The shadow zone and its mysterious nature

[NFunky]

Hey all,

I've been cogitating on some of the aspects of serious sub-to-sub confrontations and thought of something nobody seems to have ever discussed here. It concerns the ever mysterious 'shadow zone' effect. This effect has been well described on this forum as it pertains to above-to-below layer detection, but what about the other way around? I would assume that there is a mirror of the shadow zone above the layer when talking about a sonar that is below "looking up" as it were. Does anybody know if this is true and if so, is it present in DW?

If this effect is present, it seems to me that it would open up some very interesting ASW tactics. For instance, a player who suspects his opponent is under the layer could sit in the shadow zone (i.e. as close to the top of the layer as possible) and dangle his TA underneath for a good long listen. Then, if his opponent goes above the layer it would be only a matter of seconds to duck under the layer and be in the shadow zone on the other side. Hell, the two subs could play hide and seek like this all day without firing a shot... Hmm, sounds a bit dull now I come to think about it, but dull in an interesting way if you know what I mean.

So in a nutshell, is the shadow zone effect limited to being below the layer for some reason or is it a mirrored effect that depends on which side of the layer the listener is on?

[Shearwater]

What do you mean by the "shadow zone"?
The layer works both ways (i.e. being either above or below the layer has the same consequences in regards to your detectability and your own sensors). I think you can actually stay above the layer and let your TA sink below, but you have to adjust your speed accordingly.

[NFunky]

Well, here's one of the threads which includes info on the 'shadow zone', but I think there are a couple more. This one was just the first one I could find with a quick search, but I'm sure you'll find the rest if you just search for "shadow" in this forum alone.

http://www.subsim.com/radioroom/show...ghlight=shadow

A very interesting and edifying thread BTW. Oh and yes, unless you are traveling at 7 kts or more, your TA will sink below your depth (the lower your speed the deeper it will dangle).

[Dr.Sid]

Hi .. the shadow zone is bellow the layer, and it starts at some distance and it lies further on. It's not symetric in it's shape. The top edge is horizontal, the side edge is parabolic (visible on those graphs I did in that other thread). It has no limit on 'the right side' and towards the bottom.

However .. in sonar there is a rule (forget the name now), if A hears B, B hears A. And it is correctly implemented in DW as much as I tested.I only did it one way, because it was easier.

So my measurements had target on the surface, and listener (array) at variable depth. But the transition loss should be the same the other way around.

So if you are in the shadow zone (considering some other listener), you can't hear him, he can't hear you. In DW, shadow zone is absolute and the transition is abrupt. So you can ping, cavitate, whatever .. the other guy won't hear a single dB of noise.

DW's model is quite simple (IIRC)

Both above the layer - good signal
One above, one bellow, in the shadow zone (far enough) - no signal at all
One above, one bellow, not it shadow zone (close enough) - somewhat weaker signal

Only one case is left. Both under the layer. I'm not sure if it is mentioned in that old thread, but it behaved same as the last mentioned case - ie weaker signal, shadow zone had no effect.

And of course it opens space for tactics, that's what the layer is good for. Good idea is especially to launch torpedoes on the other side of the layer, then steer them around, so when they finally get detected (usually by getting closer then 3nm shadow zone edge), they will come from different direction then you are.

[NFunky]

Okay, but I'm still a bit confused here. My understanding of the shadow zone, in the most basic tactical terms, is that if the listener is above the layer, I want to be below the layer, but as close to the borderline as possible so as to remain in the shadow zone at as short a range as possible. This is very straightforward and understandable to me because of the way sound will bend away from the layer. However, my simplistic knowledge of hydroacoustics leads me to believe that for a listener under the layer there would be a shadow zone above the layer. Sound would bend away from the layer on BOTH sides yes?

As an example, lets say there are two submarines, call them 'I' and 'he' for short. Assume that I do not want to get into a shooting match, but want to remain completely undetected, that the range between the two subs is between 6 to 11 nm and that both subs are traveling at more than 7 kts (so towed arrays are hull depth). Now if he were above the layer, I would duck below it, but only the slightest bit below so as to remain in the shadow zone to the shortest possible range. My question is, if he were below the layer (lets say a few hundred feet below), would I be able to be completely undetectable if I was slightly ABOVE the layer?

I understand the concept of mutual audibility and that if he were close enough to the underside of the layer, we would be invisible to each other. However, according to your data Dr.Sid, at ranges below approx. 11 nm the shadow zone has a lower limit which can be crossed by a sub. So in my scenario above, I am placing the opposing sub below the layer, but far enough below so he is NOT in the shadow zone but can hear above-layer sounds. Would there be an area above the layer in which he could not listen (i.e. a shadow zone)? Or can he hear everything above the layer that is within sonar range?

Oh and Dr.Sid, I want to thank you very much for all the work you've put into analyzing DW's sound propagation model. I would have no clue as to how sonar performed in game without having read your threads.

[Dr.Sid]

Originally Posted by NFunky

Okay, but I'm still a bit confused here. My understanding of the shadow zone, in the most basic tactical terms, is that if the listener is above the layer, I want to be below the layer, but as close to the borderline as possible so as to remain in the shadow zone at as short a range as possible. This is very straightforward and understandable to me because of the way sound will bend away from the layer. However, my simplistic knowledge of hydroacoustics leads me to believe that for a listener under the layer there would be a shadow zone above the layer. Sound would bend away from the layer on BOTH sides yes?

Oh I see .. yes. It can be demonstrated using my SoPro utility. It can be found in that old thread, but I should make proper page for it anyway, I'll post link later.

Originally Posted by NFunky

As an example, lets say there are two submarines, call them 'I' and 'he' for short. Assume that I do not want to get into a shooting match, but want to remain completely undetected, that the range between the two subs is between 6 to 11 nm and that both subs are traveling at more than 7 kts (so towed arrays are hull depth). Now if he were above the layer, I would duck below it, but only the slightest bit below so as to remain in the shadow zone to the shortest possible range. My question is, if he were below the layer (lets say a few hundred feet below), would I be able to be completely undetectable if I was slightly ABOVE the layer?

In DW, yes. As I said, the borders are discrete. Now you are in the shadow, now you are out. No transition at all. So you can be 1 ft above the layer depth and you will be safe (actually the border may be few feet of, as who know what part of the sub is used for reference).

Originally Posted by NFunky

I understand the concept of mutual audibility and that if he were close enough to the underside of the layer, we would be invisible to each other. However, according to your data Dr.Sid, at ranges below approx. 11 nm the shadow zone has a lower limit which can be crossed by a sub. So in my scenario above, I am placing the opposing sub below the layer, but far enough below so he is NOT in the shadow zone but can hear above-layer sounds. Would there be an area above the layer in which he could not listen (i.e. a shadow zone)? Or can he hear everything above the layer that is within sonar range?

This would be possible, however, I think it would be very tricky. You would have to hold very specific combination of depth and distance from the target. But then you would have to have very good solution on him, while still being undetected. I think this is very unlikely to happen.

Originally Posted by NFunky

Oh and Dr.Sid, I want to thank you very much for all the work you've put into analyzing DW's sound propagation model. I would have no clue as to how sonar performed in game without having read your threads.

Glad to help.