China has upgraded its Type 09 SSN (nuclear powered attack submarines), in an attempt to make them quieter. Chinese subs have always been noisy, and this made it possible for American subs, or surface and airborne anti-submarine forces, to easily detect them. The way submarine warfare works, whoever detects the other guy first, wins. The Chinese, apparently acting on (well paid for) advice from Russian sources, modified some of the machinery in the Type 09 boats, and added a sonar absorbing coating to the hull. Actually, two different types of coating have been seen, so perhaps some experiments are being conducted. This work is apparently also being done in an effort to make China's new SSBN (ballistic missile subs) quieter. Unless the SSBNs can avoid detection by American SSNs, the Chinese boats will be useless. In any time of crises, the U.S. will quickly put an SSN on the tail of any Chinese SSBN, and sink it if it appears that a launch is imminent (SSBNs make distinctive noises as they prepare to launch missiles.)
The U.S. Navy, for obvious reasons, won't comment on how successful the Chinese silencing effort has been. But the fact that the Chinese are working on the problem is, well, a growing problem for the United States, as well as Taiwan and Japan, both of which are very dependent on sea transportation, and vulnerable to quiet SSNs.