Noise track subtraction

C

CineG

New member
Hi guys, first post here! I'm trying to figure out a good way to on-set sound for a city night shoot in areas where there's still some lingering traffic noise. It was wondering if it would be possible to take a simultaneous recording of that background noise on its own, from a separate mic, and subtract that track (or a greatly gain-reduced version of it to match the intensity of the background noise on the other track) to create a clean recording. It's the sort of thing that, once I had thought of it, I was surprised I couldn't find some sort of "subtract file A's waveform from file B's," online anywhere. Is this something that exists, and if not, what about the idea wouldn't work?

Thanks! I'll probably have some other questions about trying to capture these noise-free recordings in the future hehe.
 
The term you probably want to look for is "phase cancellation" rather than "subtract". The way it works is that you invert the wave shape of one track. Then, when you add it to the original track, the waves cancel each other out.

That said, your "noise" track needs to be almost identical to the track you want to apply it to. You might be able to do this by using a second microphone right next to the first one but pointed in the opposite direction. However, this will require that all of the background noise be very ambient and non-directional.

You might be better served overall with lots of manual editing and volume adjusting.
 
If I read you correctly, you want to record people talking at night and not have the sound of passing cars and other unwanted noise? Your cancellation idea is doomed to failure, because to do this you need identical audio, apart from one has something different - subtract one from the other and you get just voices. The physics says yes. In practice it doesn't work because two mics only a metre or so apart, or pointing at different things will have phase errors that wreck cancellation. If you need to shoot voice or other things outside, you need proper directional mics and a good recordist to do the booming. Or you use lav mics on the subjects and use radio or individual recorders to record the audio. There is no magic cancellation of noises in the way you are thinking. Outside, even the wind can wreck this kind of thing. Inside you can use the technique - two mics a foot apart, and a very noisy room. Get the singer to sing into just one of them, and that annoying guitar can be reduced greatly in volume. The same thing outside is far less predictable.
 
CineG said:
Hi guys, first post here! I'm trying to figure out a good way to on-set sound for a city night shoot in areas where there's still some lingering traffic noise. It was wondering if it would be possible to take a simultaneous recording of that background noise on its own, from a separate mic, and subtract that track (or a greatly gain-reduced version of it to match the intensity of the background noise on the other track) to create a clean recording. It's the sort of thing that, once I had thought of it, I was surprised I couldn't find some sort of "subtract file A's waveform from file B's," online anywhere. Is this something that exists, and if not, what about the idea wouldn't work?

Thanks! I'll probably have some other questions about trying to capture these noise-free recordings in the future hehe.
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Try a differential mic pair: 2 of the same mic/model taped together (1 capsule over the other) with 1 of them connected in opposite polarity to the other. Have your subject speak/play/sing into the "in phase" one. Anything other than what's going directly into that mic will be cancelled out.
 
For a differential mic to work the two mics have to be fairly close together, and the source has to be extremely close to one of them, as in lips on grille. If this is going to be something like a boom mic, I strongly doubt it would work.
 
bouldersoundguy said:
For a differential mic to work the two mics have to be fairly close together, and the source has to be extremely close to one of them, as in lips on grille. If this is going to be something like a boom mic, I strongly doubt it would work.
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Right, which is why I said "taped together." The OP didn't specify his mic placement but if he can get away with this, it should work ok.
 
RRuskin said:
Right, which is why I said "taped together." The OP didn't specify his mic placement but if he can get away with this, it should work ok.
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I'm picturing a mic on a boom over the subject. At that distance, it won't work. Even at the distance of a clip-on mic, it won't work. It will work at lips-on-grille distance, but at that distance normal background noise isn't likely the kind of problem it was with the Wall of Sound.

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CineG said:
Hi guys, first post here! I'm trying to figure out a good way to on-set sound for a city night shoot in areas where there's still some lingering traffic noise. It was wondering if it would be possible to take a simultaneous recording of that background noise on its own, from a separate mic, and subtract that track (or a greatly gain-reduced version of it to match the intensity of the background noise on the other track) to create a clean recording. It's the sort of thing that, once I had thought of it, I was surprised I couldn't find some sort of "subtract file A's waveform from file B's," online anywhere. Is this something that exists, and if not, what about the idea wouldn't work?

Thanks! I'll probably have some other questions about trying to capture these noise-free recordings in the future hehe.
Click to expand...
What are you recording to?
 
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