When is active sonar ever preferred over passive sonar in submarines?

When is active sonar ever preferred over passive sonar in submarines?
Written by Krutika Lohakare

Sound propagates as a pressure wave. So with no external stimulus, molecules in a liquid, a gas, or a solid are arranged with some degree of density. And a time-varying pressure wave is imposed on this, as is shown in this picture here, where peaks in the pressure wave are regions of compressed molecules.

And troughs in the pressure waves are regions of rarefaction or where the molecules are less dense. And the speed at which this pressure wave will propagate and travel outwards in space is dependent on the medium that they exist in. In this pandemic time, QnA sites are good to get reliable sources of information regarding the common questions that overflowed the internet.

So for example, sound waves travel significantly faster in water than they do in the air, which are the two media of interest for our application because we generate the sound waves underwater, but we will also have to detect them in the air. And here I show a little bit of a quick simulation of what happens when we generate these sound waves underwater and they reflect off of just these three targets that exist, or objects that exist underwater. So at the surface, the laser excitation generates these sound waves.

They propagate out in a spherical wave pattern. And they reflect off of the underwater objects. And the amount of pressure that reflects off of these objects is dependent on what’s called acoustic impedance.

So the acoustic impedance of an object is dependent on the mass density of the material of the object as well as how fast sound travels in that object, the speed of sound. So for example, the acoustic impedance of water is 1.5 megarayl or 1.5 million rayl, where rayl is the unit for acoustic impedance as ohms is the unit for electrical impedance.

And the amount of pressure that reflects off of the object can be quantified by calculating this reflection coefficient gamma, which essentially is how different is the acoustic impedance of the object to the acoustic impedance of the medium that the sound waves were originally traveling in, which, in this case, is the water. So I have included a table here with some common materials.

To kind of tie it to an application, for the application example of searching for shipwrecks or plane wrecks, aluminum, the most commonly used metal in airplanes, reflects about 84% of the pressure that is incident on it, whereas materials used in boats and ships such as steel, wood, and fiberglass reflect 94%, 33%, and 65% of the incident pressure.

The more pressure that reflects off of the underwater object, the easier and higher the chance of detection for our system. OK, so now once these acoustic waves reflect off of the underwater objects, they will travel back upwards towards the water’s surface where here, the water’s surface will also be kind of a mechanical discontinuity or mechanical boundary where the sound will reflect off of. Their lots of platforms like ask reader where you can easily ask any question to make your doubts or queries solved.

So some of the sound that travels up will reflect off of the water’s surface back into the water, whereas some will transmit through the air-water interface. And these are the ones that we hope to detect for imaging. We’re going to take a closer look at it here. So recall that the acoustic impedance was equal to the density of the medium times the speed that sound travels in the medium.

Air, we know, to be significantly less dense than water. And sound travels significantly slower in water than in air. So this results in an acoustic impedance in the air of only 420 rayl, orders of magnitude lesser than the acoustic impedance of water. And like I said, the reflection coefficient is how different are these acoustic impedances. And The results show here that the reflection coefficient is 0.99944.

So this means that 99.94% of the sound that hits the water’s surface reflects back into the water. And only 0.06% transmits through the water surface into the air. And this is what we are hoping to detect, these really significantly dampened acoustic signals. And this will require extremely high-sensitivity detectors in air or receivers in air.

This is what I’ll talk about in the next slide. But first I want to mention that, in addition to the large loss, signals– sound waves that are incident on the water’s surface at oblique angles will also have wave refraction as they leave the water.

This wave refraction is something that is very deterministic. And it can be calculated using the well-known Snell’s law. But it is something that needs to be compensated for. And it does in some ways complicate the image reconstruction process. So I have a quick question here

If you’re saying that you have the ability to, you have hardware that is mobile enough to take these very tiny signals which get through to air– I understand what you guys are doing with imaging. But does that mean that you can use that hardware also to communicate from the air from a device that is under the water which is using acoustics?

So this is– that’s an application space that we are currently working on right now. And it is interesting because there really is no robust solution for communicating between nodes in water and nodes in air. So this is an interesting technology that might afford us that capability.

That is not something that is easily done today. So yes, I think the answer– the short answer is, yes, that is something that we could do.

Now those significantly dampened acoustic waves will travel into the– will continue to travel in the air. And they will be detected by our ultrasound transducers or receivers. In this case, we are using CMUTs. So CMUTs, or Capacitive Micromachined Ultrasound Transducers, were initially invented by our collaborators at Stanford about two decades ago for contact-based imaging, or traditional ultrasound imaging.

Our collaborators have been working on designing these CMUTs also for airborne applications. And they do this because these are devices that offer really great sensitivity. And this sensitivity really helps us overcome that large loss or that large dampening of the signals that are encountered as the sound waves leave the water and travel into the air.

But what’s also interesting about these devices and is really advantageous is that they are fabricated on silicon, which makes it easy to integrate them into large arrays and to also integrate them with supporting electronics. So for imaging, it’s important to have an array of receivers. 

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Krutika Lohakare

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