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Figure 1 Schematic presentation of principles of wave intensity analysis: (A) Fluid inside a tube is set into motion by pushing an upstream piston by a constant force. The graph to the right displays velocity (U), pressure (P) and the product of increments in pressure (dP) and velocity (dU); i.e., dP x dU. Because there is a transient rise both in fluid pressure (+dP) and in velocity (+dU), dP x dU is positive. The +dP x dU indicates a forward-going wave and the +dP indicates a compression wave, thus a forward-going compression wave. (B) Fluid is set into motion by pulling a downstream piston. This causes a decrease in pressure (–dP), but an increase in velocity (+dU), resulting in a negative dP x dU. The –dP x dU indicates a backward-going wave, the –dP indicates an expansion wave, thus a backward-going expansion wave. The components of each of the four possible waves are outlined in Table 1.