DW sound propagation model measurements

[Pisces]

Originally Posted by Dr.Sid

... But upgrading SoPro is number one priority now (shorter name for sound propagation .. how do you like it ? lol).

Cool! But how about "SoProp-er"? Proper in a UK dictionary means better or right. "It's the proper way to simulate sound propagation"

[Dr.Sid]

Uh .. so exact filling of area between rays showed little more complicated than it seemed son I only solved some basic cases and SoPro 1.1 is not ready for release yet. But it looks promising !

[Pisces]

Pisces, the reason I selected ‘surface bounce’ equal to zero was just to simplify the display. I assumed that surface refection would only become relevant in the creation of the 2nd. CZ.

Took some time to think about this longer. But I don't thinks so. The convergence zone area at the surface is a small area. Which means that rays that strike the surface on the near end of (even the 1st) CZ reflect back down and immediatly reinforce the rays that come up to strike at the far side of the CZ. Which results in a lesser signal if reflection is off.

As an example see the following images:
With reflection:

No reflection!!!:

Strangely enough the center of the starpattern doesn't show increased signal level, but some of the arms do have higher dB levels

But to understand the paths the rays take it is smart to simplify this way and not get distracted.

[Dr.Sid]

dB measurement is not much exact .. just wait for the next version.
You are right .. with no reflection half of the sound energy is wasted, but the pattern will be more or less same.

[feld]

Dr. Sid,

Do you happen to recall water depth during these tests?

-feld

[Dr.Sid]

Don't have missions anymore but IIRC it was 10.000 to 12.000 feet and in different sets of test I verified that water depth has absolutely no effect on sonar (at least passive one).

[feld]

Originally Posted by Dr.Sid

Don't have missions anymore but IIRC it was 10.000 to 12.000 feet and in different sets of test I verified that water depth has absolutely no effect on sonar (at least passive one).

That's deep enough: I think your own data shows some water depth effects on sonar. My source for much of what follows is here. But I think that Ulrich's Underwater Sound has similar discussions and I seem to recall that you mentioned the book in a previous post.

Spherical to Cylindrical Spreading:
This means that the exponential tail on the left side of the graph is probably spherical spreading. Ulrich mentions it in the "transmission loss" chapter of his book. Around pg 120 I think but I don't have mine handy. Anyway spherical spreading is used to compute transmission loss when the range is short (for example, my source link above uses less than half the water column height) in simple SONAR equation based propagation models. 10,000-12,000 feet is 3 to 4 kiloyards or roughly 1.5 to 2 nm which matches up nicely with the transition from exponential tail to linear slope at the far left side of your graph. I'll bet that, if you went someplace really really deep (like marianas trench) that this change from exponential (actually 1/R^2) to linear would occur at a longer range. The transition should occur at the

holy Bottom Bounce Batman!
More exciting, I just realized that the large "hump" on the Bottom Limited curve and the smaller hump on the "Surface Duct" curves could be bottom bounce transmission loss!

Take a look at this picture from the FAS website below:
It's a curve of transmission loss (dB) versus range (km). Note that the zero for TL is at the top of the graph. This means that TL numbers near the bottom of the graph mean higher losses and lower probability of detection for a source/receiver combination at the range on the bottom.


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

Note the "hump" structure at ~10km. That's bottom bounce "gain" i.e. a range at which less sound energy is lost in transmission. If you imagine the TL vs range line above subtracted from a horizontal line measuring signal strength and plotted it might be easier to see...

@Dr. Sid: Do you remember how the "Surface Duct" curve looked when the source and receiver were below the layer? There should be a BB hump there too...if I'm right.


-feld

edit: to add the stuff on bottom bounce

[Dr.Sid]

Cool .. somebody really looked at my data

Unfortunately I can't remember about the situation where both listener and source are under the layer. This calls for a another test.

As for the bulge .. it could be even bottom bounce, in case that reflection angle would be taken into account (or phase interference). Otherwise the bottom effect would be uniform for all distances. Tests with different water depth would solve this. I recommend you try it, it's great fun, but little time consuming.
I'm quite busy and I invest all spare time in the 'other project' (see my signature).
It's a pity we most probably will never know how exactly DW model works, and it still can surprise.

[feld]

I'm in the midst of writing a pretty big East China Sea scenario. Some of the water is deep enough that I should be able to do decent testing. It'll be a while but I'll report what I find.

real life is definitely beginning to interfere with my gaming...
-feld