Lysia wrote:I've read that a longer Noise Sample will give better result for Noise Reduction.
That was true of the old "Noise Removal" effect, but less so for the new "Noise Reduction" effect.
More important for the new effect is that the noise is representative of the noise that you wish to remove.
In both effects, the "noise profile" is created through statistical analysis of the selected noise, but the method for doing this has changed between the two effects. The ne effect is able to obtain its statistical data from a shorter selection than the old effect.
There is a minimum length for the noise sample (the length depends on the sample rate). The effect tells you if the selection is too short.
Generally it seems to give no benefit to extend the noise sample by repetition.
If you don't have a long enough section of "noise only" to make the noise profile, then you can create a new track (must be the same format as the track that you want to process), and copy/paste short sections of "noise only" from different parts of the original track into the new track, then use the new track for the noise profile.
Lysia wrote:the recording is very weak compared to the back noise
That's going to be tough. Don't expect to be able to remove the noise completely - attempting to do so will make the remaining sound horrible. You will just have to try and strike a balance between "noise" and "not too much damage".