Apparatus features a container and a transducer. The container is made of a selected material and has a container wall with a selected thickness, and configured to hold a fluid therein. The transducer is configured on the outside of the container wall, and is also configured to provide a standing wave into the fluid. The selected thickness and material of the container wall is chosen to ensure about a ½ wavelength of a desired frequency exists within the container wall, so as to substantially reduce back reflections toward the transducer due to any mismatch in acoustic impedance at the interface between the container wall and the fluid, and so as to substantially maximize the amount of energy delivered to the fluid, thus improving the operating efficiency of the apparatus.