sound cancellation by phase inversion

i'm looking for a way to cancel out the sound exiting from a room into a hallway. the room is a sensitive area and i would like to keep people from hearing what is going on inside. are there any commercial products that i can buy to accomplish this task.

i was thinking of placing a microphone just inside the door with the speaker just oiutside.

does anyone know where to start?
there are some devices that you can plug in that produce a 'white noise' type of thing. I know that some shrinks offices have them near the doors, so you may want to look into that kind of thing......
I'd say you'd probably have to determain where the sound is going to "leak" from the most and position your mic near there inside the office pointing into the office where the unwanted noise will originate, and put the speaker near the "leak" area, pointing out of the office eg. (under the door). for sound to cancel, it must be exactely the same, and 180 degrees out of phase. probably a good omni mic will work best. :?:

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