Indian Navy News & Discussion - 15 Dec 2016

[Singha]

btw is it true that if a radar has a effective range of 100km, a passive RWR can pick it up out to 200-300km ? is the same true for passive sonar vs active sonar too?

[brar_w]

Same as with a jammer. Your RF emission only need to travel one way i.e from a transmitter source to an RW receiver a given distance away . For a radar you need the two way path from the transmitter to an object and back to the receiver to be able to detect and classify.

Here is the one-way equation - http://www.phys.hawaii.edu/~anita/new/p ... ne-way.PDF

and the Radar Range Equation - http://www.radartutorial.eu/01.basics/T ... on.en.html

[Singha]

real data on sonar systems seems highly classified, which accounts for the huge flap over leaked scorpene data.

[brar_w]

The performance of systems might be, but the physics behind the active and passive sonar detection are most certainly known.

For active Sonar equation - https://www.usna.edu/texis/search/redir ... 252016.ppt

For passive - https://www.usna.edu/Users/physics/ejtu ... apter9.pdf

[shiv]

So I'm not surprised coteries exist on BR and shiv & vina being pally with moderators get away with pages after pages of irrelevant whale analogies.

Sir please do not construe my disagreement with you as a plot with someone else to put you down. If I disagree with you I do it on my own and do not need assistance or allies.

Here are two screenshots from your 1984 paper. Both you and I have stated that helicopter noise is to be filtered out. And that is what is done. If helo noise is filtered out what difference does helo loudness make? Can you produce one reference that says helicopters for dipping sonar are selected for quietness?


I don't know if you did or did not read the links I posted but I certainly read yours. The noise produced by the downwash of a helo is stated in your reference as sounding louder than a sea state 2. Would a sub be able to differentiate between a heavy sea state and a helicopter?


It is your prerogative to make false accusations about conspiracies and personal friendships but please note that these allegations only serve to divert focus from the main issue - an act that I term as fudging. I have already stated that your explanations do not address my question and I remain unconvinced. I am yet to see a single paper from you from the 1950s, 1980s or later that submerged submarines can detect helicopters .

[shiv]

btw is it true that if a radar has a effective range of 100km, a passive RWR can pick it up out to 200-300km ? is the same true for passive sonar vs active sonar too?

I am no expert either way - but here's a guess. This would be true because the absolute range that radar signals reach is far higher than the range from which coherent reflected signals can be detected in order to build up an "effective" picture of the radar reflecting entity.

[vina]

My first response is, "because I know". However, as a student of science, that response is inappropriate

Ah. As a good student of science, it is best to understand what you actually DON'T know and to put pen on paper and think , for which the substitute these days is Google search. However, you are searching for the "wrong" thing. What you should be Googling for is "sound impedance reflection" .

Noise travels across mediums. And it travels quite a distance.

So, I did that Googling favor for you and I am pasting the Class Room Lecture that Google retruns (the 2nd one).
Now, this is very very easy to read (even a lay man can do so), starting from basics, on what happens when sound hits the water. They derive the basic formulas and actually put numbers and answer the question, what happens when sound hits the water ? Well, a massive majority is FULLY reflected back , and only a small portion penetrates. And how small is that? Well, it is 4/3500 or a HUGE (drum rolls please) 0.11 % of the sound impinging on the water surface penetrates. So , this to me , doesn't seem like "Noise travels across mediums" .. It looks like a very very very small % of noise penetrates into water ! The attenuation is of the order of 30db (remember db is a logarthimic scale).

And believe me. Though the attenuation in water is orders of magnitude lower than in air, the higher frequency sounds will die out very quickly indeed and wont travel far.

Again as a "Good Student" you should pay more attention when I post analogies of how how fishes can look out of the water in a cone. The inverse is also true. I did the numbers given the standard speeds of sound in air and water and it turns out, that sound only in a cone with a half radius of around 13deg can penetrate the air water barrier. All the other sound outside of the 13 deg cone is fully reflected away because it is greater than the critical angle. (no this is not in the class room link I posted).

Now tan(13) is 0.23. So for a helicopter hovering at 30m above the surface, the ONLY area the directly refracted sound exists (which is the strongest that can be read, the reflected sound and other multipath will be orders of magnitude less in strength) can penetrate is a circle directly under the helicopter with a dia of approx JUST 13.85m and the Sub has to be DIRECTLY underneath to detect it. Out of the total noise radiated by the heli, this 13.85 m dia circular patch is a very small area indeed and even out of that sound in that area, just 0.11% penetrates.
Gives you an idea of the figures you are talking about. The ambient sea noise is of the order of 60db in most cases.

So lets get some BASIC facts right. Sound DOES not penetrate the air water boundary. A very minute fraction does so , and that too within a small circle directly below the copter (the closer the copter is to the water, the smaller the circle gets). The sub to read the strongest sound radiated (ie, the refracted sound) has to be DIRECTLY under the copter.

This is analogous to a seal swimming under very thick ice (so no sunlight filters through the ice), with light penetrating from a circular airhole in the ice. The sub & helicopter sound is exactly like seal & light through a hole. Difference is the seal can see the ray of light underwater due to Tyndall scattering, but the sub cant "see" that ray of sound because it doesnt get scattered by water.

The direct corollaries are
1. A sub can detect (the 0.11% noise transmitted, through the 14m dia cone) only if the heli is directly above or , by some very lucky circumstances in absolutely best ideal conditions, that "ray of sound", undergoes multiple reflections from the ocean bottom, and reaches the sub. Trouble is , every reflection from bottom, will exponentially degrade the signal, and the sub should have extreme S/N noise ratio to pick it up given the ambient noise. For all practical purposes , this wont happen.

2. The heli need not do any favor by flying just 30m or at tactical dunking altitude always to even allow this kind of thing. If it flies at say just 500m normally and drops down to 30m just for dunking, the sound on the surface will be so low (give just 0.11% transmits of what it puts out directly below), you simply cannot hear the heli underwater. PERIOD.

3. Even for a PASSIVE dunking sonar, for even perfectly favorable long distance propagation conditions , the amount of sound put into the water by the helicopter (remember the 0.11% and that too only in the cone?), is FAR FAR less than what the sub puts out. So, by reciprocity , if the heli sound can reach the sub, the sub's noise can reach the heli as well by the same path. The sub sonar has a FAR more difficult theoretical problem in finding the heli, THAN the heli sonar (assuming both are silent and the only noise is due to the heli and sub). Net result, Heli finds the sub FIRST and kills it.

In summary. This entire stuff of "hearing a Bear or Kamov" underwater due to "contra rotating screws" is bunkum.

[Indranil]

Guys, discuss the topic without taking potshots at each other.

[Cybaru]

Thanks for that explanation Vina.

[Philip]

KA-226 naval variant only to replace old Chetaks. We will need at least 120 med sized ASW.multi-role helos like the NH-90,etc.The AW Merlin would've been great for the carriers,but sadly the AW VVIP helo scam will deny us this superb bird.

[Manish_P]

<OT> I remember reading on some forums (not to mention Wiki) that the US SOSUS grid would also detect the Bears</OT>

[tsarkar]

Both you and I have stated that helicopter noise is to be filtered out. And that is what is done. If helo noise is filtered out what difference does helo loudness make?

Dear Sir, the filtering of noise happens at the computer processing the signals received by the dunking sonar.

But other listeners can still listen to helicopter radiated noise.

Its like a person driving a noisy car in a street putting on earplugs. While he cant hear the noise, others on the street not wearing earplugs will still hear the noisy car.

Can you produce one reference that says helicopters for dipping sonar are selected for quietness?

Dedicated ASW helicopters, like ASW corvette INS Kamorta, are designed from a scratch with radiated noise reduction features. You may search and find. Rest assured, radiated noise is important enough for standards to be defined and published

NATO - STANAG 1136
STANDARDS FOR USE WHEN MEASURING AND REPORTING RADIATED NOISE CHARACTERISTICS OF SURFACE SHIPS, SUBMARINES, HELICOPTERS, ETC. IN RELATION TO SONAR DETECTION AND TORPEDO ACQUISITION RISK

Would a sub be able to differentiate between a heavy sea state and a helicopter?

In stormy seas, detection would be a function of factors including sea state. Submarines continuously keep calibrating their sonar to ambient sea conditions. So if the calibration is proper, then detecting the radiated noise in the paper quoted below is eminently possible

A more accurate noise level is some 30 to 40 db higher than sea-state 2.

However, please note that helicopter operations itself becomes difficult in heavier sea states.
https://www.indiannavy.nic.in/sites/def ... t%2016.pdf

Helo Ops upto Sea State 4

I have already stated that your explanations do not address my question and I remain unconvinced. I am yet to see a single paper from you from the 1950s, 1980s or later that submerged submarines can detect helicopters .

Well, the US Navy is convinced about Urick's paper whose abstract was posted by me and Ferguson's paper as posted by brar_w

http://www.navysbir.com/n12_2/N122-122.htm

It has been known for some time that the Navy’s P-3 Orion ASW aircraft can transmit sound into oceans and be detected (reference 1).......Additional articles (reference 5) show that it is possible to determine aircraft parameters from the signals projected into the sea.

REFERENCES:
1. Urick, R.J. (1972) Noise signature of an aircraft in level flight over a hydrophone in the sea. J. Acoust. Soc. Am. 52 pp. 993-999.

5. Ferguson, B. G. & Speechley, G. C. (2009). Acoustic Detection and Localization of a Turboprop Aircraft by an Array of Hydrophones Towed Below the Sea Surface, IEEE Journal of Oceanic Engineering, Vol. 34, No. 1.

What is important is what the practitioners believe.

No one gives a damm what you believe or want to believe since you've moved this discussion from knowledge to nuisance.

Hydrophones work as hydrophones, whether positioned in deep sea in Urick's paper or whether positioned on a submarine.

One may might ask for a reference patient with Behçet's Syndrome or one may ask for a reference patient with Behçet's Syndrome in 20-30 age group residing at Jayanagar T Block. The symptoms will be the same in both references.

[sankum]

Ka 226 will not carry dunking sonar but will carry depth charges and torpedoes to attack submarines located by other helicopters or ships just like MATCH Chetaks.

[tsarkar]

Noise travels across mediums. And it travels quite a distance.

So, I did that Googling favor for you and I am pasting the Class Room Lecture that Google retruns (the 2nd one).
Now, this is very very easy to read (even a lay man can do so), starting from basics, on what happens when sound hits the water. They derive the basic formulas and actually put numbers and answer the question, what happens when sound hits the water ? Well, a massive majority is FULLY reflected back , and only a small portion penetrates. And how small is that? Well, it is 4/3500 or a HUGE (drum rolls please) 0.11 % of the sound impinging on the water surface penetrates. So , this to me , doesn't seem like "Noise travels across mediums" .. It looks like a very very very small % of noise penetrates into water ! The attenuation is of the order of 30db (remember db is a logarthimic scale).

The entire premise of your post on why submarines cant hear aircraft is based is sound impedance when traveling from air to water.

Thank you for posting this fine example that so aptly illustrates your lack of real world knowledge

You see, what you completely missed is that your theory of impedence is based on pure water / pure glass. Sea water is a completely different medium

As an aside, pure water is a poor conductor of electricity. Pure water, infact is a good insulator.

However, so many people die of electrocution in India in monsoons because of power transmission lines exposed to water. Why?

Because there is no pure water in real world outside labs. Water in the real world has ionic solvents in it that make it a good conductor of electricity that results in electrocution of people during monsoons.

Forget Google, I'm sure you would've watched the movie 3 Idiots and its reference to "salt water is a good conductor of electricity"

So shoo off and figure out impedance in Sea Water.

Not just that, your impedance theory is based on homogeneous medium of water or glass. The Sea is anything but homogeneous. It has thermoclines, gradients, mixed layers, salinity changes, currents, deep sound channels, thermohaline circulation

There is a whole cadre of officers and Department in Indian Navy called Hydrography that focuses on this science under Chief Hydrographer to the Government of India.

Infact the 8 ships of the Sandhayak class + 4 ships of old Makar class + 1 Sagardhwani (13 in total) outnumbered the 3 Delhi + 5 Rajput class destroyers before Kolkata class was commissioned.

India's only Underwater Autonomous Vehicle - the Konsberg HUGIN - is operated by the Hydrography Office.

Ever wondered why IN invests in such a large number of ships and expensive UW drones prioritizing them over frigates and destroyers?

Because these ships gather knowledge about the sea. The behavior of thermoclines are observed and measured in all times of the year as is other oceanographic phenomenon.

Before a ship or submarine leaves for patrol, it collects the latest hydrographic data. This helps in understanding channels in the ocean where sound propagates and channels were sound insulates. This helps in both enemy sub hunting as well as own submarine operations.

And believe me. Though the attenuation in water is orders of magnitude lower than in air, the higher frequency sounds will die out very quickly indeed and wont travel far.

I completely believe your ignorance and pompousness based on what you wrote here.

If you read the Ferguson paper posted by brar_w https://www.docdroid.net/Q7c2bI9/ieee-a ... n.pdf.html

At low frequencies (<300 Hz), the sound spectrum of a propeller driven fixed wing aircraft is dominated by a series of line component occurring at the blade rate and its harmonics. For instance, the blade-rate of a P-3C Orion turboprop aircraft is 68 Hz, which corresponds to each four-bladed propeller rotating at a shaft rate of 1020 r/min.

So why are you even discussing high frequency when aircraft and helicopter radiated noise in water is measured in Low & Ultra Low Frequencies?

So lets get some BASIC facts right. Sound DOES not penetrate the air water boundary. A very minute fraction does so , and that too within a small circle directly below the copter (the closer the copter is to the water, the smaller the circle gets). The sub to read the strongest sound radiated (ie, the refracted sound) has to be DIRECTLY under the copter.

No, your BASIC FACTS are HORRIBLY WRONG, because your BASIC facts does not account for salinity in seawater and other ocean sound propagation factors.

I would rather believe Urick

http://asa.scitation.org/doi/abs/10.1121/1.1913206

An aircraft flying over a hydrophone in the sea produces a noise signature whose magnitude and duration depend on such parameters as hydrophone depth, water depth, and flight altitude. This signature represents sound reaching the hydrophone in four different ways: via a direct refracted path, via one or more bottom reflections, via the so‐called lateral wave, and via sea scattering.

This is analogous to a seal...

When you run out of facts, whales, fishes and now seals to the rescue

Go learn some more on what happens in the real world...

[Viv S]

<OT> I remember reading on some forums (not to mention Wiki) that the US SOSUS grid would also detect the Bears</OT>

From "Soviet Cold War Weaponry" by Anthony Tucker-Jones

[tsarkar]

Can someone download this paper?

Title: Ray Theory Solution for the Sound Intensity in Water Due to a Point Source above It
Authors: Hudimac, A. A.
Publication: The Journal of the Acoustical Society of America, vol. 29, issue 8, p. 916 (ASAJ Homepage)
Abstract: The sound intensity in the water due to a point source above it has been determined by the ray theory. For a given height of source and depth of point of observation, the intensity and lateral range are given by a pair of parametric equations in terms of the angle of incidence. An example is given for which iso‐intensity lines are plotted. A striking feature is the way in which intensity increases appreciably with depth (except directly below the source).

This has the actual equations to calculate sound intensity due to a source in the air, and its propagation.

[hnair]

Am going to ignore tsarkar's innuendo about "moderators siding with vina and shiv", both of whom have been warned off, posts deleted and banned for short periods of time for baiting, by me as well as other admins. tsarkar, on the other hand, did not even get a warning, when he did some fine personal attack on three other posters, just last week. All, please stick to the topics and let the arguments stand on their merits.

Service experience beats book knowledge often times.

Japanese admiral does not need to impress anyone.
....
....
Why are we getting so argumentative?

ramanaji, various posters in this forum has summarily called professionals from various services, some real fine names, including ACMs, Shree Ajay Shukla, Shree Matheswaran, Shree Shyam Saran, Dr Chidamabaram etc (to name a meagre handful), based on what the poster perceives, as the absurdity of their pronouncements or work output. I did no such thing, other than a typo about his title, Rear Admiral vs Captain, for which I apologise. Service experience does not preclude an observation about their communication or contents of the same. My view still stands, that the flourescent lamps info he was giving are "hollywood sound bites", a common PR method for getting over DDMitis related apathy and prevent them from writing only negatives about a subject. The Japanese admiral was successful in that, we are discussing this point. If he said "we dont use normal hairdryers when underwater, but special absorbing clothes wrapped around our heads", we would still read it with interest because it grabbed the attention of a DDM. But does not mean we should not raise our brows, politely.

A Japanese admiral is cool, but not that cool such that he gets a free pass where our senior service folks gets none.

[JayS]

Can someone download this paper?

Title: Ray Theory Solution for the Sound Intensity in Water Due to a Point Source above It
Authors: Hudimac, A. A.
Publication: The Journal of the Acoustical Society of America, vol. 29, issue 8, p. 916 (ASAJ Homepage)
Abstract: The sound intensity in the water due to a point source above it has been determined by the ray theory. For a given height of source and depth of point of observation, the intensity and lateral range are given by a pair of parametric equations in terms of the angle of incidence. An example is given for which iso‐intensity lines are plotted. A striking feature is the way in which intensity increases appreciably with depth (except directly below the source).

This has the actual equations to calculate sound intensity due to a source in the air, and its propagation.

Meanwhile, this paper talks about air-to-water sound propagation phenomenon. Its a review paper and gives some references to earlier research done.

http://cpb.iphy.ac.cn/fileup/PDF/2016-12-124306.pdf

Interesting comment from the paper:

In 1992, Chapman et al.[14] presented that if the height
of the airborne source is sufficiently small, the air-to-water
transmission problem can be modeled by replacing the source
in air with an underwater source at a depth which is much
smaller than the acoustic wavelength in water. In principle,
any standard propagation code in underwater acoustics could
be modified to treat air-to-water sound transmission problems.

From a cursory look at this paper, its indeed as Vina said, that there is very little sound energy goes in the water. However that much energy is sufficient to be picked up by a sensor and detect the source of sound in air. However whether it works only when the sensor is right below the source or it can pick up the sound at sufficiently large (to be of any practical importance in ASW) lateral distance away from the source is not clear to me.

[Ankar]

Can someone download this paper?

Title: Ray Theory Solution for the Sound Intensity in Water Due to a Point Source above It
Authors: Hudimac, A. A.
Publication: The Journal of the Acoustical Society of America, vol. 29, issue 8, p. 916 (ASAJ Homepage)

http://asa.scitation.org/doi/pdf/10.1121/1.1909097

[tsarkar]

Can someone download this paper?

Title: Ray Theory Solution for the Sound Intensity in Water Due to a Point Source above It
Authors: Hudimac, A. A.
Publication: The Journal of the Acoustical Society of America, vol. 29, issue 8, p. 916 (ASAJ Homepage)

http://asa.scitation.org/doi/pdf/10.1121/1.1909097

Hi Ankar, can you download the full paper? The abstract is too brief and does not contain the equations.

[brar_w]

Here you go : http://docdro.id/Ggsn14q

[vina]

Ah. Question time folks. Multiple choice answers only.. Just three questions.

Question 1. Why do they put a gel on you when they do an ultrasound scan ?
a. The gel is cold and nice and helps your skin remain smooth and relax and be less tense, so that the ultrasound works better
b. Well, ultra sound rooms are air conditioned, so the gel has moisturizers and prevents skin cracking
c. The gel is useless and the western medical companies are cheats. Baba Ramdev's shudd- desi cow ghee can be substituted instead of the gel. Far cheaper and 100% indigenous.
d. The gel actually is sound conductive and bridges the impedance air gap between the transducer and patient's body so that the sound waves can actually flow into the body and not reflect off it right away due to the air gap!

Question 2. Why do they make you drink a LOT of water before an ultrasound scan ?
a. You are advised to drink a lot of water (6 glasses/day recommended). It is good for you, so drink it
b. Doing su-soo is easier when you have a full bladder. So you should drink it.
c. Water helps the dried up cells in your body rehydrate quickly so that whatever you need to see is fat and plump and full and not withered like a dried up lemon
d. Water serves as the propagating medium. By avoiding any empty air gaps in your bladder, you remove impedence mismatches and allow the sound waves to propagate freely , and you avoid reflections from air /tissue boundaries

Question 2. Why is the sensor (film/detector) in an X ray behind the subject, while the Ultrasound receiver right next to the ultrasound source ?
a. X rays penetrate through tissues and fluids and stuff, so it makes sense to put the object being imaged is between the source and sensor. While in an Ultrasound, sound is NOT penetrative and it reflects from boundaries with impedance mismatches
b. Well, as proven by Hydrophones receiving sound signals from aircraft (only because it is salt water mind you, wont work with fresh water we are told, because it doesn't conduct electricity), sound propagates across impedance boundaries, and since body cells largely have fluid which conduct and have salts, you can actually put the receiver in the ultrasound machine BEHIND the object like you do for X rays
c. The major medical device manufacturers (GE, Philips , Siemens and Toshiba) are cheats and are colluding. The indigenous Ultra Sound machine BPL actually had the ultrasound receiver placed like X ray machines
d. Front or back doesnt matter. IF you put the ultrasound receiver like an X ray, you need a BIG room like in an X Ray machine. By putting it next to the source, you shrink the size of the room needed and that is why Ultrasound rooms tend to be small and cramped with just about enough space for a single bed and ultrasound machine. Very efficient.

Thanks for thinking over these questions folks. Once we have common sense anwers to these, we can graduate to bigger questions of if sound penetrates or reflects from big impedence mismatches and how it will work only for fresh water and not in sea water why lot of people in India get electrocuted in rains , and finally how in 3 idiots Movie one protaganist gets an electric shock in his gonads (is this even possible , of course it must, you saw it in the move didn't ya..hint.. electrical impedance mismatch between metal and air gap) because he peed on a spoon connected to a live wire.

As always. With Regarsds,
Vina (A member of the IIT AND IVY League Cliques)
ps. Is it possible to have membership of two or more cliques at the same time?

[shiv]

No one gives a damm what you believe or want to believe since you've moved this discussion from knowledge to nuisance.

If you did not give a damn you would not be trying to make so many posts to refute what I have said. Questioning you became a nuisance only because I called you out for trying to pass off your personal opinion as fact and failing to back it up.

You still have not provided one single reference to say that submerged submarines can detect helicopters. In fact how about providing some references for the Ka 226 being too noisy for dunking sonar - other than claiming that you know and that the US navy believes the same things that you do. I did say that you are coming up with every possible excuse for getting out of actually coming up with a specific answer - the latest one being "No one gives a damn".

[ramana]

Folks to second Indranil, don't get personal.
So far its learning experience.
Lets not spoil it so early in the new year.

[nirav]

<OT> I remember reading on some forums (not to mention Wiki) that the US SOSUS grid would also detect the Bears</OT>

From "Soviet Cold War Weaponry" by Anthony Tucker-Jones

Hmm.
I think one needs to harrumph real hard and tell the Americans that they didn't really detect those bears with their SOSUS grid in the 60s..

We have a whaleology and sealology expert who has authoritatively called it "bunkum".. that too in bold..

Wasn't aware IITs have added the whaleology/sealology as a major...

[ramana]

Ok. No more.
Thanks, ramana

[Indranil]

1. The question is not whether a whirr is audible in water or not. Is there a signature of the noise that can be picked up. Seems like there is.
2. How much does this signature depend on the magnitude of the noise, and hence a Kamov being more detectable than say a Merlin. Tsarkar ji, can you show us anything that says that the Kamov is more detectable on the sonar vis-a-vis other aircraft. Otherwise, your initial premise itself is moot.

[yensoy]

Ah. Question time folks. Multiple choice answers only.. Just three questions.

Great ideas from this questionnaire! We should get the Chinese and Pak subs to drink a lot of water and apply gel on their outsides.

See the problem with this entire bunch of "reasoning" is that enemy subs are by definition non-cooperative subjects. The reason all these defence gizmos are so expensive is because you are trying to detect minute patterns which survive the air-water interface and the high attenuation within the water medium. You are absolutely correct that most of the signal doesn't make it past the boundary, and whatever remains is highly damped as one descends. Yet we try, because we don't have an alternative. We try with huge arrays of sensors and huge processing to discern the tiniest of patterns that remain.

[shiv]

IN Kamovs
https://www.youtube.com/watch?v=iGngFSwNQqY

[manjgu]

tsarkar... how good are subs at shooting choppers, planes when they are submerged? from what depths can they do it.

[Rakesh]

I posted a link on the question above, in the previous page of this thread. I am reposting it again manjgu...this is the only system I know of.

IDAS missile
https://en.wikipedia.org/wiki/IDAS_(missile)

Added Later: did a Google search and came across some more links...

this would go nice in a glossy brochure...
https://www.youtube.com/watch?v=pPxM5y-t4FA

A3SM: A True Game Changer for Submarine Self Defence against Threats from the Sky
http://www.navyrecognition.com/index.ph ... e-sky.html

[Cain Marko]

Holy acrimony bats!!! My take on this entire sound made in air not propogating in water is that.. while under certain conditions such sounds can't get too far, practically in sea water this is not only possible but measures are taken to alleviate such issues. The citations by TSarkar make that much very clear.

[manjgu]

tsarkar... looks to me as technology stands today, ASw choppers or fixed wing a/c are at not too much risk from subs. So even assuming what u r saying is right about noise propagating to the sub..the choppers or fixed wing a/c are not at a risk... is that a fair assesment?

[shiv]

To try and continue by simply stating what I know (or think I know) and hoping that no one feels he/she is being personally insulted - sound waves in air are movements of air molecules and they do not transmit well into liquids. The opposite is also true - sound from fluids does not get transmitted well into air/gas filled areas. This is routine ABC level knowledge of medical ultrasound. If the intestines are full of gas they interfere with ultrasound because the sound will not be transmitted through the gas. Ultrasound is therefore best suited for solid organs like the liver, kidneys and bladder and not the air filled intestines.

Very loud sounds in the air mean very high energy and transmission of even that into water is low and governed by what we learn in school physics - ie dependent on the angle of incidence. In other words a Tu-95 bear flying directly over a sound receiver in water may be detected. I have no references to prove this but if the sound receiver is deep underwater or if the receiver is well off to one side - i.e. hundreds of meters or a few kilometers off to one side rather than directly underneath, the loudest sounds will not be heard underwater.

Forget sounds - it is the downwash of the helicopter that agitates the water surface and this surely generates noise that sounds like rough seas. One of the refs posted earlier characterized this as louder than "sea state 2"

When there is a helo with dunked sonar producing noise in the water from downwash the dunked sonar itself will be the first to be affected by interference from that sound and for that reason the noise must be cancelled out or ignored. Of course a submarine in the vicinity may hear the noise of the perturbed water surface. very little information is available about what significance can be attached to this.

The point that I have been trying to make is that passive listening devices can be buoys or even ships - these listen silently and passively for days or hours for submarine noises. But when there is a need to actively locate a possible submarine threat that may be lying still and silent underwater or moving very silently it needs to be bombarded with sound waves - or "pinged". Like an ultrasound scan these pings reflect off various surfaces - sea floor, fish, rocks etc and hopefully off the submarine as well. The skin of the sub is coated with sound absorbent stuff to stop such reflections but even so the sub will throw a "shadow" where the objects beyond are invisible (like gall bladder stones do on ultrasound). The source of these "pings" can be from dunked sonar or from ships. So the helo could be in active pinging mode or passive listening mode.

However since a helo can stay in the air only for a matter of minutes or hours but passive listening needs to be conducted 24x7 - helicopters are not the best passive listening devices. An array of underwater sensors, buoys and ships are far more suited for constant passive listening. So when a helicopter is sent out to dunk a sonar it is likely that it is on an active search for a sub and it is coordinating with other assets in the area - some of which will actively send out pings while the other assets are listening passively for echoes from those pings. Under these circumstances a submarine will hear ever single ping - will hear the noise of ship screws and maybe even helicopter downwash - but all that it can do is to remain as silent as possible and hope that it will not be detected.

[manjgu]

Rakesh..from the A3SM link it does look subs can detect objects in the sky .both choppers and MPA's.

[Cybaru]

Rakesh..from the A3SM link it does look subs can detect objects in the sky .both choppers and MPA's.

Only if Heli based assets are actively pinging, MPA would be tad difficult and dangerous as the location of sonobuoy and the plane have no corelation. I don't think the video above states that the sub listens to prop wash and fires in that direction, it is painting missing in the general area of where the ping came from in hostile waters. It's easy if the Heli based platform is advertising itself by pinging. That doesn't change the physics situation being discussed above.

Brochure for sharpeye submarine radars. Radars like these also known to secretly slew AA missles in Tom Clancy novels!
https://cdn.kelvinhughes.com/upload/pdf ... /naval.pdf

[shiv]

Holy acrimony bats!!! My take on this entire sound made in air not propogating in water is that.. while under certain conditions such sounds can't get too far, practically in sea water this is not only possible but measures are taken to alleviate such issues.

Read the citations carefully.

The "alleviation" is not so that submarines do not hear them. The alleviation is so that the helicopter sonar and detecting ship sonars are not confused and drowned out by the nearby sounds of the helicopter and downwash.

The newest linked paper is from 1984. Since 1984 technology has moved a long way in sound cancellation technology - and those papers are all early papers where researchers are still trying to classify what sound is what and trying to make rules to ignore the helo sound which interferes with their own listening devices

NONE of those papers say that submarines are hearing the helicopters or that helicopters need to be silent. They only speak of the interference produced by helicopter noise to their own listening devices and how to recognize/ignore/suppress

[vina]

Great ideas from this questionnaire! We should get the Chinese and Pak subs to drink a lot of water and apply gel on their outsides.
See the problem with this entire bunch of "reasoning" is that enemy subs are by definition non-cooperative subjects.

Yes. However it is indeed a brilliant idea to expect the same non cooperative Chinese and Pakistani helicopters to hover over exactly where your sonar located is so that you can get a noise blip and design and build your systems around that.

You are absolutely correct that most of the signal doesn't make it past the boundary, and whatever remains is highly damped as one descends.

. Didn't you read the research on underwater Hydrophones reading the noise from a P3 orion ? Didn't you hear about how SOSUS detects Bears (only because of contra rotating screws mind you, you should assume that SOSUS can't detect a quiet P3) ? Now you are saying that the signal 99.9% of which doesn't even penetrate , and out of the remaining 0.11% signal , that too degenerates very quickly ? You MUST be wrong of course. The signal penetrates the air/water boundary like a Paki diving into Baksheesh (and that is backed by scientific proof of course, and you saying signal reflects is "unscientific", because, sea water conducts electricity , while fresh water doesn't, got it?)

Yet we try, because we don't have an alternative. We try with huge arrays of sensors and huge processing to discern the tiniest of patterns that remain.

Now you are talking about totally different things. What was posted here was that the helicopter noise is audible to a sub (sort of like hearing an approaching helicopter to a person on the ground) and that information is tactically usable for a sub like hiding/whatever. What you are saying is that like the SOSUS, I will put the array where aircraft usually passes and get the signal to me over a long distance via a communication cable!


And for that the "proof" that was posted was signal to noise reduction efforts in dunking sonars, and well known papers about a hydrophone underwater able to record the passage of a plane overhead. This "proof" puts a very limited point yes, which is a very small residual noise 0.11% gets transmitted downwards into the water, and that too within the sonic cone (13 deg half angle) below the aircraft , and that is detectable by hydrophone. Yes. If you have a 2d grid /multiple grid like the Sosus, you will be able to get bearing and speed , by doing math that aggregates/calculates from the time information of the blips on the individual hydrophones as the sound cone from the plane over head passes over it.

The problem comes when someone puts a spin and extrapolate this basic limited fact in the proofs to "A submarine sonar can hear a hovering sub from afar and take tactical action" against all known basic facts (this mind you is post fact after pointing out that it doesn't make sense, and then you desperately google up sonar SNR improvement efforts and this paper and that... that is called confirmation bias) , which is plainly rubbish.

[Singha]

what is the battery life of a sonobuoy dropped from a LRMP?

helis are generally useless for ASW unless a firm contact has been located by other means - slow, short legged, 2 puny LWTs, it can merely go to location of last contact, dip its sonar and fire a LWT in a spiral pattern....not great.

a LRMP, supported by LR HWTs from ships with towed arrays is a far more dangerous and persistent beast, which is why JMSDF invested in some 80 P3.

[Cybaru]

All this conversation about SOSUS tracking bears is interesting, but way besides the point and moot. The primary detection units for bears are probably the 4 radars that cover the GUIK gap out of iceland and the NATO base in germany which flies AWACS patrols and command center that connects with long range radars.

https://en.wikipedia.org/wiki/NATO_Air_ ... lenkirchen
http://www.heritage.org/research/report ... c-security

http://www.al-monitor.com/pulse/politic ... z4UrNjGQuM
"The system is controlled from the NATO air base at Geilenkirchen, Germany, which also monitors all aerial moves."