Abstract: An audio signal energy level detector for use with audio signal processing is described. The energy level detector includes a high-pass filter series-connected between an audio input signal and a series-connected rectifier and integrator the output of which is connectable to the control input of a variable gain device that processes the audio input signal. A characteristically -10 db/decade amplitude versus frequency slope audio input signal is high-pass filtered, rectified and smoothed to produce a constant-energy, DC gain control signal. When the audio input signal is processed in accordance with the invention by the variable gain device to produce an audio output signal suitable for dynamic, downstream processing, the tonal balance and timbre of the audio input signal are preserved. Passive and active, half-order, five-stage high-pass filters are described that provide the desired +10 db/decade amplitude versus frequency slope over the audio frequency range.