Audio envelopes describe aspects of sound over time. ADSR or Attack Decay Sustain Release envelopes describe varying volume levels, which can help shape sound generators to mimic musical instruments in an audio synth.

Here's a look at the ADSR envelope filter in the Teensy audio library.

Sketch & Wiring Diagram:
https://github.com/GadgetReboot/ADSR_...

Teensy Audio Design GUI:
https://www.pjrc.com/teensy/gui/

ADSR and misc info:
https://www.wikiaudio.org/adsrenvelope/
https://en.wikipedia.org/wiki/Envelop...)
https://en.wikipedia.org/wiki/Piano_k...
https://www.onemotion.com/chordplayer/

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