Originally Posted by
JD-EE 
The submarine had no chance of reception unless it's crush depth is WAY below that of anything I could find on the Internet a couple years ago.
Hum, does this statement also apply to submarines equipped with
towed array sonars (which can be towed lower than the ship's depth)?
So the implication of this is that the French Navy are idiots then? Otherwise they were just tooling around the ocean for a bit of fun, since they obviously have a better idea of whether or not there was any chance of reception.... I vote there WAS a
reasonably good chance.
A typical FAA approved pinger or beacon attached to the FDR will send out a continuous acoustic 160.5dB, 9ms pulse at 37.5 kHz for a minimum of 30 days, usually up to 90 days or longer at a slightly reduced dB level.
Sound propagates in all directions from the source - sound traveling vertically will be largely unaffected by thermal layer effects, so would suffer attenuation due to salt water (chemical content affects absorption), which at 37.5kHz would be roughly 5.5dB/km. Now I have no idea of the acoustic noise floor of the ocean or the sensitivity of the detection array on the sub, so I can't even guess what might have been the theoretical picture, but there seems to a very good chance of picking up a fully working pinger assuming the sub passed nearly overhead... at greater angles of incidence then thermal layers start to hide sounds and bounce them around, so may be there is significantly less chance of finding them at say greater than 30 degree off-vertical.
However I do wonder why the pinger frequency is so high?
37.5kHz = 5.5dB/km attenuation
9kHz = 0.5dB/km <<< 10 times less, or potentially 10 times the detection range.
- GY