Comments

kdm wrote on 11/19/2007, 1:32 PM
It has to be consistent noise with very little fluctuation, oscillation, etc, or captured at the same time as your scene, but without voices, etc (not really practicle or possible in most cases). If there is too much variation in the noise color, content, level, slight change in location when recorded after the fact, etc, then it won't work. (e.g. If you put two pink, brown or white noise generators on two tracks, and invert one, they won't cancel - too random and independant in time).

In many cases, you may be better off using some light noise reduction instead.
daryl wrote on 11/19/2007, 4:15 PM
Thanks, that all makes sense, but the noise is quite consistent (very faint air conditioner hum), and was captured at the same time. Actually, I did the test in my office just to see if it would work. I made a recording, and ran about 30 seconds of extra (no voices) time. I cut the extra recording time off, pasted it into a track below the original, inverted, no difference.
However, if I copied the whole recording, pasted it in second track, inverted, it worked great, except it also canceled out the voice.
Could not do much with EQ either.
Chienworks wrote on 11/19/2007, 6:10 PM
It can't be just similar and consistent. It has to be darn near identical! Noise is by it's very nature random. The reason this works for some sounds is because when inverted, you end up with negative values that correspond with the original sample for sample. If you don't have exactly the same noise, sample for sample, then inverting and mixing will just end up with .... random noise.
daryl wrote on 11/19/2007, 9:03 PM
Dang, I just knew it couldn't be that easy.

Thanks much Chienworks, appreciate ya!
Bill Ravens wrote on 11/20/2007, 6:07 AM
If it's cyclical noise, like 60 cycle hum, you may be able to get it to cancel out by inverting the phase then playing with the phase angle.
smashguy37 wrote on 11/20/2007, 7:47 AM
Adobe Audition (well, I use Cool Edit 2) has an excellent noise reduction plugin built in. Assuming the voice has a good signal over top of the hum, select a portion of hum, don't set the slider too high (it might cause audible artifacts) and it should do a good job. Works for me all the time, but I'm sure there are other applications out there that do this.
plasmavideo wrote on 11/20/2007, 8:00 AM
You might be referring to a post I made a long time ago regarding phase inversion and noise reduction.

Actually, there have been several threads regarding phase inversion.

The one I posted was about a little trick I discovered using the NR2 plugin. I found that I could use the plugin and save a resultant file of the noise only, and use that noise only file on a seperate track, flip the phase and use a level envelope to allow me to have more control over the noise cancellation than I thought possible within the NR2 plugin itself.

Another thread used phase flipping with compression to attempt to cancel out room echo. That trick works well. You can do a search for that thread here and on the Sound Forge forum. It might work somewhat on random noise. It acts as a sort of "gate" during pauses in speech, by phase cancelling audio during lower portions of the audio.

Tom
craftech wrote on 11/20/2007, 8:01 AM
There is a preset you should try first. In the default Track EQ plugin there is a "60HZ hum notch using four stacked filters".

John