During metal-removal processes, acoustic emission energy variation in certain frequency bands is correlated with tool wear. Energy estimation in real-time, therefore, is potentially important in the monitoring and control of machine processes. In practice, a commonly used energy detector is simply a linear, time-invariant filter followed by a magnitude-square operator. Such a conventional energy estimation technique is limited by an unavoidable time- frequency resolution trade-off due to properties of the linear time-invariant filter. Furthermore, the result of this estimation is sometimes physically uninterpretable. In this paper, we describe desired properties for a quadratic acoustic emission energy detector, then formulate an alternative energy measure which is more generally valid than the conventional technique. It is also more flexible than quadratic systems based on Teager's energy operator. The features extracted by the quadratic detector, such as position and energy of the main resonance, preserve the details, in both time and frequency, which are useful for machine tool monitoring. Examples of relative performance are demonstrated on both synthetic signals and on data from metal drilling applications. We conclude that the generalized quadratic energy detector provides superior energy estimation.