This paper describes the construction of two circuits capable of measuring the energy of transient signals, in particular, stress wave emission (SWE) signals. Energy of SWE signals cannot be measured by conventional energy meters since these meters require a repetitive signal, and by nature SWE signals are nonrepetitive. Compared to the more popular threshold-crossing technique, the energy processor gives a more accurate indication of the energy released from a system under stress. The constructions are based on the theory that there is a definite linear relationship between the true signal energy and the area under the envelope of the squared signal. A theoretical development of this relationship and experimental results showing the linearity for one of the circuits is also included.