Power is a measure of the rate of the work being done by an ultrasonic field. Power is measured in watts. Power describes the potential for the field to interact with the medium in which it is propagating. It is the rate at which energy is transformed from one form into another. It is the major determinant of heat production by the ultrasound beam.
Intensity is both space and time dependent.
Accurate measures of beam area are essential for intensity measures. Ultrasound directed into a thermally insulated vessel. All of its energy converted to heat. Rate at which water temperature rises converted to intensity. Measure of average intensity.