Sound speed propagation and detection

[shchuka]

I was reading the Notfallmappe_06 manual and stumbled uppon this:
Quote:
For subs at different depths, which one will detect the other depends upon the SSP profile (assuming that there is no layer and the subs acoustic radiation are equal): If the SSP speed increases with increasing depth - then the deeper sub will hear the shallower sub first. The sound from the shallower sub accelerates towardthe deeper sub; the sound from the deeper sub decelerates toward the shallower sub.-----------------------------------------------------------------

This doesn't seem right. As far as I understand, if sound bends towards the slowest speed, then the shallower sub should hear the noise from the deeper sub because the sound in this case bends upwards. The noise from the shallower sub decelerates downwards towards the deeper sub because the speed of sound there is higher than at the surface and wants to go upwards.

Is the manual wrong?

[USS Moose]

From my basic understanding of physics and SSP and sub positioning in this particular scenario, time would be negligible (or the same, depending on how you look at it). Looking at the graphs of common SSP types, velocity is more or less linear over any given amount of time in a function of depth over temp, therefore we can assume that acceleration is constant. By way of analogy, lets imagine two cars:

Car A: initial velocity (Vi) is 20 m/s; final velocity (Vf) is 10 m/s

Car B: Vi is 10 m/s; Vf is 20 m/s

let's say the distance they have to travel is 150 m.

from the kinematic equation (remember acceleration is constant so it's not in this formula), d = [(Vi + Vf)/2] * t, we solve for time giving us

(d * 2)/(Vi + Vf) = t

Both cars have to travel the same amount of distance and since the sum of their initial and final velocities are the same, the answer for time is the same. In this case, it would be 10.0 seconds.

So back to subs, the time it takes for the sound(s) of Sub A to be detected by the sonar of Sub B (and vice versa) would be the same because even though one accelerates and the other decelerates, the EXTENT of their acceleration is the same. Therefore, I believe SPPs are generally used only to determine shadow zones/depths of detectability.

Either my assessment is correct or I just wrote an entire thing of nonsense. Either way I hope that it opens up a new perspective.

[Pisces]

A user of this forum named Dr.Sid made a little program to visualize soundrefractions:

http://www.subsim.com/radioroom/showthread.php?t=120487&highlight=sopro

The download is small, but awesomely slow.

Sorry, time of arrival difference of the sounds at each sub doesn't play into this. Well, not unless you consider wave interference patterns of several rays of the same sound mixing together. It's not about who heard the other first. Remember, your engines and screw and whatever emit continuously. So that is in no way an advantage over the other guy.

It's only if you send .... one ping only! (simultaneously) ;)

[shchuka]

Hmm I'm confused now. What role does does SSP play then? I though it was all about being in the slowest speed region where you can pick up a contact much clearer and faster than in the fast speed region

[Pisces]

Sound bends it's direction when it pierces levels with different sound speeds above and below it. So if the sound speed is reducing with depth then each feet of depth it penetrates it slightly bends downward towards the lower speed region. If the ray is already pointed downwards then it goes steeper down. If it is pointed upwards then it turns shallower.

If the sound speed increases again with depth then every feet of water it passes the soundrays bend toward the lower speed region and thus upwards. This can create a pattern of sounds trying to climb to the surface and keep bouncing down at the water-air interface. This is a surface duct, and increases the detection range. But it is very much dependant on the surface state. Stormy surface doesn't reflect the noise very nicely.

If you have a minimum soundspeed at some depth then the soundrays start oscillating around it and create a (deep) sound channel. But you need quite a depth for this to happen, as sounds hitting the bottom will distort it.

As far as I understood, soundspeed layers are not really reflective or like a opaque curtain, as Hollywood would like to make us believe. Maybe more so with extreme salinity and temperature changes will it have reflective properties. Sound will still penetrate it, but it will have less intensity. And due to the bending it will head the other way again within some distance and never reach the listener until it is too close. This is where our tactical advantage comes in when staying below the layer. The trick is to be in a depth in which your sound doesn't reach your target, and most of his does reach you.

[FPSchazly]

Yeah, it's all about the "layer". Being able to detect faster would theoretically only help if both targets were dead silent and then became noisy at the same time. Even then, one would detect the other only fractions of a second faster. I'd wager that being able to get enough data and generate an effective solution is more important than detecting someone first, unless you have a wide aperture array :D

[banryu79]

@shchuka:
The "Sound Propagation" utility by Dr.Sid, it's very useful in understanding what kind of sound patterns are generated from a sound source at a given depth in an underwater environment described by a given SSP.

You can play with this utility and with the small pack of SSP shipped with it or you can create your own custom SSP. This last feature is very useful in the context of DW: at the start of a mission you can look at the values of the SSP profile and use them in "Sound Propagation" to generate the sound patterns of a given source located at a specific depth and even export it as an image.

For example: use "Sound Propagation" to compute sound patterns for a source under the layer and above the layer; then you can anlyze the results to find the shadow zones and understand better the ditance/depth/detectability from the source...

[Pisces]

Yes. But with the added caveat that the game is probably a severe approximation of it. Dr. Sid's measurements and tests ( search for his threads) suggest that it's modeling is fairly simple. But does reflect the various sound transmission path types. So use this tool to understand what happens in real life. The game does not do this raytracing

[banryu79]

Yes. But with the added caveat that the game is probably a severe approximation of it. Dr. Sid's measurements and tests ( search for his threads) suggest that it's modeling is fairly simple. But does reflect the various sound transmission path types. So use this tool to understand what happens in real life. The game does not do this raytracing
Good points, I should have added this considerations (I red Dr.Sid thread mounths ago...)
Even this, SoPro is still a very useful, educating tool, especially for total newcomers in the undewater sound propagation world :)

[banryu79]

Btw, I'd like to include here below the links to two intresting threads that could be relevants to shchuka and other interested readers:

DW sound propagation model measurements - by Dr.Sid (http://www.subsim.com/radioroom/showthread.php?t=117814)

Seeing the SPP in action? - by Markhot (and Sonoboy) (http://www.subsim.com/radioroom/showthread.php?t=124973)

EDIT: fixed second link.

[Pisces]

The last link doesn't work.