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I'll to simplify my conclusions.
There are 3 SSPs in DW. Bottom limited (BL), Surface duct (SD) and Covergence Zones (CZ).
In BL there is no layer and there is no difference no matter what depth you or the other ship is. Signal gets weaker with distance and that is all.
In SD a CZ there is layer. In CZ it is more shallow, but otherwise CZ behaves very much like SD. Layer can TOTALLY hide you. To achieve this, you must be under the layer, and you must be distant enough. My measurements shows this distance is about 6 nm with SD and 4nm with CZ. Also the one who is trying to detect you must by above the layer ! I call this situation 'being in the shadow zone'.
If you are closer than this distance, but under the layer, your signal will only get weaker, and for subs, it can complicate identification. However you will be detected (as you are very close now).
This applies for both active and passive sonar. If you are trying to evade active sonar into the shadow zone, get under the layer and get distant enough. You know that you are hidden when you no longer hear the pings.
It is possible to detect target under the layer, but the listener must get under the layer too. And even then the transmission of sound under the layer is much worse then above the layer. I guess the detection range bellow the layer can be even half of that above the layer, and ID of good sub can be certain only at very short distances (under 5nm).
Note that all transitions between these states (normal signal, signal weakened by layer, shadow zone) are sharp. There is no gradual signal weakening. Once they hear you, in the moment you cross the magic border they don't.
Depth also plays no other role in detection. You either are below the layer or above it.
There are some gradual changes in signal strength in DW, but they are not caused by SSP.
With CZ there is also minor signal increase for surface to surface at 30nm and 60nm.