The paper reports on an array of Acoustic Emission (AE) sensors used to detect the relative variations of the input (AE1 coupled at the nozzle) and utilised (AE2 coupled at the workpiece) jet energies. Firstly it is shown that the system is able to detect variations of jet energy caused by pressure and abrasive mass. Apart from developing dedicated signal processing methods, the paper also presents a new measure, Transfer Rate of Energy (TRE) that allows the link between the jet energy (AE2), area of abraded footprint (Af) and jet feed speed (Vf). With TRE being a constant of material, the process can be conducted in such a way that, while monitoring AE2 and knowing TRE value, adjustments of Vf can be done to control the jet penetration (i.e. Af). This strategy has been calibrated for single trenches while varying the Vf both in steps and continuously. This, errors in the Af have been managed to be less than 20%. The proposed sensing method has been integrated into a closed-loop control system for abrasive waterjet machining to enable the generation of freeforms with minimal human intervention.