Undetectable by what? IUSS can detect a sub at much lower radiated noise levels than a tactical array, and active sonar doesn't need any radiated noise at all.
Besides which you will never ever ever find out any platform's real life radiated noise levels outside of speculation. It's very secret information.
In game I can get detections with the LA on Typhoons at 20+ miles while they are moving at 2 kts, so there is probably no "undetectable" speed for nucs.
__________________
|