sound opposition phase in real time

Hi, I’d like to know if it is possible to inverse microphone input in real time in order to cancel ambiant sound by using Audacity ?
It’s easy to do by post processing datas, but in real time ??

Audacity doesn’t do real-time processing.

That’s how active noise canceling headphones work. And, there are cars that have active noise cancelation.

For “ambient” sound it’s not so easy because phase is related to time and distance. The cancellation speaker needs to be placed near the noise source, and of course the microphone can’t pick-up the net canceled sound. The mic has to be placed to pick-up just the sound that should be canceled (such as the sound outside of the headphones).

For example, if you flip the polarity on one speaker (or invert one channel in Audacity) and play a mono file you get pretty-good bass cancellation but otherwise is just sounds “phasey” and “spacey”. As you move your head, different frequencies go in-and-out of phase. If you place the left & right out-of-phase speakers closer together you’ll get better cancelation but you won’t get silence.

There is totally a way to do that by writing checks. You strap two news-gathering microphones so their heads are together and talk over only one half. You need to make a special microphone “Y” cable with one microphone backwards. That’s where your cancellation comes from. Plug that into your field mixer and go.

Doesn’t work with computer microphones.

You can make good recordings in very difficult conditions like that.


You can also work a shotgun microphone as hand-held. That’s not cheap, either. I got a pix of that somewhere.



Those work by interference-tube techniques. Long tube with holes in it.

This is the two microphones wired wrong technique (click on the image). That’s a commercially available “Y” cable. They don’t look like it, but those are two matching Electro-Voice 635A microphones. That thing with the “X” is a phase reverser. I built that one, but you can buy those , too. You put a longer microphone cable between the field mixer and the “Y” adapter if needed.
This technique appears in Burroughes "Microphones, Design and Applications."