So quiet that they’ve been undetectable by US and Russian subs.
So quiet that the Russian navy has ceased encroaching in Norwegian littoral waters.
So quiet that the US has contracted one of these subs to provide OPFOR and training.
So quiet that the US Navy has considered buying one of these subs for evaluation and littoral work.
THAT quiet.
Especially compared to a nuclear sub.
The diesel engines built for submarine use are so finely balanced that one can stand a nickel on-edge on one running at full throttle.
Add soft engine mounts and the vibrations propagated out the hull are essentially nil.
Yes, as Phishfry says there are other noise sources as well, but if you care enough to do that much engineering on a 110 year old basic engine design, you also care enough to do all you can to suppress any flow noise sources as well.
I’m given to understand that some of our subs produce less noise than an equivalent volume of ordinary sea water.