Stock DW (1.01+Hotfix and later) playable PSL's:
Seawolf: 63+(speed*10/40)
688I: 65+(speed*10/35)
Akula-I(imp): 65+(speed*10/33)
Vepr: 64+(speed*10/35)
Gepard: 65+(speed*10/35)
Kilo 877: 60+(speed*5/17)
Kilo 636: 55+(speed*5/20)
The Akula-I(imp) is louder than the 688I at all speeds >0 knots.
The Kilo 636 is the quietest at all speeds.
As for reality...here's some public source info:
