Abstract: A transducer assembly capable of radiating and detecting energy over a controlled beam width around a selected axis is formed by a piezoelectric element mounted in a cylindrical resonant cavity defined by a Helmholtz chamber. The resonant chamber has an energy emitting end wall positioned normal to the selected axis and is arranged to have a single aperture ring which emits energy symmetrically around the axis at a predetermined radial offset distance therefrom. The energy emitted from the chamber through the end wall sums to form along and around the selected axis a beam-like pattern of controlled width, the beam width being controllable as a function of the offset distance and the energy wavelength. In one embodiment, circular apertures which operate to emit spherical radiation patterns are formed in the chamber end wall. In another embodiment, an annular aperture is formed in the chamber end wall concentric with the selected axis.