hi there - we have a large group of excellent cassette audios I must digitize - what is the best combo of noise reduction i can use after conversion to mp3 (or aup) so i may set up a template to run with each job? thx so very much.
MP3 is an end-product. That’s the format you put on your Personal Music Player or upload, not a production, filter or effects format. MP3 reduces the quality of the music and makes some repair jobs very difficult.
AUP is not a sound format. That’s the text file that manages an Audacity Project which lives in the _DATA folder of the same name. Don’t mess with the stuff inside the _DATA folder. Let the AUP file do it.
You can Export as a WAV file and that’s a good compromise. That’s an uncompressed, high quality, “perfect” format and all three major computer types understand it. That’s why it’s the Audacity default.
Now the hard part. Noise Reduction isn’t perfect and it’s best if you individually tune it for each tape or performance. Noise Reduction works by getting a “taste” of the noise alone (the “profile” step) and then, in a second pass, tries to remove that exact “taste” from the rest of the show. Unless each of your tapes is of the same maker and tape formula and was recorded on similar machines, the noises are going to be different.
That was the good news. Noise Reduction can’t do a very good job with tape hiss. One of the odd characteristics of hiss and other similar noises is they have some characteristics of all musical tones. So when you set up to remove hiss, Audacity tries to remove everything.
We published some fuzzy rules about using Noise Reduction. A common mistake is trying to make a studio production out of a poor recording when the best you can usually do is reduce the noise a little before the voices or music start to sound funny from unintentional reduction damage
http://manual.audacityteam.org/o/man/noise_removal.html
Koz
It all depends what you mean by “excellent cassette audios”.
If you mean to say that your cassette tapes play with “excellent” sound when they are played in a good cassette deck, then you won’t need to “reduce noise”.
I have digitized more than 100 cassette tapes of chamber classical music. The results are superb: no hiss, no external noise, no background electronic noise. But this has to be done from the Line-Out of a good stereo cassette deck to the Line-In of a PC with good audio hardware.
Or from the Line-Out of a good stereo cassette deck to the Line-In of a good external USB soundcard to the USB input of a PC (particularly if your PC doesn’t have a line-in as many don’t these days - especially modern laptops). 
WC