FWIW I've used rifles in various calibres with barrels from 16" - 30". I really notice a sharp difference down at 16 - 18", not so much in noise but in concussive blast. Even with proper ear protection it is quite obtrusive. You also get a lot of flash in low light. Frankly I don't like these short barrels (other than in pistol calibres) for these reasons.
In non-magnum chamberings once you get to 20" any extra length makes little difference to noise, save that it moves a bit further away perhaps, but you don't seem to get that blast (unless the rifle's braked).
