A vactrol is a light depending resistor (LDR) placed next to a light source (previously an incandescent bulb; nowadays an LED). As the light gets brighter, the resistance goes lower. This is a great way to add voltage control to what would otherwise be a resister in a circuit.
What makes people speak about vactrols in hushed, excited tones with the lights lowered and incense burning (or is that smell a module plugged in backwards?) is that the relationship between the light and resistance is nowhere near linear or instantaneous. In particular, when you flash an LDR at a light sensitive resistor, it does not change the resistance instantaneously and stay there – instead, it takes some time to slew down to the desired resistance. When you turn the LED off, the resistance may not go instantaneously to full; instead it might take a brief moment to decay. (Some refer to the result of these delays as ringing, but a trusted engineer tells me they’re just slow.) These characteristics are useful for creating percussive sounds and attacks. It may also take awhile for maximum voltage on input to equal resistance on output, or after the LDR has been on for awhile, it might take longer for the resistance go to maximum when light is turned off. Oh, and there’s a chance no two will work exactly the same due to manufacturing and component tolerances), meaning two “identical” modules may behave differently. Some digital modules try to emulate the most desirable characteristics of vactrols.
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