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.
search1st Reviewed by search1st on . 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 Rating: 5