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.

[Dr.Sid]

As for SoPro .. I have some troubles with CADC wiki at the moment, so here are direct links to my site (sometimes unreliable).

Documentation: http://subsim.questions.cz/sopro/doc/sopro.html
Binaries: http://subsim.questions.cz/sopro/sopro.1.9.zip

[NFunky]

Ahh so there is an above-layer shadow zone! Thanks again Dr.Sid.

Yeah, it would be fairly unlikely to be a useful combat tactic. I was thinking more in a situation where I'm pretty sure the enemy sub is below-layer, what the best place to remain undetected would be which appears to be just above the layer. It would definately not guaranty remaining undetected, but it would reduce the chances of being detected. Even if he popped above-layer to have a listen, it would only take seconds for me to slip into the shadow on the other side of the layer. This would effectively reduce detection range to something like 6 nm or whatever the minimum shadow range is. This might be useful in a situation like Akula vs. Seawolf. The Seawolf would detect the Akula before the Akula got even a whiff unless it was able to hide in the shadow zones until minimum shadow range. The one big problem with this is that the Akula driver has to guess which side of the layer the Seawolf is on, so yeah, all in all it's not that great of a tactic, but it is another option to consider.

And I'm just about to download SoPro. I don't know how I missed it the first time I read through that old thread.

[Dr.Sid]

To make it clear .. in real world the shadow zones does not have concrete shape and zero transmission. Even SoPro is not totally correct since it does not take many effects into account, especially diffraction.
Shadow zone is simply zone with low sound transmission. Those zones changes with depth, and if you are under the layer, most of the area above the layer becomes 'the zone'.

But in DW, where it is much simpler, I think it is better to say there is 'some zone' bellow the layer, starting at some distance, and if either of both participants is there, there will be no sound transferred at all.

[NFunky]

Cool, thanks again Dr.Sid. I think I've pretty much got it now and yes, the sharp edges in DW's hydroacoustic model has always annoyed me, though not quite as much as some other game issues. I also wish there was a way to set the layer depth in the mission editor or at least a way to change an AI sub's depth based on layer depth. It would have been a valuable feature for mission designers.

On a slightly different note, I seem to remember reading somewhere on this forum that the SOFAR channel is modeled in DW. Is this really true? If so, I would assume the scenario SSP has to be set to convergence zone for it to work cause surface duct just seems to have a linearly negative thermocline from the layer all the way down. I would guess that a sub would only be able to reach SOFAR depth in pretty high latitudes, but I'm not sure, it might still be to deep even near the poles.

Also, are there any other major acoustic effects I should know about in DW in any of the three SSPs?

[Dr.Sid]

There was some occasional occurrence of 'second layer'. Check this thread:

http://www.subsim.com/radioroom/showthread.php?t=131383

Other then that, I'm afraid nothing. But you might still try, DW did surprise me few times. They don't have things they say they have, but sometimes also they have things they don't talk about.

AFAIK latitude has no effect on layer depth, and there is no sound channel. Also depth has no effect (at least on passive sonar). Bottom type does have effect, but very straightforward, it simply scales everything down as the bottom gets softer.

Edit: my bad, latitude DOES affect layer depth, it is mentioned in that thread.

[Dr.Sid]

I thought a bit about the shadow zone shape from the 'other perspective'.
And I came to this:

First situation I consider 'normal'. Listener is on the surface (the black dot) and he is interested about transition loss for different points in vertical plane. This is the situation I measured and described in 'measurement thread'.

Bright means normal signal, middle blue means somewhat worse signal, dark blue means shadow zone.



Now speculation begins. If you reverse the logic, and put listener into the shadow zone (ie. under the layer), and you test to which sources he would appear in the shadow zone, his 'TL map' would look like this:



And if you revert this again, you must get this results to listener at any depth above the layer:



It would need some more testing but I guess it should look like this.

[Molon Labe]

Originally Posted by NFunky

This would effectively reduce detection range to something like 6 nm or whatever the minimum shadow range is.

That's a bit too strong of a statement, unless you are absolutely sure the target will not change depth--which really, is expecting deliberate blindness on your target's part, since the shadow zone works both ways and the opponent (or mission designer) knows it. Something to keep in mind is that when you are sitting above the layer, you're in a surface duct in both the SD and CZ SSP types; but when you're below in a SD SSP your sound is curving down and detection range is quite short.

EDIT: Sorry, wrote that in a hurry. You did address target depth changes, which means you've got the right idea. The caveat is that it's impossible to time your depth change perfectly, which means after a target depth change there will be several seconds of same-layer-side detection risk, but by controlling your speed you might still come close.

[NFunky]

Yeah, that's why I realized it's not a particularly valuable tactic, but it's a good place to start when deciding where best to avoid detection as long as possible.

Now I recently thought of another question related to the shadow zone. It's simply how much, if at all, the steepness of the thermocline (above and below the layer) effects the range and/or shape of the shadow zone. Dr.Sid, did you happen to notice any differences during your tests? I would assume that IRL the steeper the thermocline the more effective the shadow zone would be, but I don't really know how this relationship would work. But is there actually any relationship in DW?

[Dr.Sid]

My tests only showed that shallower layer means shallower shadow zone, as is to be expected. As for steepness, both my test showed very similar shadow zone shape, while the SSP profile was rather different. So I'd say there is no difference, but I did not test it properly. So I suggest you to test it. Oceanology gives new dimension to DW, and I don't have it even installed at the moment.

Btw. thermocline is border of two bodies of water with different temperatures. What can be steep is sound speed profile, or gradient of temperature, not thermocline.