Measurement of the properties of liquids based on the dispersion of Lamb waves in an acoustic waveguide

An acoustic waveguide sensor consisting of two identical parallel plates on which surface acoustic waves are propagating allows the measurement of proportion of liquid properties taking advantage of the mode conversion of SAW via the measurement of the transmission time and amplitude of short wave pulses [1]. A favourable realization of such a sensor consists of thin plates of non-piezoelectric material with a thickness in the order of the acoustic wavelength for example glass plates of about 1 mm thickness at excitation frequency of 1 MHz on which Lamb waves are excited and detected by means of piezoelectric interdigital transducers attached at the rear surfaces [2]. Due to the dispersion of Lamb waves for a given material and excitation frequency, their propagation velocity depends on the thickness of the plates. Correspondingly for the lowest order symmetric and asymmetric modes of Lamb waves the Rayleigh angle at which mode conversion occurs and the acoustic velocity difference to the liquid will be different and will change in different ways with changes of properties of the liquids. This allows a novel access to the investigation of such liquids with enhanced sensitivity.