Noise cancellation

I know there are programs, like Audacity, that will null out background noise.  You give them a sample of just the noise and they subract that from the whole audio track. Does anyone know what these programs do if the noise sample is louder than whats in the audio track? Suppose, for example, you shot a video walking around a store while the manager talks about the various items.  The noise you want to null out is a generator at a building site across the street.   But this noise will be louder when we're walking near the front of the store than when we're at the back.  If the noise sample being used to null out the noise is louder than  the noise in the audio,  will noise get added back in, or will it the track noise just go to zero and stop?

Thanks
stevaAsked:
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KimputerCommented:
Depending on how "smart" the program is, the result might be less satisfactory during the "louder" sections.
But as with all digital things, sometimes, what's not there (clear recording of the voices), cannot be retrieved.
Just like a fuzzy picture can't always be made sharp again.
You're also providing a noise SAMPLE. The program will always ONLY TRY to cancel the noise the best it can with the sample. But as you know, the real noise in the recording is not exactly the same as the sample.

In your case, you should just try the noise cancellation function several times, each time providing an edited version of the sample (where you lower the levels every time)
stevaAuthor Commented:
After trying this with Audacity it appears that the noise sample being subtracted will not get added back in to the signal if it's greater than the noise in the signal.  However, I was not successful in completely removing this noise with Audacity.  I believe that the problem was that the noise varied alightly across the video.  The noise  source was a generator that was driving miscellaeous tools, so it varied slightly depending on the load.  I think you can only get 100% noise cancellation with Audacity when the noise is somethng like an air conditioner or a hum, where a 3-secnd sample of the noise from anywhere in the track accurately represents the noise everywhere in the track.

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