Very nice read, there are few other interesting papers on same page, one of them is PDF version of "The Third Battle" I mentioned. Your article describes that after noise levels of soviet subs decreased by about 35dB from mid 60's to mid 90's, and similar quietening was reached by third-world conventional subs, detection ranges decreased from previous HUNDREDs miles down to FEW kilometers at best (!! - and the people still vote for several miles of det ranges for creeping quietest subs in the world against each other :-/ was this realism mod or not ?), and passive sonars, even best ones, developed in Cold war against deep water nuclear subs, are seen as not enough to fight with such threats... Even when they can be improved still, still quieter subs can be build too and det ranges would vary from few hundred meters or less to few kilometers which is knife fight no one would want to get into... something like new generation super-maneuverable fighter planes with high-off boresight missiles and helmet mounted sights - if you get into merge in such fight, you have roughly 50% chance to die, no matter how good your plane is... who sees first wins, no matter what plane, $120M or $20M. You don't want to get into that close fight, you want to fight at distance. So new sensors, for example low-frequency, LPI, wide aperture active sonars are needed... to get those det ranges back into at least torpedo-range class (>10-15nm).
Here's another interesting one - describes just this concept:
THE EMERGENCE OF LOW–FREQUENCY ACTIVE ACOUSTICS AS A CRITICAL ANTISUBMARINE
WARFARE TECHNOLOGY