I gotta say, I love this whole idea. Noise canceling techniques could be used in many places. Cruise ships, jets, train & subway stations, etc. Ever been inside a powerhouse? You can hardly hear yourself think. My dad can't hear anything over 4k htz due to working at one for 35 years.
70 db wouldn't cut it though. They'd need something a little more powerful.

CP, How much room do you have to work with?