The difference in noise level between SSN-774 (Virginia) and SSN-751 (Improved Los Angeles) is given as 13dB. For passive sonar 15dB difference results in maximum detection distance of thirty-one meters.
The difference in noise level between SSN-774 and SSN-688 (Los Angeles) is given as 18dB. For passive sonar 20dB difference results in maximum detection distance of one hundred meters.
The difference in noise level between SSN-774 and SSN-637 (Sturgeon) is given as 35dB. For passive sonar 35dB difference results in maximum detection distance of three kilometers.
That's how submarines see their targets. You see where I am going with it? If not then here's the answer:
The reason why American submarines could hunt Soviet submarines was because until the end of the cold war NATO held significant technological advantage over Soviet submarines that was decisive. American subs were so much quieter and American sonars so much better than American subs could play "air battles" with Soviet subs and Soviets countered it with speed or diving depth.
The ideas for stealth aircraft came from studies of submarine tactics. But that advantage came from significant difference in noise levels and sonar capabilities. Once Soviets matched it - with Improved Akula - scenarios such as "The Hunt for the Red October" were no longer possible. Instead what you get is the British and French SSBNs crashing into each other because neither could hear the other until it was too late.
There is some space for maneuver with conformal and flank array sonars and increasing sonar gain in tower arrays but once noise levels gets cut to background noise level - standard is 90dB - submarines become effectively invisible to passive detection because they become quieter than the sea itself.
The reason why there's a SSN with every carrier group is because that's how you can keep a towed array hidden at depth to counter disturbances caused by thermal layers and other environmental factors. A surface ship can also drag a TASS but it can be spotted by long-range detection while a submarine can't. So the sub is a stealthy way to put a passive sensor ahead of the group or keep it at 200-300m depth while the surface ASW escort has it at 50-100m.
The main reason for having nuclear submarines is the ability to quickly (in nautical terms - at 15-20 knots) move an asset from one place to another without revealing its location to sensors other than passive sonar - so no satellites, no patrol aircraft, no radar etc. Then they lay in wait like snipers. That's what subs are - snipers. Not berserkers running naked into battle.
Because of that to counter US submarines PLAN has surface ships and aviation and not submarines. Submarines are offensive weapons and ambush tools. If you want to think about competition between USN and PLAN in this field you have to rethink your entire understanding of which naval asset does what.
As for China being unable to match US and Russia on technical level, that is soon going to be in the past as well.
What you need to know is what causes the greatest reduction in noise levels:
The reason why Soviet subs were significantly louder before USSR acquired numerical machine tools which introduced greater precision and quality for moving parts and propellers. It happened in the 70s and 80s and those revolutionary tools which resulted in noise reduction from Victor III levels to Improved Akula levels are antiques compared to modern industrial machinery. Improved Akula is inferior to latest US SSNs but it nevertheless is a threat that requires that the submarine be deployed to counter it.
- quality of manufacturing of key mechanical elements,
- ability to measure and model ship noise in practice,
So the only thing that China needs is time to learn. They needed to catch up with the 70s, 80s and 90s while having equipment and computing power of the 00s and 10s and the Soviet case study to learn from.
USS San Juan (SSN-751) was laid down in 1982 and entered service in 1988. Considering the production rate of the boats in the class it means that its mechanical design is the product of late 70s and its electronics including sonar was the product of early 80s with minor updates until the MLU overhaul in 2010. If PLAN had San Juan type of submarine it would be a nightmare for USN. Even if the advantage is still on American side San Juan forces USN to go into the field and do the hard work.
PLAN doesn't need to match USN sub for sub and dB for dB. It needs to be able to put to sea a fleet of 20 or 30 "good enough" SSNs and keep the production line going for the next generation of vessels and USN is in deep trouble with current numbers. Anything more or better and it's a completely novel situation that the USN has never been in before.
And this is why USN is so desperately trying to get more Virginias or wants XLUUVs. Because the submarine game is a numbers and positions game, not a "one good swordsman is worth ten bad ones." If it was then USN would be gunning for top performance at the expense of numbers and it isn't.
Markoz81, could you comment on this ?
"The sale of the Toshiba propeller milling machinery to the Soviets, and other submarine intelligence furnished by the Walker spy ring resulted in significantly quieter Soviet subs by the later part of the decade. As writer Neal Stevens wrote about the Akula-class Soviet boats, "The combined results generated a steep drop in broadband acoustic noise profiles." Prior to this betrayal of the Western Alliance, NATO antisubmarine warfare specialists could pick up the sounds of Soviet submarines from 200 miles away, or roughly the distance from Washington to New York). After the Toshiba incident, the distance decreased to 10 miles."