The artificial SH3 layer just shuts off sonar access below a certain level. That's called arcade level and I surely wouldn't be caught using it. The fact that you do not know exactly what the depth of the level is doesn't help much.
In SH4 they actually simulated a thermal layer. This layer isn't a soundproof door though. It only attenuates sound, it does not shut it off. It might make you harder to detect at silent running or let you run a little faster without being detected. It will shorten the detection range. But if you have three DDs up there, one on top of you, another off to the side pinging, and the third wandering around with too many depth charges, that thermal barrier and a dollar might buy you a cup of coffee if you're careful where you try it. I have been at 445' below a thermal layer at 260' and been badgered to death. As a matter of fact that is my situation in my career right now. It's three against one and they've got my number. Unfortunately for them, about an hour ago it was four against one and they had my number.
I'm doing the twisty turnies, running out from under any charges dropped my way and trying to get all three behind me so I can show em my fanny to sneak away. Then I'll come up to periscope depth and see if I can give one a bloody nose or surface and end-around if I get really lucky. That's the only way I'll get to the merchies they guard with this convoy because the escorts are darn good this time.
Trigger Maru + Run Silent Run Deep + Natural Sinking Mechanics