One would think (at least this one thinks) that active noise cancellation would be an ideal technology for quieting projectors.

For those who haven't heard of active noise cancellation it is, in a nutshell, a microphone that listens to the ambient noise, a little dsp that basically produces and inverted copy of the ambient noise, and then a speaker to emit the inverted copy of the noise. The net effect is that the noise is cancelled out.

Pilots have been using ANC headsets for years to cancel the engine and air noise while in flight. ANC works best in two cases, placing the cancellation close to the ears (as in the headset itself) and when placed at the source of noise itself. This second case would be ideal for a noise projector - most of the noise would be emitted from the cooling vent(s), so placing the ANC at the vent would kill the noise at its source.