Active vs passive noise control (of fan noise).

Sound insulation (a massive barrier which blocks sound transmission) or sound absorption (a porous material that "sponges-up" sound) are passive noise controls (sometimes called "soundproofing").

Active noise control (noise cancellation or anti-noise) uses a speaker (either near the noise source or near the person's ear) emiting sound of the same amplitude, but with the opposite phase (polarity) of the noise. The two sound waves "cancel" each other out, thus reducing the noise significantly.

Bose first introduced noise-canceling headphones. The headphones don't work well on random noise such as speech, working best on non-random noises like the roar of a jet engine, or the noise inside a moving car (tire/road and wind noise). These noises are similar to those produced by a fan (continuous and not rapidly changing).

Actively canceling the fan noise would be easiest at its source, rather than where it is heard, since a person might be anywhere around the stealth grow-room. All you'd need is a microphone, phase shifting electronic circuit (whatever that is...), an amplifier and a speaker, with the mic and speaker located right next to the fan.

Aside from the expense and the power consumed by the amplifier, I wonder why no one here has mentioned trying active noise reduction.

That sounds very technical and complicated for many stoners to grasp onto right away! :stoned:

But, if you can manage to get it to work, then you'll be heralded as a pioneer, and will have many follow in your footsteps. So if you have the means to try an experiment like this, then by all means, please do! And report back to us when you've gathered your results.

Take care. :jointsmile:

I was thinking of mounting the fan in an insulated wooden box. Right next to the fan, I could put fiberglass insulation up against walls made of thick plywood or particle board.

The fan noise would be absorbed by the insulation, hit the heavy wall, and reflect back through the insulation (thus absorbing more of the sound energy).

I noticed that photos and videos of grow rooms sometimes show a fan mounted in a box, or someone even buying a fan already in such a box. Do you see those around the USA, or are these boxes more commonly found in shops outside the US?

What danger is there for the fiberglass insulation igniting and causing a fire? Seems like foam rubber or styro-foam would be MUCH more flammable.

I've also been thinking about a duct muffler. What is inside of duct mufflers, and how can I build my own? I'm guessing it's just a baffle of insulation, but it might just be a pass-through system for all I know.