Lots of really interesting stuff here. I had completely forgotten that ship-to-ship communications were still rather primitive compared to the radio networks we can set up today. Seems I learn something new every time I come here (in this case, a bunch of new stuff) :up:.

Just to clarify my thinking behind this question, the idea behind it is not to actually detect a specific sub but to act as a deterrent. If a lead escort is burning up the ocean with sonar waves a sub might be more reluctant to place itself in the path, not to mention there would be 3 or more others listening passively for any sound. I would think any kaleun would be aware of the passive escorts, and be wary about approaching. I suppose I'm placing too much emphasis on the idea that escorts only needed to keep subs away to achieve their objective. I can definitely see why a hunter-killer group would use passive sonar almost exclusively.

I believe that the technique was known as 'herding' but needlessly putting any additional noise in the water would make the hydrophone operators jobs more difficult. Although oscilloscopes and paper traces had supplimented the human ear on active and passive acoustic systems, the techniques were rudimentary compared to the computer generated visual waterfall displays of modern sonar.

Would secondary echos from a target be identified as such on third-party ships?

What to do when contacts with fish, thermal anomalies, flotsam etc cause multiple false alerts during the course of a crossing? Eventually one can see that it would be easy to cry wolf once too often with the inevitable slower and less effective escort reactions this might cause.

Much more is known about underwater sound propagation today and still research continues. The herding technique may have scared off timid captains but also may have given the bold and the skilled CO valuable insight into the convoy's defences and actually facilitated his approach and attacks.

Just $0.02

It has allready been said that passive sounds carry further than active sounds. I forgot why, but now Randomizer mentioned 'waterfall displays' I get a flash of insight again (seeing 'narrowband in Dangerous Waters' before my mind's eye). Active sounds are high frequency (ultrasound) as others allready have said, but passive sounds are predominantly (very) low noises, like low rumble. The really low frequencies are made by vibrations of the sub's long hull and screwshaft and carry much energy, but those are also in-audible for the human ear. The somewhat higherfrequencies like fans or pumps or just rattle of smaller hull-sections (a bit less energy but still much) can be heard by the human ear. These low frequencies can carry much further than higher frequencies. As higher frequencies get absorbed more easily because they carry less energy to begin with. Anyway, that's what I learned from below links and community written manuals for DW.


http://en.wikipedia.org/wiki/Sonar

http://www.fas.org/man/dod-101/navy/docs/fun/part08.htm

@ ridgewayranger : Ok, I must admit was underestimating the sonar equipment and thinking too simple. They may not have had transistors or signal processing computers like they do have now, but such techniques like filtering for specific frequencies (or that heterodyne stuff whatever it is... 'voodoo magic', that's what it is!) can be done with vacuum tubes and simple electronics.