Sonar experts: How much sound crosses a layer?
 

Have you tried 1.03 public beta yet? The SSP effects are MASSIVE. It seems like even very loud sounds cannot be heard across the layer.

Back in SC, when the layer effect was barely perceptible, it was a bit dissapointing but soon people explained to me how the SSP was supposed to work, and eventually I could use it a bit and see a small effect. I'm happy to see a stronger effect, but I'm in need of re-convincing that this is as it should be.

Are there any sonar experts that can comment on this? Thanks.

It really depends on how much the water temperature (sound speed) changes across the layer. I remember in the Med, during the summer, a very strong layer at about 100-150 feet. It was so strong that we often couldn't hear nearby surface ships until we came up to periscope depth. Had more than one 'Emergency Deep' as a result :o

I'll have to play around and see if there are "strong" and "weak" layers in the game. Does the game choose the exact SSP profile at random within the specified SSP type, or does the exact shape of the SSP depend on things like depth and time of day or any of that sort of thing?

"Strong" and "Weak" is determined by the "bend" of the SSP. The greater the bend...the stronger the layer. I've seen examples of SSP's where it "jacknifed" at the layer and it was really strong vrs. one that doesn't bend at all produces no layer basically.

It will be interesting to see how or if this "new" aspect of DW effects torp evasion tactics.

Re: Sonar experts: How much sound crosses a layer?
 

I haven't looked at the patch yet, but in addition to the temperature gradient, how effective the layer is should depend a lot on the frequency of the sound. Do you see the effect in narrowband or broadband?

Low frequency noise should require a lot deeper duct to be trapped because the cutoff frequency of the duct decreases with depth. Any sound waves with a frequency lower than the cutoff frequency won't "fit" in the surface duct and won't be trapped. Similarly, high frequency noise should not require as deep a duct to be trapped.

At some distance, even with a strong duct, you should still be able to detect because you're getting direct path energy. Also, if the duct is weak, it will leak more energy than a strong duct.

Actually trying to estimate the effectiveness of sonar in the ocean can get you a PhD in oceanography and physics because the answer is almost always, "it depends on a lot of stuff.."

From what I can tell, it depends quite a bit on the bathymetry. I'm sure there's also a stochastic component as well.

It ought to also depend on the season and windspeed.

If you can imagine, in a summer month, the sea surface temperature is much higher, and it cools as you go deeper so the sound speed will be higher near the surface and drop as you go deeper. That would drive the sound waves down, so unless the water is deep enough so that pressure can dominate temperature enough to push the sound speed higher again, you end up with a bottom limited case. In the winter, the sea surface temperature is not so high, so there might not be so strong a temperature gradient. So maybe it's not as bad.

Additionally the surface duct depth probably ought depend on the wind speed some how too. Strong winds would create a deeper isothermal mixed layer, while low winds ought to make it almost go away.

My suspicion is, though that a lot of the interelatedness of the environmental variables isn't there in DW. I know there's no correlation between sea state and wind speed, for example. One could spend a whole lifetime developing a sonar model. With a little bit of research online, you can do a lot to make sure everything is set up the way it "ought to be." In order to have as accurate an SSP for the given scenario as possible.

Speaking as a layman and with respect to those who I suspect know far more in real life about this topic,
it simply amazes me the extent to which we all endlessly speculate about what real-life elements of sonar
performance are included in the sim. Unless and until SAS expand on their somewhat brief manual exposition
we are indulging in a rather pointless exercise. :yep:

Yeah... that's why I get really annoyed when people say things like, "the sonar ranges are too short [long]" and you never see anyone say what the SSP looked like, what season it was, what the seastate was, how deep the water was, etc.. The truth of the matter is that any estimate of sonar performance has to be qualified with an extensive description of the environmental conditions in which that performance was achieved.

Unfortunately, too often the only environmental qualification I see is that the detection took place in Tom Clancy land.

And if you really want to get into it, any good sonar model has to go hand-in-hand with a good global climatology.

In truth, outside of classified discussions, nobody really knows how good a given system will perform against a given target and that's how it should be.

Here, Here! :yep:

let me give you a very good link about it :

http://www.fas.org/man/dod-101/navy/...P/snr_prop.htm

suggest you read that, than we continue talking about it
for our own culture and to know how to use it in DW

That's a good read, OKO, for sure.

I've read it before and will read it again, now that it is very relevant to the sonar model we are now working with.

I've said it before and I'll say it again here... thank you SCS! :up: :rock: :arrgh!:

Check this one out also!

http://www.fas.org/man/dod-101/navy/...s/uw_acous.htm

on your link, we could see the increase of dB when increasing speed is NON LINEAR =>

http://www.fas.org/man/dod-101/navy/...s/IMG00006.GIF


What I said for a long time ... when lots of people said it was linear ...
So we can consider the increase of the dB should be not as linear as the one in actual DW (with sound fix) nor than the one of LWAMI.
it's not + 1db per knots or per 2 knots, but rather near no difference at slow speed and huge one when increasing speed
like 2dB beetween 5 and 10 knts, and 10 dB beetween 15 and 17.

I remember how peoples said DW stock wasn't good here.
We could see now, stock DW was probably more close to the real thing than the actual values .....

THIS need a real new work.

So, if we combine the new values in 1.03B with the original sonar sound vrs. speed issue we'd be in business! :rock: :rock:

:-) this is graph of sonar self-noise, so it's conected with hydrodynamic, "flow" noise. Well, it's even not flow noise itself but "sonar self noise" ! Flow noise at zero speed should be rather zero don't you think ? So I guess on your graph it's combination of sonar self noise with flow noise.
First, nobody said flow noise is linear :-). But the noise generated by a sub so combination of few kinds of noise, al low speeds machinery noise predominates, at high speeds in modern subs rather flow noise predominates. The flow noise is not linear. But this graph don't look good for me. Because from what I know the flow noise in general is proportional to sixth power of the speed (may be other power at lower speeds maybe). So it's raising curve as you think. BUT it is not raising curve if you plot it on a logarythmic graph !! It becomes the opposite then, because logarytm or a^x is stronger function than even x^6. On your graph the noise is in dB and speed in kts - and the curve is raising. This would lead to an absurd on the very right side of the graph, because more you are going to the right (higher speed) faster the noise (in dB) raises, so at some point increase of speed by 1kts would lead to several dB of noise increase so noise would double or raise 10 times with 1kts of speed increase ! Don't you feel something is wrong here ?
The graph of flow noise can't raise higher and higher on the right.
So even if this graph of sonar self noise is correct (because for example it's determined not only by flow noise but other things) it has not much to do with submarine self-noise which is combination of flow noise and few other kinds of noise. If sonar self noise is combination of constant base 50dB + flow noise, then it could look like on your graph but you'd be surprised how it looks more to the right :-).
Well ok I have found my Excel spreadsheet with noise calculations and made a combination of constant base NL and a flow noise. After a while I have found a combination of parameters that give quite similar result, look (click on the lower graph to enlarge):

http://www.fas.org/man/dod-101/navy/...s/IMG00006.GIF

http://img373.imageshack.us/img373/2...tant2ou.th.jpg

Look at the red line, looks quite similar. But the first impression is wrong, when you take a second look at what happens more to the right and understand what's happening - it's actually a dB scale sum of constant 20dB noise with a flow noise. Look at second graph, it's same function but showed more to the right and with the flow noise part showed in black, as you see the red line is starting from base 20dB but later it becomes more and more similar to the black line.

http://img373.imageshack.us/img373/2...ant28gx.th.jpg

The black line is flow noise and I guess it's quite different from what you could expect. But the red line, sum of 20dB and flow noise at first look quite different than it becomes later. So not always looking at begin of an graph can give you good understanding of what will happen later, if you don't know what function or sum of functions the graph shows. The begin may look quite different from the rest of graph.

But I think the graph you showed is not very precise and maybe also the sonar flow noise at low speeds is little different function because it don't look like speed^6 function. So it's either more complicated (and even less connected with submarine noise) or not very precise...

sure Amizaur : my graph wasn't very accurate, it was just an example.
But personnal experience abord sailing ships and my love for physics (even if I'm not a physicina myself) just show me there is real gaps in this matter
For example, on my ship, well ... my father ship, a catana of 44ft, I could see it's quite easy to go to 10 knts, even with low wind speed (15 or 17 knts), without near any perturbations at the tail of the ship, and the perturbations will start around 11 to 12 knts.
From that speed, we really need more wind to get over this gap (our personnal best speed is 21.3knts on this boat, with 40/45 knts of wind "au portant" and the smallest sail)
It's simply due to the hull shape.

I'm convinced there is some gap also for sub hulls.
Because they are also subject to physical laws.
So, gap changing drastically the noise generated.
There is a before and after the gap
after the gap, water perturbations start and make much more noise than the noise generated by the sub itself.
This gap is not modelised at this time
and this is domageable for tactics.
because I'm convinced a good commander of a real sub KNOW this gap and know when he can go over this or when he can't, because he will risk counter-detection

This gap should be VERY different, depending on sub tech
Well all read Akula is as quiet as US subs AT LOW SPEED and NOT on high speed.
I think it's just because this gap is reached earlier with the Akula than with a 688i and furthermore with the Seawolf.

Seawolf is known to be "silent until 20 knts"
20knts is his gap, before he is really stealthy, after he make much more noise.
I think this gap should be around 12 or 14 knts for 688i, and beetween 8 to 10 knts with Akula.
At this time this isn't modelised at all
We have a progressive and linear curve that doesnt' reflect this kind of real problems.

Of course, these data should be one of the most secret ones.
But with some talk and tought, we could find something much closer to the real thing than what we have actually.
I'm still convinced the original data were closer to the real thing than what we have from the sound vs speed fix ... or with the LWAMI.

I say you again that original data was ZERO, not any noise increase with speed at all :-D. The SCS noise-speed fix changed this to 10 points (so 20dB) of linear function of speed for SSNs.
I'm not sure about what "gap" you are talking here. If you have water drag in mind (which is quite complicated function of the speed in the water) or difference between laminar and turbulent flow maybe ?
The noise generated is related to both drag and flow kind (turbulent being more noisy) but don't know if it's related so directly to make same plot and to make assumptions about noise created by an object in water from the knowledge of it's drag characteristics. And even if you have plotted function of object's drag in the water vs speed on logarythmic scale for drag, you'd be surprised how the graph looks :-). It bends in the other side than you expect :-).
For sure the Seawolf (and every modern sub) is designed to reduce turbulence and reduce flow noise (or rather to increase the speed at which the flow noise becomes noticeable). And if the machinery noise is low and not increase much with speed, and the flow noise is reduced so that it becomes to show up only at 20kts, then you get a quieter sub than can run faster being quiet. But this would also mean being quieter from competition at lower speeds :-). The situation you describe would occur if there is a minimum amount of machinery noise that CAN'T be reduced even at stop (why?), but on the other hand it is dampened so good that it don't increase with speed almost at all and stays on this lowest level up to 20kts. And then at 20kts the flow noise shows up and raises further with the speed. So base noise level at 0, 5, 10, 15 kts, and then flow noise becomes to show and kicks in at 20kts and higher. Well little strange, if there is some machinery that makes some noise at idle (and you can't reduce it to non-measurable value) then the machinery would make increased noise at half speed or full speed... Or do we assume that every piece of Seawolf's propulsion (reactor and it's cooling, steam generators and turbines, reduction gear and shaft, propulsor) is working or rotating with same speed at 5kts than when at 25kts ?
What is true that the machinery noise raises slower than flow noise and that's why the flow noise predominate at some speed. But I don't believe that it's 110db, 110db, 110db and suddenly starts to raise 120 130 140. It may raise slower at low speeds, and become to raise faster when flow noise predominates. It may be at 20kts in Seawolf and 10kts with 688I maybe, and maybe not. We don't know at what speed the flow noise predominates for modern subs. But I have assummed that it's about 20kts for Seawolf and about 10kts for 688I and set the machinery and flow noise graphs to reflect this. Currently I can't model this in speed-noise curves (because they ARE linear in DW from the very start, it was SC were they were non-linear) but I know it CAN be done, because the FFG has non-linear curve, or more precisely have some flat area on the beginning of the later-linear graph. I can't make sub curves same way because this affects sub minimum speed. It can be compensated and it is compensated for FFG (it can stop) but I don't know exactly how and can't do this. But I plan to find how it was done for FFG and then I could add flat area on beginning of sub's noise graph, even 15kts of it :-). I will plot supposed sub noise profile and then find what possible to achieve DW noise curve would fit best to it. Possible to achieve in DW are linear with some flat area on beginning. Non linear noise function was in Sub Command only and was not very fortunate because even flow noise curve raises sharply at some speed range, but then becomes to flatten in dB scale. In SC it raised faster and faster with speed hitting max already before flank speed.
I think current model is better and more accurate, if I knew how to compensate the minimum RPM value effect on minimum speed it would be even better with flat beginning possible.

I didn't find again the post when I tested this (dated, as my screens, from mars of this year)
but still have the screens that prove there WAS change depending on the speed =>

With Stock DW
KILO at 12 knts

http://okof4.free.fr/images/DW/screenshots/KILO_12.jpg

KILO at 17 knts

http://okof4.free.fr/images/DW/screenshots/KILO_17.jpg

you can see a real difference here.
So, it wasn't 0 Amizaur ...
As I tested it, I remember there was a real huge gap at some speed with ALL subs, not only the KILO.

I think pictures speak by themselves.

it's a fact known for all nukes
even at complete stop they still make noise, and there is very few difference beetween a nuke at stop and a nuke at 5 or 7 knts

But I don't think you get my point :
As I told you, there is always, on every hull, a time where you start creating hydrodynamical perturbations, when the laminar flow start to become not so laminar, and here turbulence are created.
As I told you, I could see that each time I go sailing, on the rear of my boat.
that's because my hulls (its a multihull) are optimised for cruise at 10 to 11 knts.
If I go over this speed, even if I could go much faster (well ... its still a sailing boat anyway ...) I will have much more turbulence in my wakes.
but before 10 knts, if you look at the wakes, you have the impression that the ship is stopped
and suddenly, if you go over that speed, you see wakes coming.
It's the time the hydrodynamical flow stop to be absolutly laminar along my hulls.
Not only this slowly brake the increase of speed, but also from this point, I start to need more energy (from the wind) to increase the speed than I needed before 10 knts.
Just because I've got out of the optimal flow, starting turbulence.
And turbulence for subs is noise.

So, for subs, there is also a critical point like this.
A point where a real commander will know he will produce much more noise.
And this is an important tactical thing.
Tactical thing that doesn't exist at this time in DW, but existed in stock DW, as the screens above could show you