Active Noise-Canceling Mufflers
There have been a few experiments with active noise-canceling mufflers, especially on industrial generators. These systems incorporate a set of microphones and a speaker.
The speaker is located in the pipe, which wraps around the exhaust pipe so that the sound from the exhaust comes out in the same direction as the sound from the speaker.The computer monitors the microphone in front of the speaker and the microphone behind the speaker. By knowing some information about the length and shape of the pipe, the computer can generate a signal that drives the speaker. This can offset most of the sound from the generator. The downstream microphone lets the computer know how it works so that it can be adjusted as needed.