied theoretically and experimentally. It was found that by varying the height of the gap, the phase shift through a 3.2-MHz QSH wave delay line can be varied by more than 230/spl deg/. We have also theoretically investigated the influence of a thin layer of arbitrary conductivity on the velocity and attenuation of the QSH wave. Calculations show that the variations in these parameters can be as high as 18% and 5 dB per wavelength for a change in layer surface conductance from 10/sup -7/ to 10/sup -5/ S. Results obtained in this paper confirm the attractive properties of QSH waves for a variety of sensing and signal processing applications.