envelopefilter

An envelope filter is a signal processing technique in which the amplitude envelope of an input signal is used to vary the response of a filter applied to the signal. The envelope is typically extracted by rectifying the signal and smoothing it with a low-pass filter to produce a control signal that follows the loudness of the input.

This control signal modulates a secondary filter stage, often a band-pass or shelving filter, producing a time-varying frequency response that mirrors the signal’s dynamics. In many audio applications, envelope filters are used to create auto-wah and other dynamic filter effects, where the envelope follower controls a filter’s cutoff or resonance to produce expressive, rhythmically responsive timbres.

Common implementation choices include how the envelope is computed and how the modulation is mapped to the filter parameters. The envelope can be derived from peak or RMS detection, and smoothed with attack and release (or decay) times to shape responsiveness. The modulation function may be linear or nonlinear, and the target filter topology is typically a biquad-based stage for precise control of frequency and Q.

Applications of envelope filtering span music production, guitar effects, and dynamic equalization, where the goal is to produce a filter effect that reacts to instantaneous signal level rather than applying a static frequency response. It is distinct from a plain envelope detector, which aims only to recover the amplitude envelope, rather than to drive a filter. See also envelope detector, envelope follower, auto-wah, and dynamic filtering.