AEN007 wrote:2) Users can move the REAfir waveform up or down
which I guess is similar to the AudNR Noise reduction (dB) slider?
I think that is more like the new "Sensitivity" slider control.
AEN007 wrote:3) REAfir has an FFT level option. It seems that the higher the FFT level -
the more precise the noise waveform. I suppose the AudNR attack/delay slider
functions like the REAfir FFT level?
If you mean the FFT "size" parameter in ReaFir, then that does create a similar effect to the Attack/Decay control, but only as a side effect of the FFT size rather than being an independent control. I'm not totally sure here (you would need to check the Audacity code for a definitive answer), but I think that the Audacity Noise Reduction uses a fixed FFT size and the Attack/Decay settings operate directly on the gating function.
AEN007 wrote:4) I'm not sure what the REAfir equivalent of the AudNR Frequency smoothing is ...
I don't think it has one.