A transducer for non-invasive measurement includes: a piezoelectric element; a base plate; and driver electronics. The piezoelectric element is mounted to a first face of the base plate. A second face of the base plate is mountable to a wall of a vessel that holds a liquid. The driver electronics drive the piezoelectric element at a plurality of activation frequencies. When the second face of the base plate is mounted to the wall of the vessel, the transducer when activated excites acoustic waves in the base plate and to launch an acoustic wave into the liquid. The transducer is designed such that an angular divergence of the acoustic wave launched into the liquid varies as a function of at least the activation frequency and a dimension of an emitter.