🚀 go-pugleaf

I am assuming that all of you are talking about doing what is called active
noise control. This is not so easy as switching the leads of the speaker or
microphone. Because

1) The speaker has to be placed very very close to the microphone
2) the 180 degrees delayed waveform called "anti-noise" shows up in the
   reference signal. Because of 1)
3) The amplitude needs to be adjusted and limited.
4) The acoustics of the room will keep changing and that means one has to have
   an adaptive system.
5) The noise will only be cancelled at the error microphone and may add to the
   existing noise at other points in the room.

If you have some of the last few issues (October) of EE Times you may 
read about the acoustic noise control muffler for cars that is being put 
out by Walker mufflers. It is supposed to provide as good cancellation as 
existing passive mufflers and also provide better mpg from your car as 
the fuel burns cleaner when there is no back-pressure induced by the 
passive muffler.

Also, acoustic noise control was used by headphones designed by Bose which
were used in the Voyager flight couple of years ago.

Some active companies in this area are Noise Cancellation Technologies and
Active Noise & Vibration Technology.

There are a whole bunch of papers available on this.... I have a list of some
of the good ones. If someone is interested, email me.

-- Girish Govind
   govindg@ucunix.san.uc.edu