Physics of Underwater Sound

[Overdog]

Sonar is nothing at all like Radar. Radio waves can be assumed to travel at a constant speed, and are not significantly affected by such things as temperature, salinity, and density (or depth) as underwater sound is.

 

Temperature of the water, and the temperature gradient with increasing depth, the surface layer, depth, salinity, and thermocline all combine to profoundly affect the way sound propagates in ocean waters.

 

Oceanographers refer to the surface layer with uniform hydrographic properties as the surface mixed layer. This layer is an essential element of heat and freshwater transfer between the atmosphere and the ocean. It usually occupies the uppermost 50 - 150 m or so but can reach much deeper in winter when cooling at the sea surface produces convective overturning of water, releasing heat stored in the ocean to the atmosphere. During spring and summer the mixed layer absorbs heat, moderating the earth's seasonal temperature extremes by storing heat until the following autumn and winter, and the deep mixed layer from the previous winter is covered by a shallow layer of warm, light water. During this time mixing is achieved by the action of wind waves, which cannot reach much deeper than a few tens of meters. Below the layer of active mixing is a zone of rapid transition, where (in most situations) temperature decreases rapidly with depth. This transition layer is called the seasonal thermocline. Being the bottom of the surface mixed layer, it is shallow in spring and summer, deep in autumn, and disappears in winter, when heat loss at the surface produces instability and the resulting convection mixes the water column to greater depth (Figure 7.1). In the tropics winter cooling is not strong enough to destroy the seasonal thermocline, and a shallow feature sometimes called the tropical thermocline is maintained throughout the year.

 

Sonar then, to be accurate, must be calibrated for the ocean conditions which control how the sound pulse will behave.

 

In some cases, you will have what is called a surface channel. This is a channel of water near the surface where the temperature is constant down to the beginning of the thermocline. The sharp temperature change at the beginning of the thermocline will actually reflect a sonar pulse, thus channeling a hull-mounted sonar sound pulse in the surface channel for long distances. In other conditions, since sound travels perpendicular to its wave front, if the top of the sound wave is in warmer water, the sound pulse will bend downward, behaving as though it were a beam. So if the thermocline is right up near the surface, the sonar pulse will bend sharply down toward the bottom...

 

If the water is shallow enough, and bottom conditions are optimal, it may then be possible to bounce the sound pulse off the bottom of the ocean, where it will return to the surface some distance away. The sound may then reflect back off the surface, bouncing back and forth between the surface and the ocean floor as it propagates through the ocean.

 

In other conditions, the temperature gradient causes the sound pulse to bend down toward the bottom, but if the water is deep enough, the bottom of the wave front will begin to travel faster than the top of the wave front as the sound beam goes below 2000 feet, where density becomes the controlling factor. This will cause the sound beam to bend back up, where it will eventually hit the surface and bounce off, continuing to propagate in this fashion for long distances.

 

Just wanted to start this thread in case anyone is interested in discussing this topic...

[freeztar]

In other conditions, the temperature gradient causes the sound pulse to bend down toward the bottom, but if the water is deep enough, the bottom of the wave front will begin to travel faster than the top of the wave front as the sound beam goes below 2000 feet, where density becomes the controlling factor. This will cause the sound beam to bend back up, where it will eventually hit the surface and bounce off, continuing to propagate in this fashion for long distances.

 

Can you elaborate on the bolded part?

 

Also, I'd be quite interested to hear about the questions you may have about this subject (for discussion purposes).

[Overdog]

Can you elaborate on the bolded part?

 

Also, I'd be quite interested to hear about the questions you may have about this subject (for discussion purposes).

 

Ok, I will, but I'll have to do it tomorrow...as I've already stayed up past my bedtime...

[modest]

I also have a question that I've thought about before (I think while reading The Hunt for Red October).

 

If we're talking only about active sonar, is there not a frequency high enough to easily penetrate the thermocline? Does such a frequency (if existent) have drawbacks?

 

Excellent thread by the way and your experience on the subject shows.

 

~modest

[Overdog]

Can you elaborate on the bolded part?

 

Ok, the sound propagation scenario you are asking about is called "convergence zone" propagation. This, from wiki, describes it a lot better than I could...

 

Sonar operation is affected by variations in sound speed, particularly in the vertical plane. Sound travels more slowly in fresh water than in sea water, though the difference in speeds between fresh and salt water is small. In all water sound speed is determined by its bulk modulus and mass density. The bulk modulus is affected by temperature, dissolved impurities (usually salinity), and pressure. The density effect is small. The speed of sound (in feet per second) is approximately equal to:

 

4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in feet)) + salinity (in parts-per-thousand ).

This is an empirically derived approximation equation that is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. Ocean temperature varies with depth, but at between 30 and 100 meters there is often a marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. This can frustrate sonar, for a sound originating on one side of the thermocline tends to be bent, or refracted, through the thermocline. The thermocline may be present in shallower coastal waters. However, wave action will often mix the water column and eliminate the thermocline. Water pressure also affects sound propagation. Increased pressure increases the sound speed, which causes the sound waves to refract away from the area of higher sound speed. The mathematical model of refraction is called Snell's law.

 

Sound waves that are radiated down into the deep ocean bend back up to the surface in great arcs due to the increasing pressure (and hence sound speed) with depth. The ocean must be at least 6000 feet (1850 meters) deep, or the sound waves will echo off the bottom instead of refracting back upwards, and the reflection loss at the bottom reduces performance. Under the right conditions these sound waves will then be focused near the surface and refracted back down and repeat another arc. Each focus at the surface is called a convergence zone (CZ). This CZ forms an annulus about the sonar. The distance and width of the CZ depends on the temperature and salinity of the water. In the North Atlantic, for example, CZs are found approximately every 33 nautical miles (61 km), depending on the season. Sounds that can be heard from only a few miles in a direct line can therefore also be detected hundreds of miles away. With powerful sonars the first, second and third CZ are fairly useful; further out than that the signal is too weak, and thermal conditions are too unstable, reducing the reliability of the signals. The signal is naturally attenuated by distance, but modern sonar systems are very sensitive, i.e. can detect despite a low signal-to-noise ratio.

Sonar - Wikipedia, the free encyclopedia

 

So if you transmitted and wanted to listen for an echo coming back from the second convergence zone (66 miles), assuming an average speed of sound of 4,800 feet/sec, the total distance the sound would have to travel would be 66 x 2 = 132 miles = 696,960 feet. So you would need to configure your sonar to wait about 145 seconds after you transmit before you even start to listen for an echo coming back from the second CZ.

 

Here is an article describing the AN/SQS 26 Sonar, I served on the Knox class destroyer escorts. We also had a towed variable-depth sonar I worked on, called IVDS (Independent Variable Depth Sonar). The idea of this active sonar was to lower the transducer below the sound "layer", where we could transmit beneath the barrier of the thermocline.

 

AN/SQS-26 Sonar

The long range AN/SQS-26 sonar, installed on FF-1052 and FFG-1 class ships, is designed to use direct, bottom reflected and convergence zone acoustic propagation paths where they occur in the oceans to achieve maximum effectiveness. Active modes include bottom bounce, bottom bounce track, bottom bounce triple frequency, convergence zone, omni-directional. Passive modes include audio via headphone, video via plan position indicator (PPI), B-scope, A-scan recorder, graphic indicator, sector scan indicator, azimuth recorder, and numerous dial indicators.

 

The SQS-23 was too large to be fitted to all but the largest fleet destroyers of World War II, and the follow on to the SQS-23, the 3.5 kHz SQS-26, made matters even worse with an even larger transducer and greater power requirements. This demonstrated that new construction was going to be necessary to create an adequate surface ASW platform, but that even if designed as austere, ASW-only Frigates they would be larger and much more expensive than the largest World War II DD. Indeed, starting in 1960, 58 SQS-26 ships were authorized of the Bronstein (2), Garcia (10), and Knox (46) classes, but the SQS-26 also experienced problems similar to the SQS-23 and was not fully accepted for service use by the Navy until 1968.

 

The eventual success of the SQS-26 get well program led the surface community to finally abandon DASH in 1968 and embrace a manned helicopter option. This led to the LAMPS I program, a conversion of a lightweight, commercial utility helicopter, which first deployed in 1972. LAMPS I was small enough to operate from many DASH ships and it gave SQS-26 ships a reliable, first convergence zone weapon. LAMPS finally gave surface ships a final location and detection system to go with its convergence zone sonar so they could finally use SQS-26 and towed arrays.

 

The LAMPS/SQS-26 combination was widely deployed in the Bronstein, Garcia, and Knox class destroyer escorts (later retyped as FFs-1037-1098), 58 of which were deployed between 1960-1967. These were arguably the first truly successful, postwar ASW ships, and at the same time, certainly the least popular members of the destroyer community. This inverse relationship between ASW effectiveness and acceptance in the destroyer community highlights the fundamental doctrinal turmoil that community was experiencing during this period.

 

The surface force slowly evolved from a sensor/weapon suite based on the SQS-26, ASROC, and LAMPS I to one that included by the mid 1980s a passive towed array and the longer range LAMPS III. The LAMPS/towed array combination revolutionized surface ship ASW capabilities by combining the detection ranges heretofore only achieved at the tactical level by submarines deploying large, below layer, passive arrays with the rapid, long range prosecution capabilities provided only by air ASW assets.

AN/SQS-26 Sonar

 

EDIT:

 

Oh yes, the SQS-26 was huge, and took up maybe a quarter of the space on the ship. It used so much power to transmit that it had to have it's own dedicated power generator. You did NOT want to be in the water anywhere near the ship when that thing went "ping".

[Overdog]

I also have a question that I've thought about before (I think while reading The Hunt for Red October).

 

If we're talking only about active sonar, is there not a frequency high enough to easily penetrate the thermocline? Does such a frequency (if existent) have drawbacks?

 

The higher the frequency, the better the resolution of the returning data, but the usable range is greatly reduced. Mine sweepers and such use much higher frequencys that can resolve much smaller objects, but have much shorter effective ranges. As for penetrating the "Layer", (the barrier produced by the thermocline), I don't think frequency would make much if any difference. Some of the sound transmitted always penetrates the Layer, it's just greatly attenuated depending on how much of the sound energy is reflected...

 

It's been 25 - 30 years since I worked with Sonar, and I'm sure it's been improved a lot.

 

I wonder if we'll ever have Sonar as good as a Dolphin's. A Dolphin can tell the difference between an identical fake plastic fish and a real fish with it's sonar from the far side of an olympic-sized pool.

 

The sonar employed in medicine these days (which I know next to nothing about) utilizes high frequencies and gets amazing resolution, but if you've ever seen some of those sonagram pictures, you pretty much have to be an expert to interpret them.

[Overdog]

I also have a question that I've thought about before (I think while reading The Hunt for Red October).

 

Oh, speaking of The Hunt for Red October, that book/movie was one of the first to expose a lot of the cold war naval technology to the general public. I remember having trouble with suspension of dis-belief in a number of those scenes, however. Those torpedoes that Sean Connery was dodging right and left employ a combination of active and passive sensors to home in on the target. Once one of them picks you up, the movie is over. Unless, of course, your name is Sean Connery....

[Overdog]

I'm curious, how many have ever heard the acronym SOSUS?

 

SOSUS - Wikipedia, the free encyclopedia

[modest]

Once one of them picks you up, the movie is over. Unless, of course, your name is Sean Connery....

 

Right full rudder, 30 degree down angle - your move

 

 

Seriously though, I do remember thinking the book was based on some good technical knowledge. There was a part in the book (if I recall - a long time ago this was) where the Red October was jamming a torpedo's active Sonar by producing a ping of its own, which I found interesting.

 

But, I also recall incredulity at a lot of things. For instance, the Red October got hit point blank with a torpedo with little ill effect - in the keel at that! If I'm not mistaken that would split a sub in 2 directly. But hey, it is fiction after all :shrug:

 

Thanks for the reply above by the way - makes good sense.

 

~modest

[Overdog]

Seriously though, I do remember thinking the book was based on some good technical knowledge. There was a part in the book (if I recall - a long time ago this was) where the Red October was jamming a torpedo's active Sonar by producing a ping of its own, which I found interesting.

 

The MK-46 Torpedo is what we had, and probably what Sean Connery would have been dodging...

 

 

There are countermeasures you can try against these torpedoes, I suppose it's possible, but the scene you're talking about struck me as more like something taken straight out of a Star Trek episode.

 

Submarines do have various countermeasures they employ when attempting to evade active sonar, such as a high-speed turn that produces what we called a "knuckle", (a swirling vortex of water that reflects sound). Also they can release canisters of something like Alka-Seltzer, that produce lots of bubbles. More sophisticated and expensive decoys are slow moving "torpedoes" which are equipped with electronic sensors which pick up the ships sonar transmission and actually generate a fake response...these are mainly designed for evading an enemy ship's sonar.

 

There was also a very sinister little device, I don't remember what it was called, that could be exploded in the air above the suspected location of a submarine. The explosion scattered thousands of little swiveling magnetic hammers that would attach to the submarine's hull and hammer away as the sub tried to creep away at slow speed.:shrug:

[modest]

There was also a very sinister little device, I don't remember what it was called, that could be exploded in the air above the suspected location of a submarine. The explosion scattered thousands of little swiveling magnetic hammers that would attach to the submarine's hull and hammer away as the sub tried to creep away at slow speed.:shrug:

 

 

To quote another fictional mariner (Aubrey):

"What a fascinating modern age we live in"

 

~modest

[Overdog]

:shrug:

 

To quote another fictional mariner (Aubrey):

"What a fascinating modern age we live in"

 

~modest

 

Oh, this is all ancient history. I'm sure the modern stuff is a lot more sophisticated. Imagine a Tomahawk cruise missle with a Mk 46 warhead and a 2000 mile range that flys in a pattern above a section of ocean releasing sonobouys along it's route.

 

Sonobuoy - Wikipedia, the free encyclopedia

 

The missle monitors the sonobouys, and if a sub is detected, drops it's torpedo right on top of it.

[Overdog]

Now a Dolphin's sonar, compared to ours, makes it look like we're feeling around in the dark with a stick.

 

Dolphin Echolocation

 

The sensitivity of Dolphin sonar is simply astounding.

 

The biosonar of the bottlenose dolphin is a very sophisticated high performance system that continues to outperform any man-made system within its operating regime. It does this by employing an array of techniques not fully exploited outside of the biological sphere. While it is believed that all members of Cetacea (whales and dolphins) have at least some echolocation capability, that of the bottlenose appears the most optimized.

 

The capabilities of this biosonar echolocation system have not been discussed in a comprehensive but detailed report in the technical literture. The system employs a very sophisticated signal generating and receiving system and an equally sophisticated information extraction system. The attached case study, "Dolphin Biosonar Echolocation" will explore many of the features of the system at a level not previously documented.

 

The phonic and aural systems of the bottlenose dolphin have evolved well beyond that of other chordates, including the bat.

 

Sound is generated primarily in the nasal passages (and does not emanate from the mouth)

Sound generation has been broadened in frequency (into the UHF region of 150 kHz or higher)

Sound generation has evolved into two parallel systems that can operate independently or simultaneously

Sound reception is supported by two conformal lens-type receivers (unrelated to the "outer ear")

 

The overall system measures range and azimuth using two distinct operating modes

pulse mode for short range and high precision (less than 100 meters)

swept continuous tone for longer range (out to about 600 meters)

The system can determine the azimuth of active sources at much longer ranges

There is growing neurological evidence that the dolphin can "see" sound patterns just as it and other chordates see light patterns.

It is likely that it can merge the information that is sees in the visual and acoustic bands...

 

...In toto, the sonar of the bottlenose dolphin is considerably more sophisticated than any current man-made sonar in the world. It rivals the most advanced airborne radars available today. Merely listing the most obvious properties of this biological sonar (biosonar) attests to its fantastic capabilities. It is fundamentally a multi-band, multimode (including Doppler detection), frequency-hopping, steerable beam, binaural receiver, camouflage penetrating, single-pulse (when required) system with properties at least as sophisticated as the latest stealth fighter plane, the F-117, and latest stealth bomber, the B-2. It is not known how many targets the dolphin can track simultaneously. However, the capability is probably similar to the above man-made systems.

Dolphin biosonar (sonar)echolocation case study

 

The echolocation system of the dolphin is extremely sensitive and complex. Using only its acoustic senses, a bottlenose dolphin can discriminate between practically identical objects which differ by ten per cent or less in volume or surface area. It can do this in a noisy environment, can whistle and echolocate at the same time, and echolocate on near and distant targets simultaneously - feats which leave human sonar experts gasping.

Echolocation