Abstract Phononic crystals offer an innovative platform for acoustic liquid sensors. Based on a longitudinal cavity mode, we introduce an acoustic sensor system using a two dimensional phononic crystal with in-plane wave incidence. The phononic crystal is made up of a steel plate having two regular arrays of holes and a cavity in-between. The holes and the cavity are filled with the liquid of interest. We both theoretically and experimentally demonstrate that the transmission peak caused by the cavity mode can be used for sensor purpose. Theoretical simulation and experimental measurement show good consistency in the response of the transmission peak frequency. This frequency is primarily sensitive to the speed of sound thereby sensitive to the composition of the liquid mixture.