Does thermal layer mask sub from active sonar aswell?

[Castout]

I know that noise or detection by passive sensor is inhibited by changing layer. Does it also inhibit active sonar? I heard from somewhere that in RL it does...:hmm:

[Dr.Sid]

I think it is simulated. At least the pings are not heard under the layer (and correct distance).

[TLAM Strike]

You bet ya! Check out Nordic Hammer in the RSR Campagin for a good example of this.

[Castout]

It is...? Well quite a suprise...a good one

[Dr.Sid]

Why ? If you have it for passive, why not use it for active too ?

[SeaQueen]

Originally Posted by Castout

I know that noise or detection by passive sensor is inhibited by changing layer. Does it also inhibit active sonar? I heard from somewhere that in RL it does...:hmm:

Actually, in real life, the thermal layer isn't a big deal to low frequency passive sonar. Typical surface ducts found in nature are not fat enough to support low frequency passive ducting so the sound waves act like the duct is hardly there. You might see some of it if there was a double duct, but that doesn't happen often enough to make a difference. Active sonar is what's most effected by the thermal layer because the operating frequencies are typically higher.

Unfortunately, in video game land, they always show you these ray traces. The thing is, ray traces are only an approximate solution and are really only good in the medium to high frequency domains. So... people in video game land think surface ducts effect sonar uniformly across the frequency spectrum. It's not true, though.

I wish that there was a naval sim that captured the frequency dependent behavior of sonar but my suspicion is that would have two problems:

A) It'd be computationally expensive because you'd have to be constantly computing the wave solution to the sonar equation

B) Players would freak out because they'd see a ton of behaviors they'd never seen before in a sub sim and figure it HAD to be doing something wrong.

It'd be interesting, though.

[DrMilton]

Originally Posted by SeaQueen

Actually, in real life, the thermal layer isn't a big deal to low frequency passive sonar.

So, does that mean that you can pick up that first 50Hz no matter which side of the layer you are...?

[SeaQueen]

Originally Posted by DrMilton

So, does that mean that you can pick up that first 50Hz no matter which side of the layer you are...?

Maybe. It still depends on the usual suspects (water depth, sound speed profile, source depth, target depth, bottom composition..). It's just that unless the sonic layer depth is really deep, a 50Hz signal not going to get trapped in the surface duct the same way a 5000Hz signal might.

[Dr.Sid]

Hm .. 50Hz has wavelength of 30m. From what I've read surface duct can be shallower, but even deeper. From theoretical point of view this is really not easy to tell. 5 Hz would not be trapped at any time. 500 Hz will be trapped practically always.

[SeaQueen]

It's more complicated than that because of the nature of the way waves and boundary conditions work. If you calculate the normal modes of the duct, the cutoff frequency of that duct is related to the wavelength of the first normal mode. It turns out that it goes as the 3/2 power of the sonic layer depth, so the cutoff frequency of a 30m surface duct works out to be about 1 kHz if I did my arithmetic correctly. That means for the sound to be trapped by a 30m duct, you need it to have a frequency GREATER than 1 kHz. For 500Hz to be trapped you'd need a duct at least 50m deep, that's over 164ft. Typically mixed layers don't usually extend that deep so for low frequencies to be trapped you'd need something like a weird cold core eddy but even then I don't think it's common for these sorts of features to form. You'd almost need a double duct due to strange variations in salinity or maybe a big storm mixing masses of water in strange ways. It happens, but I wouldn't plan around it.

You can find formulas in Computational Ocean Acoustics by Jensen et al. or in Urick. The formula in Jensen is just the formula in Urick rearranged from wavelength into frequency and converted to metric.

Originally Posted by Dr.Sid

Hm .. 50Hz has wavelength of 30m. From what I've read surface duct can be shallower, but even deeper. From theoretical point of view this is really not easy to tell. 5 Hz would not be trapped at any time. 500 Hz will be trapped practically always.