static subs still emits noise?

[magicstix]

Originally Posted by Dr.Sid

I did some simulations .. just in 2D, and just with masking object of shape of circle (ie approximation of sphere in 3D) .. and it seems that even waves twice the size of the masking object leave some shadow. At wavelengths at same size the shadow is rather well defined and protrudes many wavelengths behind the masking object.



Sound comes from left, as planar waves. I've tried point source too, but it looks about the same. Te wavelength is 8 pixels, so is diameter of the masking object. The brightness is average amplitude over time. Ie you don't see individual waves, without any interference the picture would look like flat are with brightness slowly dropping with distance.
In these kind of simulation, sound reflects of any part which does not 'move' .. ie. also from borders of the simulation. To prevent it, border 50 pixels are 'attenuation zones' .. amplitude there is artificially attenuated so the sound is absorbed, rather then reflected. Even so some of the sound can be seen being reflected from top, bottom, or even right border. The circles in front of the blocking object are interferences of incoming and reflected waves, and they nicely show wavelength of the sound.

If we take Kilo as 10m diameter, and if we look for frequency with such wavelength, it would be about 150Hz. IMHO we can take the picture as rather good simulation of sound at 150Hz bending around Kilo sub.
150Hz is rather present in usual background noise. Higher frequencies would be masked even better, but background noise drops quickly with frequency.

I know sub guys usually go ultra silent when someone talks about 'how does Kilo look on sonar' .. especially when someone mentions 'hole in the water' .. but my bet is at 130m and silent, Kilo would be spotted as shadow on BB. Or anything at that size.

Sure my simulation is pretty crude, and I don't have anything 'official' to compare it with .. it might be totally off .. but it's my 2 cents ..

A few minor problems:
- Close in sound does not behave as a planar wave at all, wavefront curvature effects are extremely important.
- The amount of shadowing and diffraction is very frequency dependent.
- Broadband sonars don't operate on a single frequency, but instead the average amount of energy in a band.
- Broadband sonars have complex autogain and normalization algorithms that affect how the data will appear to the operator.
- You're ignoring beamforming and array effects.
- You're mostly ignoring multipath effects around the occluder.
- Your sim treats the kilo as a perfect occluder with no transparency.
- Your sim assumes noise sources occluded by the kilo behave with strong directional correlation. This is not the case in ambient sea noise.
- Ray based models don't simulate low frequencies properly at all, especially at your 150 Hz test case. Sound doesn't behave like a ray at low frequencies, it behaves more akin to electrons in a waveguide, with "fuzziness" and a distinct lack of directionality.
- The "kilos are a black hole in the ocean" thing is a myth, started by a nefarious source linked in a wikipedia article.

That said, there has been research into using ambient noise as a sort of "acoustic daylight" to "see" quiet objects through shadowing and correlation coming from reflections off the object, but those are all very special arrays with complex processing that isn't employed in your run-of-the-mill passive broadband sonar, and they don't work very well at all yet (and tend to only work at extremely close ranges of tens of feet).