A new acoustic microsensor principle is introduced for the first time. It is based on acoustic band gap materials, so-called phononic crystals. The sensor employs features of the band gap to determine properties of one component that builds the phononic crystal. Here the dependence of the frequency of specific resonant modes within the band gap is correlated to liquid material properties. The sensitivity to variations in speed of sound and density of the liquid confined in a small gap has been found to be much higher than achievable with traditional ultrasonic measurements.