Is there a way (or a plugin) to amplify a sound progressively: I mean a sort of fade in ora fade out which starts or stops at a certain amount of volume…
You can drag-select and apply Effect > Fade In or Effect > Fade Out.
You can also use the Envelope tool (two white arrows and bent blue line) to create a custom fade and there you can define both ends.
http://kozco.com/tech/audacity/pix/CrossFade.jpg
Those blue lines are rubber bands and you can push them around to do whatever you want.
Koz
Maybe I don’t explain well what I meant.
The “Fade in” effect increases the volume from silence to “X”.
I have a track with “x” volume and I want to amplify it progressively (not instantly, so I can’t use the “amplify” effect) from “x” to “x+7”…
As Koz suggested: use the envelope tool - see: http://manual.audacityteam.org/man/Envelope_Tool
WC
There is also this Nyquist plug-in that seems to match your description: https://forum.audacityteam.org/t/simple-plug-in-for-creating-partial-fades/16055/1
See here for how to install plug-ins: http://audacityteam.org/download/plugins
Fade by db seems perfect! Thanks!