In determining the efficiency of a transducer it is desirable to know whether some of the electrical energy is expended in a pumping action at the transducer surface, as distinguished from the hydrodynamic flow due to absorption of energy in the medium. The force due to the total unidirectional flow from a plane‐wave transducer in water, acting upon a thin membrane placed at various distances, can be measured, together with the intensity of radiation passing through the membrane. The transducer is also suspended in the water in such a way that the total force reacting upon it can be observed. From these data and the losses in crystal and mounting, one can calculate how much of the applied energy goes into radiation. Experimental results are discussed.