This is an article worth reading. You can also learn more by attending ATI’s sonar and acoustics courses.
This is a quote from the summary.
There are 39 nations operating a total of 400 diesel electric subs. Only three of these nations (China, Iran, North Korea) are likely to use their subs against the U.S. or its allies. China has fifty of these boats, Iran has three (plus 25 much smaller mini-subs) and North Korea has 20 (plus 50 much smaller mini-subs). So the U.S. has to worry about 73 diesel electric subs and 75 mini-subs. But about half the full size subs are elderly, obsolete, and noisy. The same can be said for at least half the mini-subs. That leaves about 36 full size subs and 40 mini-subs that are a clear threat (though the older stuff can be a threat if you get sloppy). That’s a lot of subs, and they make the East Asian coast and the Persian Gulf dangerous places for American warships.
Moreover, the North Korean and Iranian fleets (and governments) are in decline, while China is pouring more cash into their armed forces. If there’s any diesel-electric boats the U.S. Navy has to be extremely concerned about, it’s the Chinese. While China continues to try and develop world class nuclear subs, they are also moving ahead in creating world class diesel electric boats.