Their subs are very noisy.
Also from Wired:
http://www.wired.com/dangerroom/2011...na-submarines/
On the other hand, the flurry of undersea activity gives American forces more opportunities to tail and examine Chinese subs. And U.S. analysts
discovered a silver lining in the gathering strategic storm clouds. Chinese submarines are a hell of a lot noisier than anyone expected. The sound you hear is the Pacific balance of power tipping in Washington’s favor.
As recently as 2007, China’s diesel-powered subs and a handful of nuclear-propelled models managed just
a few patrols per year, combined. Two years before that,
none of Beijing’s undersea boats went on patrol. For years, the majority of PLAN submarines remained tied up at naval bases, sidelined by mechanical problems and a shortage of adequately trained crews.
As long as the PLAN’s submarines were idle, the U.S. Navy’s spy planes, surveillance ships and snooping subs had few opportunities to assess China’s undersea capabilities — and, most importantly, how much noise the Chinese generate while submerged and moving. Navies can use passive sonars to track submarines by the sounds they make. The louder a vessel, the easier it is to detect. And destroy.
With little information to go on, American intelligence officials had to guess. In cases like that, “you guess conservatively,” a respected U.S.-based naval analyst tells Danger Room on the condition of anonymity. The
conservative estimates placed the latest PLAN subs roughly a decade behind the state-of-art for Russian submarines — and potentially 20 years behind U.S. undersea technology.
Now Chinese subs are patrolling more frequently. “Within the last year or two the Chinese have begun to deploy diesel boats more frequently into places like the Philippine Sea,” the analyst reveals. More and better data is flowing in from U.S. forces. With that data, the Navy conducted a fresh assessment of PLAN submarines. The unnamed analyst attended a classified briefing based on the assessment.
The assessment’s biggest surprise: Leaving aside the PLAN’s dozen imported Russian subs, new Chinese submarines can be detected at what’s known as the
“first convergence zone,” a ring approximately 25 miles from an undersea vessel where outward-traveling sound waves pack close together