You may be wondering how my idea of Active Walls might work and what is taking place. Imagine taking one of your loudspeakers and placing it against a wall, facing into the room. Next to it place a microphone pointed in the same direction. If the microphone is connected to a clever circuit, the sound picked up can be rebroadcast by the speaker out of phase, actively canceling sound. Once cancelled, the sound that would formerly have reflected off the wall and returned to your ears is cancelled. You have created a non-reflective wall. Active noise cancelation is nothing new. Bose and others have used it for years in their noise canceling headphones. The headphone is a small powered loudspeaker and outside of the ear covers are tiny microphones that pickup sound, reverse its phase, and rebroadcast it to your ear. Thus, any sound entering the headphones is nulled by the out of phase loudspeaker output. Here are two diagrams I found on the internet describing how this works: In the destructive interference picture you can see as one wave goes up, the other opposing wave goes down, creating a sum of zero. Think of a vessel with water and a pump. If the pump is pulling water out of the vessel it drains quickly. If another pump pushes water in, the vessel fills. If both pumps push and pull at the same time and in equal measure, the water level never moves. The push of one pump cancels the pull of the second pump. That is how active noise canceling works. One of the challenges of mechanizing such a scheme with loudspeakers is the microphone will also hear that which the speaker produces as well as what's around it, a problem not encountered when using headphones.
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