Active sonar and submarines, acoustics

[Nikitin]

Hi, a newb-question here. How does it work?

Let's say an active sonar from a ship sends out a sound-wave at the suspected location of a submarine.

How does it actually detect the submarine? I mean, why is the echo the active sonar receives back so special? I mean, if the sound wave hits the flat bottom of the ocean a nice echo comes back as well...

Is it because submarines have a specific sound signature? So that their echo is unique?
or
Is it maybe because the sonar operators try to spot an irregularity when "mapping" out what is below them? So in that case the submarine should be practically invisible to the active sonar when lying at the bottom of the ocean?

 

[Lsos]

I'm no sonar expert, but depending on how far the object you're looking for is, the echo will come back sooner or later.

You send out one ping, but many different echos come back at different times from different directions. The sooner the echo comes back, the closer the object.

I'll guess that a sub is difficult to spot if it's resting on the bottom, but I'm not sure what the resolution of sonars is. I'm sure if you're close enough and the bottom is flat enough, you'lll be able to spot an irregularity.

I'll venture to guess that each sub has it's own specific echo signature as well...

 

[Nikitin]

thanks for the help, you sure know more than me :)

hmm but aren't active sonar sound waves moving in just one direction, like laser waves (light/IR waves) move in one direction? sure when they hit an object the sound waves would disperse in all manners of directions (that's the echo I believe), but the listening device can only pick up an echo from the direction that it sent the pulse, so you'd get only one echo I think but I certainly may be wrong

Excuse me for my English

 

[russ_watters]

Subs don't use active sonar much - you can't be stealthy if you're blasting your stereo. But if they already have a direction to their target using passive sonar, active will give them distance.

Also, unless they are very near shore, they don't need to worry about the sea floor messing up the signal.

 

[Nikitin]

no I was talking about ships trying to find submarines.. I am wondering why the echo from subs is so much more special than, say, the one from a round rock at the bottom of the sea.

I thought it was maybe because a ship is mapping an area with active sonar where it thinks there may be a sub and finds the sub as an irregularity. or that subs have a special echo signature or something

 

[Vanadium 50]

If the sub is 200 feet down, and the sea floor is 2000 feet down, which echo will arrive first?

 

[Nikitin]

allright thanks, so basically the active sonar is detecting irregularities when "mapping" what is below it? Would this mean that if a sub is maybe 2-3 meters above the ocean floor it could survive active sonar bombardment undetected?

 

[rollcast]

You aren't working for the Russians are you??

The best way to understand it would be to get a fishfinder and understand how that works, then scale up.

As for resting on the bottom, hard to say without knowing the complexity of the system

 

[DragonPetter]

OP there's a sonar equation, analogous to the radar equation.

You have a source level SL
This is the intensity of the acoustic energy from your transmitting hydrophone. Say you send out a pulse and wait for a pulse to come back, the sent pulse intensity level is your SL.

A target strength, TS which is the intensity of the acoustic energy that is projected by any object you might see with a submarine's detectors. In active sonar, this is how much intensity of the signal hitting the target gets reflected back, like the energy a submarine would send back at you. In the passive sonar, this is basically how high the intensity of the sound that the target is making. Say a whale swims by and makes a noise and you're listening for it, the intensity of the noise the whale makes is the TS.

A transmission loss, TL, this is how much intensity is lost of an acoustic wave as it travels over a distance, so as the distance gets larger, TL gets larger. TL is dependent on spreading loss, and more complicated mechanisms like sulfate relaxation and shear stresses in the water that the acoustic signal is dissipated as heat.

A noise level NL, and other factors such as reverberation. This is a noise floor that any detectable sound needs to be stronger than, or else it will get lost in the noise. This can come from wind on the sea surface, marine life, ships in the distance, turbulence and machines in your submarine, etc.

Then there is a detection threshold that is dependent on all of the above factors.
DT = SL + TS - 2TL - NL

In the case of a passive sonar, the SL is not present and it is TL instead of 2TL because the sound only travels 1 way instead of roundtrip. So if a submarine reflects a noise with an intensity louder than 2TL + NL - SL -DT, it will show up as a waveform on the submarine's sonar. Different objects make different waveforms, and so classification of the waveforms is necessary to determine what kind of object your sonar detected. A submarine will reflect a lot more energy than a fish, so you can see the strength of the reflection and determine what the size of the object was.

As far as your other question, there are some hints.

The sea floor distance is already known, or atleast it is not changing so that you can determine it pretty fast. The time delay from when you send the pulse til you receive from the sea floor is basically a known, and if you get a reflection with a different time delay, you will know it came from somewhere else other than the floor.

Also, the frequency at which the pulses are sent determines the wavelength of the acoustic wave. A very high frequency is needed to detect smaller objects, and so an appropriate frequency is chosen to find objects of a certain size. Think of this as tuning your sensor's frequency to pick out the size of a submarine best.There are different sensors, single-beam, split-beam, multi-beam hydrophones. Split beam uses an array of transducers with 2 beams on different axes that let the hydrophone determine the direction that the object came from. More complicated beams are available that even allow imaging with acoustics. The hydrophone array allows you to steer your beam and focus it at certain places, so it is easier to guess what you're looking at if you know where the sound is coming from.

 

[DragonPetter]

thanks for the help, you sure know more than me :)

hmm but aren't active sonar sound waves moving in just one direction, like laser waves (light/IR waves) move in one direction? sure when they hit an object the sound waves would disperse in all manners of directions (that's the echo I believe), but the listening device can only pick up an echo from the direction that it sent the pulse, so you'd get only one echo I think but I certainly may be wrong

Excuse me for my English

You will get multiple echoes from the dispersion you mentioned. But the echo that reaches you the first is the direct echo from the object if reflected off of. All the other echos will take a longer time to reach you, which will distort the waveform.

 

[olivermsun]

Let's say an active sonar from a ship sends out a sound-wave at the suspected location of a submarine.

How does it actually detect the submarine? I mean, why is the echo the active sonar receives back so special? I mean, if the sound wave hits the flat bottom of the ocean a nice echo comes back as well...

If the submarine is not actually on or near the bottom, it could stand out as a large 'hard' target. A Doppler sonar might also detect that the large hard target is moving.

Is it because submarines have a specific sound signature? So that their echo is unique?

The sub hull might "ring" (although that's something you'd want to suppress in the design). of course, passive sonar might detect other funny noises as well, like the sound of machinery or screws if the sub is trying to escape.

Is it maybe because the sonar operators try to spot an irregularity when "mapping" out what is below them? So in that case the submarine should be practically invisible to the active sonar when lying at the bottom of the ocean?

This game depends a lot on how well the bottom is known. I would tend to doubt that a small sub sitting motionless on some irregular part of the continental shelf would be "visible." Then again, it would be a mean trick for the sub to park itself that way without using active sonar rangefinding (without crashing into the bottom, anyway).

Also, the frequency at which the pulses are sent determines the wavelength of the acoustic wave. A very high frequency is needed to detect smaller objects, and so an appropriate frequency is chosen to find objects of a certain size. Think of this as tuning your sensor's frequency to pick out the size of a submarine best.

I'd guess that any sonar with a frequency chosen to give decent range (attenuation is roughly per-wavelength) will be able to detect a large hard target like a submarine floating in the middle of the water column.

 

[DragonPetter]

I'm fairly sure any sonar with a frequency chosen to give decent range (attentuation is roughly per-wavelength) will be able to detect a large hard target like a submarine floating in the middle of the water column.

Frequency selection is still important and can help your chances to see a submarine. Higher than necessary frequency is a poor choice for 2 reasons:
1. Like you said, range is decreased
2. You will start to see small objects, which ruins your SNR if you consider a submarine as your only desired S. I was referring specifically to the backscattering relationship developed by Rayleigh, and choosing an appropriate wavelength that will maximize the backscattering cross section of a submarine while minimizing the backscattering of objects smaller than a sub.

 

[Nikitin]

thanks guys, i have lots of reading now ))

and no i don't work for the russians lol

 

[Nikitin]

hmm, you are using a high frequency sound pulse so that the wavelength of the sound pulse gets smaller? and thus the dispersion of the sound when it hits the target is smaller?

But basically, the active-sonar operator is looking for "irregularities"? Like you said, if the echo from an active sonar pulse gets back way too quickly, it is obvious that there's something floating there?

The submarines don't really have any "echo signatures", at least not the modern ones?