This article is from the Active Noise Control FAQ, by Dr. Chris Ruckman
Active noise control is quite different from noise masking. Acoustic masking is the practice of intentionally adding low-level background sounds to either a) make noise less distracting, or b) reduce the chance of overhearing conversations in adjoining rooms. In active noise control, the system seeks not to mask offending sound, but to eliminate it. Masking increases the overall noise level; active control decreases it, in some locations if not all.
 
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