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Deamon
Joined: 30 Apr 2002
Posts: 2302
Location: Germany
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 Posted: Sun Apr 23, 2006 2:56 am Post subject: @Sonalysts: Reverberation question |
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Back then as i played sub command there was a feature where when you appraoch the surface or bottom then at a curtain distance from the surface the ownshipnoise starts to radiate back and increase the ownship noise resp. the background noise. Wasn't this called valume reverberation ? Anyway my question would be how exactly do this work ? What formula should i use to calculate this ?
This would be especialy a question to the sonalysts guys.
regards,
Deamon
Last edited by Deamon on Wed Apr 26, 2006 8:48 am; edited 1 time in total |
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compressioncut
Joined: 21 Jun 2003
Posts: 238
Location: Canada
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| Posted: Sun Apr 23, 2006 8:04 am Post subject: |
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Volume reverberation is caused by junk (suspended particles, bubbles, etc) in the water causing uneven and unpredictable sound propagation and return. I've only ever used the phenomena as it relates to active sonar transmissions and returns, not ownship noise or passive sonar. I don't think it's much of a concern at the low frequencies involved in passive sonar.
What you're talking about sounds like bottom bounce and surface bounce, with OSN reflecting back directly onto OS sensors. Sounds like something unique to subs, and certainly new to me. |
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Deamon
Joined: 30 Apr 2002
Posts: 2302
Location: Germany
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| Posted: Sun Apr 23, 2006 8:47 am Post subject: |
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| compressioncut wrote: | | Volume reverberation is caused by junk (suspended particles, bubbles, etc) in the water |
It's a long time ago, but i thought this effect was called surface or bottom referberation. But i migh error here.
| Quote: | | causing uneven and unpredictable sound propagation and return. |
Becose of an uneven concentration of junk in the water ?
| Quote: | | I've only ever used the phenomena as it relates to active sonar transmissions and returns, not ownship noise or passive sonar. |
Don't know how it is with ships but for submarines the volume referberation als throws back part of the OSN back at the sub and increase the ovrall interfering noise that way, if i remember right.
| Quote: | | I don't think it's much of a concern at the low frequencies involved in passive sonar. |
Why not ?
| Quote: | | What you're talking about sounds like bottom bounce and surface bounce, with OSN reflecting back directly onto OS sensors. Sounds like something unique to subs, and certainly new to me. |
Yes some sort of a resonance effect near the surface or hard bottom.
Deamon |
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SeaQueen
Joined: 23 Jun 2005
Posts: 358
Location: Washington, DC
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| Posted: Sun Apr 23, 2006 9:11 am Post subject: |
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| Deamon wrote: |
It's a long time ago, but i thought this effect was called surface or bottom referberation. But i migh error here.
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You're right. The difference between a bottom bounce and bottom reverberation is that bottom bounce is sound you want and bottom reverberation is sound you don't, basically.
Technically, though, usually people refer to reverberation when they're talking about active sonar. There's things called sonar equations, and the reverberation level in an active sonar equation refers to energy from the ping scattered back to the receiver off of any random stuff that's not a target. Essentially, it's just "noise."
In passive sonar, if that actually was an effect, it probably isn't a big deal in most situations. It's reverberation in a looser sense of the term. It shouldn't really be a big deal in most situations, though.
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Becose of an uneven concentration of junk in the water ?
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They're just objects with a density other than that of water. That causes some energy to be reflected back. If there's an awful lot of these little objects out there, the effect adds up and you have to account for it.
My favorite volume scatterers are schools of fish. Their swim bladders are extremely efficient scatterers of sound. You can see them come out at night, and go to sleep during the day. It's kind of neat. There was a neat article in the most recent issue of Physics Today about that actually. It made me happy.
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| Quote: | | I don't think it's much of a concern at the low frequencies involved in passive sonar. |
Why not ?
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Because the wavelength of the sound is a lot larger than the features responsible for most of the backscattering (mostly waves from the surface and the irregularity of the bottom). There almost certainly is some, but I'm not sure there's enough to worry about most of the time. |
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compressioncut
Joined: 21 Jun 2003
Posts: 238
Location: Canada
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| Posted: Sun Apr 23, 2006 10:40 am Post subject: |
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| SeaQueen wrote: | | Deamon wrote: |
It's a long time ago, but i thought this effect was called surface or bottom referberation. But i migh error here.
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You're right. The difference between a bottom bounce and bottom reverberation is that bottom bounce is sound you want and bottom reverberation is sound you don't, basically.
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Bottom reverberation is not volume reverberation. Bottom mostly causes absorbtion and scattering losses, but volume reverb concerns us most.
The major cause of volume reverberation is suspended particulate and air bubbles, and yes, fish, in some locations. And there are reverberation-limited formulae for use in active range prediction, but not in passive (to my knowledge). |
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SeaQueen
Joined: 23 Jun 2005
Posts: 358
Location: Washington, DC
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| Posted: Sun Apr 23, 2006 10:56 am Post subject: |
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| compressioncut wrote: | | And there are reverberation-limited formulae for use in active range prediction, but not in passive (to my knowledge). |
Same here. The whole effect he described struck as something that MIGHT be necessary to account for if you got really close, but not likely. I imagine that if it was real, it'd be less likely to be seen near the surface, than if you got really close to a bottom type that wasn't very lossy and then made a really loud noise. It strikes me as the sort of thing that follows from the fact that if you get up really close to a reflective surface, certain nearly vertical angles would be reflected almost straight back down. It doesn't seem impossible to me, at least by intuition.
The thing is, in the passive sonar equation the source of the noise is the target. What he's decribing is almost like a modification to the self noise term.
I dunno...
As far as volume scattering goes, I thought in passive sonar that was taken into account in the transmission loss term. Damnit... my copy of Urick is at work.... *growl*
Regardless, I don't think it should be in the game anyhow. If you're that close to the bottom, you're aground and if you're that close to the surface, you're broached. |
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Deamon
Joined: 30 Apr 2002
Posts: 2302
Location: Germany
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| Posted: Sun Apr 23, 2006 1:20 pm Post subject: |
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| SeaQueen wrote: | | Deamon wrote: |
It's a long time ago, but i thought this effect was called surface or bottom referberation. But i migh error here.
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You're right. The difference between a bottom bounce and bottom reverberation is that bottom bounce is sound you want and bottom reverberation is sound you don't, basically. |
| compressioncut wrote: |
Bottom reverberation is not volume reverberation. Bottom mostly causes absorbtion and scattering losses, but volume reverb concerns us most. |
I think what i'm reffering to is best described as an resonance from a hard bottom or the surface when you are close to it. That was the point of my post. What i'm looking for is a formula that i can utilize in my sonar engine. I hoped to hear the sonalysts guy on how they made it.
This was simulated in sub command when you have approached the surface pretty close. I just did a test: Walked towards a wall and said loud and continuously AAAAAAAAAAAAAAAAAA when approaching the wall very close i notice a significant increase in valume!
Same happens when you approach the surface or hard bottom with a sub that emmits enough noise.
Now i need a formula to calculate this.
| Quote: | | Technically, though, usually people refer to reverberation when they're talking about active sonar. There's things called sonar equations, and the reverberation level in an active sonar equation refers to energy from the ping scattered back to the receiver off of any random stuff that's not a target. Essentially, it's just "noise." |
| Quote: | | In passive sonar, if that actually was an effect, it probably isn't a big deal in most situations. It's reverberation in a looser sense of the term. It shouldn't really be a big deal in most situations, though. |
Are you talking about reverberation or the resonance effect that i have mentioned ?
| Quote: | | Quote: |
Becose of an uneven concentration of junk in the water ?
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They're just objects with a density other than that of water. That causes some energy to be reflected back. If there's an awful lot of these little objects out there, the effect adds up and you have to account for it. |
And the distribution of junk is uneven, what makes the uneven propagation/return right ?
On a side note: In summer when the water becomes warm a curtain liquide chemical transforms in to gas and provides alot of junk. The reverberation increases dramaticaly and virtualy reduces the detection range by half as compared to winter!
| Quote: | | My favorite volume scatterers are schools of fish. Their swim bladders are extremely efficient scatterers of sound. You can see them come out at night, and go to sleep during the day. It's kind of neat. There was a neat article in the most recent issue of Physics Today about that actually. It made me happy. |
That is good to know. Any explicite data on the return ?
| Quote: | | Quote: |
| Quote: | | I don't think it's much of a concern at the low frequencies involved in passive sonar. |
Why not ?
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Because the wavelength of the sound is a lot larger than the features responsible for most of the backscattering (mostly waves from the surface and the irregularity of the bottom). There almost certainly is some, but I'm not sure there's enough to worry about most of the time. |
But aren't waves longitudinat ? and thus what does the frequency matter then ?
Deamon |
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Deamon
Joined: 30 Apr 2002
Posts: 2302
Location: Germany
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| Posted: Sun Apr 23, 2006 1:41 pm Post subject: |
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| SeaQueen wrote: |
Same here. The whole effect he described struck as something that MIGHT be necessary to account for if you got really close, but not likely. I imagine that if it was real, it'd be less likely to be seen near the surface, than if you got really close to a bottom type that wasn't very lossy and then made a really loud noise. It strikes me as the sort of thing that follows from the fact that if you get up really close to a reflective surface, certain nearly vertical angles would be reflected almost straight back down. It doesn't seem impossible to me, at least by intuition. |
Well, whether and how strong this effect occures depends(i guess) on the size of the submarine, noise it emmits and the proximaty to the reflective surface.
The formula consists of variables and i need to know whitch math operations i should apply to them.
| Quote: | | As far as volume scattering goes, I thought in passive sonar that was taken into account in the transmission loss term. |
Sure it is.
| Quote: | | Regardless, I don't think it should be in the game anyhow. If you're that close to the bottom, you're aground and if you're that close to the surface, you're broached. |
Again i think it depends on the values of the variables. This effect should be especialy stronger in very shallow water, when the resonance comes from the surface and bottom at once adding an additional interferance source to the already difficult hydroacoustical situation.
I remember the NWS guys talking about this effect on the NWS yahoo group. So it curtainly matters. And it was modeled in Sub Command anyway. This is especialy a concern at PD
Deamon |
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