A vactrol is a small component that pairs an LED with a photoresistor in a light proof package. Changing the LED’s brightness changes the resistance, and the resistor responds with a slight delay and a nonlinear curve.
That uneven response, which rises quickly and fades slowly, is why vactrols are used in low pass gates and give them a natural, percussive sound. The name is a trademark, and similar parts are called optocouplers.
