My method was as follows:
- New Project
Generate noise, white, level 0.01, 10 seconds
Generate tone, sine, 400 Hz, 9 seconds
Move tone track so the two tracks end at the same time
Select All then Mix and Render
Export to WAV or AIF
Select noise only portion, Effect > Noise Removal and Get Noise Sample
Select entire track
Effect > Noise Removal with Noise Reduction 12, Sensitivity 0, Smoothing 130 and Attack/decay 0.10
- Noise is removed from the first second of the track
Once the tone starts, noise is not removed, and instead "wobbles" in volume
In my understanding, this is not correct behaviour. If the NR algorithm is using FFT to sample the noise profile, then applying downward expansion to each frequency band according to the level of audio in that band, the noise should be removed even in the presence of the tone. Noise would remain the the band containing the 400 Hz tone, but would be removed in all other bands. Thus the "masking" effect would make it appear that the noise has been completely removed.
To confirm this:
- Open the exported noise+tone file in Brian Davies' DeNoise program
Set "Limit reduction to 12 dB"
Check the "Automatic" checkbox
Click the "Start" button
Import the DeNoised file in the same project and compare the two results.
The DeNoised file has the noise removed in the presence of the tone.
Virtually identical results can be achieved in DeNoise by setting the "White" checkbox, or sampling the noise profile at the start of the file per instructions on page 14 in the DeNoise manual.
1) DeNoise is superior to Audacity's Noise Removal effect.
2) The Noise Removal effect is broken.
Conclusion #1 seems obvious from this experiment.
Conclusion #2 is speculative. It all depends of how the Noise Removal effect is intended to operate. If the NR effect is intended to operate as described above, then I submit that it is broken.