Indian Navy News & Discussion - 15 Dec 2016

All threads that are locked or marked for deletion will be moved to this forum. The topics will be cleared from this archive on the 1st and 16th of each month.
Cain Marko
BRF Oldie
Posts: 3904
Joined: 26 Jun 2005 10:26

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Cain Marko » 05 Jan 2017 10:59

shiv wrote:
Cain Marko wrote: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


Read the Ferguson paper link - it is 2009, not 1984 - the abstract says it all:
We show an array of acoustic sensors towed below the sea's surface can be used for the passive detection and localization of the aircraft


While it is referring to a P3C, the disturbance caused on the surface by a chopper could be greater, and consequently, the propagation of the "acoustic energy" would probably also be greater than that of a P3C (this is my guess, but that sound frequencies make it through the water is not).
Be that as it may, it is quite clear that submerged sensors (as in a towed array) can passively listen to acoustic energy (sound?). Clear enough that the data can be used to actually pin-point the location of the emitting platform.

Cain Marko
BRF Oldie
Posts: 3904
Joined: 26 Jun 2005 10:26

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Cain Marko » 05 Jan 2017 11:11

Cybaru wrote: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."

Not to make too fine a point of it saar, but actually the conversation about SOSUS tracking Bears is exactly the point. To rehash it quickly:
TSarkarji makes the claim that IN would be better off looking at western helos rather than Ka-31s because noise generated is more by the Russki bird.
Others argued (including myself) that sounds made in air cant penetrate water although such things have been alluded to here and there including in the ulitmate jingo source on such matters - THFRO
But then, between TSarkar, Nirav and Brar, I think we have enough links to clarify that a noisy chopper could indeed be heard by a submarine, which might then take evasive action and foil the evil designs of said chopper. IOWs, if Kamov's like their Tupolev cousins, can cause great clamor and are indeed noisier than Sea hawks, IN should perhaps avoid them.

Austin
BRF Oldie
Posts: 23385
Joined: 23 Jul 2000 11:31

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Austin » 05 Jan 2017 11:43

Having heard Kamov and Sea King fly by many times , there is really not much one can argue on which is more noisier , they all make big noise and it depends on how close you are to the source of noise.

A submarine in its life time would be more worried about getting detected by a Chopper Active Pinging Sonar then trying to evade on hearing the noise of ASW aircraft or chopper unless she may be like at periscope depth that would mean she is too close to the surface and probably as good as dead if she is confronted by ASW chopper.

Submarine play they game at decent depth of 200 m atleast where noise is really immaterial and they can use the sea temperature and environment to hide itself

vina
BRF Oldie
Posts: 6046
Joined: 11 May 2005 06:56
Location: Doing Nijikaran, Udharikaran and Baazarikaran to Commies and Assorted Leftists

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby vina » 05 Jan 2017 11:59

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


Indeed. So let us summarize the "learning" we have so far.
0. Sub sonar can detect helicopter and aircraft noise from afar while underwater and take evasive action
1. Ship machinery noise is radiated underwater via air ,and not via the hull conducting it to the water
(someone asked a very sane question, if true, why bother isolating the machinery from the hull by mounting them on dampers.. far cheaper to just enclose them in an air tight cover, sort of like a burkha.. far cheaper and easier)
2. Floursecent lamp vibrations are transmitted from the sub to water surrounding it via air and not hull..
(okay suspend the fact that sub is airtight and watertight, no way air inside can communicate without the hull to the water outside)
3. Acoustic impedance applies only to "fresh water". It will not apply to sea water, because unlike fresh water, sea water conducts electricity !
okay suspend the fact that sound propagation to water is a purely mechanical phenomenon and impedance depends just on density and speed of propagation, . so in fact, sea water will have a HIGHER acoustic impedance than fresh water and will reflect more sound energy than fresh water
4. Of course, underlying all of that above is the absolutely firm belief that sound travels freely from air to sea water (since we now accept that it wont travel nearly at all into fresh water)


Now trouble with all this is, it flies in the face of every scientific fact and EVERY class in vibrations and acoustics that I have sat through.

On a whim, I googled up the name of the Prof who taught me this stuff (very enjoyable class , very interesting elective , and great teacher) to see if he was still alive after all these years as he was already close to retirement back then. Turns out, he is alive and well and is doing Absolutely Fine . I forget the name of the of the Prof who taught me the bathymetry and underwater propagation part, so cant google up his name.

One of the things that Prof Rao was always fond of recounting from his days in Karlsruhe was that the core of Louis de Broglie's PhD paper , that absolutely earth shattering work, was just 1 1/2 pages, which he came across when he was there. I remember that after all these years, because it sounds so improbable. But he always emphasised brevity and clarity because of that.

Now if someone sent these "learnings" to him and asked him for his comments, it probably would come back in one word.
Rubbish

tsarkar
BRF Oldie
Posts: 2879
Joined: 08 May 2006 13:44
Location: mumbai

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby tsarkar » 05 Jan 2017 12:33

vina wrote:Now trouble with all this is, it flies in the face of every scientific fact and EVERY class in vibrations and acoustics that I have sat through.

If Issac Newton was alive, he would have been so proud of you! That by every class in vibrations and acoustics that you sat through, you not only picked up every pebble & shell on the beach but you also swam across the vast ocean of knowledge to the other side and said, "bunkum" to others. Well done, boy!

Issac Newton wrote:I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.


Surely, the Science of Physics exceeds beyond Impedance and the Science of Acoustics exceeds beyond-
vina wrote:EVERY class in vibrations and acoustics that I have sat through.


That despite impedance, the low frequency signal remains, propagates in multiple ways as Urick discovered and can be detected.

But as per you, Newton's vast ocean of knowledge ends with
vina wrote:EVERY class in vibrations and acoustics that I have sat through.


You thereafter drop names of some of your professors. I'm sure they're fine distinguished gentlemen. But so are Hudimac, Urick & Ferguson.

Lesser mortals like me would've emailed or phoned them and asked them for more information, understand further developments in the field, discuss Hudimac, Urick and Ferguson's work, and the Chinese research paper from 2016.

But you, vina, in your greatness, are a mind-reader and can read Dr. Rao's mind and predict his reaction -
vina wrote:it probably would come back in one word.


So let me do the menial work and email Dr Rao and get more illumination on Hudimac, Urick & Ferguson's work, and how good a student you were in Structural and Acoustic Engineering, and whether Newton's Ocean of Knowledge ends at Vina & Impedance.
Last edited by tsarkar on 05 Jan 2017 15:16, edited 2 times in total.

Indranil
Forum Moderator
Posts: 7865
Joined: 02 Apr 2010 01:21

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Indranil » 05 Jan 2017 13:06

Oh no! This thread just entered a "kiski pohuch kitni upar tak hai" contest.

tsarkar
BRF Oldie
Posts: 2879
Joined: 08 May 2006 13:44
Location: mumbai

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby tsarkar » 05 Jan 2017 13:39

Indranil wrote: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.


No. As I mentioned in my very first post, my data is empirical since I worked in the operational domain, and not R&D, so I do not have studies showing the measured noise levels of Ka-28 vs Seaking. I do not have any NPOL studies. Nor have I done any market research to ascertain the opinion of IN Sonar operators.

What I know as I stated was a general understanding gathered over time that the Ka-28 is much more detectable vis-a-vis Seaking.

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


Lets expand the question to - is helicopter radiated noise important in the first place?

When hunting submarines,

DE submarines when hunting, move at 2-3 knots on batteries, and extremely silent. This is what PNS Hangor did. It spent a whole day silently stalking INS Kukhri. In such times, even the cooks on the submarine stop cooking, people hold urine, and take extreme measures like communicating via signs.

So the intent of the ASW team is to make the submarine think its not under any threat, so that it operates normally, and makes business-as-usual noise that is more detectable than noises when the submarine in "action stations". Like cruise at higher speeds and discharge battery quickly, or move on diesels while snorting.

However, the moment a submarine sonar detects any hostile noise, like a helicopter or aircraft just moving from Place A to Place B, it goes fully silent. Hence a helicopter with less radiated noise is preferable.

When we purchased INS Vikramaditya, we purchased MiG-29K, we purchased Ka-31 AEW that flies at altitude, but we did not purchase additional Ka-28. Nor is Ka-28 a part of the Naval RFP that has gone to MH-60, NH-90 and French Cougar & Panther.

With regards to a submarine engaging aircraft,

Presently, if it detects an aircraft, helicopter or ship, it goes completely silent.

If it receives radiated signals from enemy sonar at a distance, it may try to hide inside thermoclines. Alternately it may try to stalk its hunter like PNS Hangor stalked INS Kukhri.

Since transmitted noise travels more than reflected noise, it'll detect an active sonar at longer range.

It may launch a Klub/Harpoon/Exocet missile at the general bearing of noise detected. When the missile is close enough, it switches on its active radar. Where there is a helicopter, there has to be a mother ship. The submarine will attempt to take it out.

Since submarines dont have surface search radars like Garpun Bal / BEL APARNA used for cueing anti ship missiles on surface combatants, it uses sonar for the initial cueing.

Submarines recharge batteries by running diesels using air drawn from surface using a tube called snorkel. However this takes time. So submarines often surface at night to recharge.

Submariners carry Igla missiles to use when surfaced. Initial cueing on approaching aircraft is provided by 1. Sonar 2. ESM like Porpoise if the aircraft is using radar or radio, 3. IIR EO sights & 4. Periscope.

Underwater launched missiles are under development as pointed out by Rakesh. These too will use sonar cueing.


For target designation, the submarine crew needs some of the following data from the target: its bearing, its range, its speed or its course. When submerged the crew will use acoustic detection to acquire this data. If faced with a helicopter with dipping sonar, the crew will get its bearing, its speed (virtually 0) and its approximate altitude which is enough aquire the target with accuracy. If faced with an MPA, the crew will know a threat is in the area. If the submarine is detected it will employ tactics to go to periscope depth and acquire the MPA with its optronics systems.

This is all consistent with current submarine missions: When a helicopter dips its sonar, its behavior is considered aggressive by the submarine. Same thing when a MPA detects the submarine thanks to its active sonobuoys.


This is presently done when Igla is deployed when surfaced.
Last edited by tsarkar on 05 Jan 2017 15:18, edited 2 times in total.

manjgu
BRF Oldie
Posts: 2195
Joined: 11 Aug 2006 10:33

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby manjgu » 05 Jan 2017 13:51

tsarkar..ur points well taken and talking to another person in operations kind of confirmed what u have been saying ( about subs going silent at the slightest hint of someone looking for them) . the point i am making given the current state of technology its very unlikely subs shooting down choppers, MPA why the so less usage of active pinging ( though u did explain the reason ,,about range of listening only vs reflection). Passive sonar is listening to the sounds from the sub as compared to active which is looking for reflections from the subs body. Even if sub goes silent the body will still reflect waves. so your figure of 99.9% of time sonar being in passive mode is kind of surprising.

tsarkar
BRF Oldie
Posts: 2879
Joined: 08 May 2006 13:44
Location: mumbai

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby tsarkar » 05 Jan 2017 14:52

manjgu wrote:Even if sub goes silent the body will still reflect waves. so your figure of 99.9% of time sonar being in passive mode is kind of surprising.


The helicopter will first need to be within that range where it receives detectable reflections from the submarine body. That is unknown to start with.

However, the distance at which the submarine will detect the transmitted signal is far greater than the distance at which a detectable reflected signal will travel.

So going active without having an idea where the submarine is in the first place has greater chances of alerting the submarine. That is how INS Khukri was sunk.

Even a general area (say off Saurashtra) is too large for a ship or helicopter to cover. The helicopter will need to hop between boxes in the search grids. The max area of the box will be defined by the range of active sonar.

We got the idea of PNS Hangor was off Saurashtra by communication intercepts and INS Khukri was sent to hunt her. However PNS Hangor was able to track INS Kukhri by interpreting Kukhri's sonar transmissions.

From the book Transition to Triumph

Thereafter "The Story of the Pakistan Navy" states: (Page 357 et seq).
"In an effort to locate the evasive enemy, HANGOR extended her patrol northwards to investigate some radio transmissions that she had intercepted on her sensors. In the early hours of the morning of 9 December, when she was off the Kathiawar coast, two contacts were picked up on her passive sonar on a north-easterly bearing. They were easily identified as warships by their sonar transmissions; radar indicated a range of 6 to 8 miles. A pursuit of the enemy began.

"When the first attempt to intercept the ships failed, the submarine began snorkelling to gain speed. HANGOR, however, failed to attract the attention of the ships and contact was lost as the range increased.

By the evening of 9 December,she was able to make out the pattern of their movement by tracking them with the aid of her sensors. The ships were carrying out a rectangular anti-submarine search. "Forecasting their movement along this search pattern, the submarine succeeded by 1900 in taking up a tactically advantageous position on the path of the patrolling frigates. The range of the ships, which were moving at a speed of 12 knots, began to close. The crucial moment which the submarine had patiently worked for since the early hours of the morning had arrived. HANGOR was finally in a position to launch an attack."At 1915, she went to action stations. Fifteen minutes later, she came up to periscope depth, but could see nothing in the dark night when the range of the ships indicated by her periscope radar was only 9800 meters. The ships were completely darkened. The Commanding Officer decided to go down to 55 metres depth and make a sonar approach for the final phase of the attack. Unaware of the submarine's presence, the frigates continued on their track. At 1957, HANGOR fired a down-the-throat shot with a homing torpedo at the northerly ship from a depth of 40 metres. The torpedo was tracked but no explosion was heard.

This was not the time to brood over the situation. The control team sprang into action and fired a second torpedo. After five tense minutes, a tremendous explosion was heard at 2019 hours. The torpedo had found its mark. The other enemy frigate came straight for the submarine. HANGOR fired a third torpedo and turned away at maximum speed.

"The first priority of HANGOR after the attack was to get into deeper waters and put as much distance between her and the position from which the torpedoes were fired, the datum (reference point) for the search by enemy units. Having successfully done that, she began her journey back home. For four days and nights, she was harassed by the enemy. The dimensions of the enemy antisubmarine effort can be gauged from the fact that about 150 underwater projectiles were fired in this period. Only on one occasion were the explosions close enough to shake the submarine.

Singha
BRF Oldie
Posts: 66601
Joined: 13 Aug 2004 19:42
Location: the grasshopper lies heavy

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Singha » 05 Jan 2017 15:02

another thing to consider is the typical LWT off a heli has a 15km range while a HWT off submarine is 50km. and many of the smaller ships carry similar LWT due to space, only a few like our delhi class cart the full-bore HWT I think (??) the DDG51 has LWT only.

so in a straight on 1:1 man to man fight the sub has both the longer range knife and higher stealth than most ships unless the ships heli is armed and available to attack the sub at close range and equalize the fight. ASROC/Klub-ASW are attempts to equalize the length of sticks. sub can go silent on electric power, ships cannot not even the diesel-electric or "all electric drive" which still needs diesel/gas turbines to run to drive the generator. and the sub can even attack with a ASM and depart while ships ASMs are useless vs subs. if the sub is a SSN it can match the ships speed too - submerged.

thats why massa with its vast fleet of SSNs (mk48 and harpoon) was not worried at all about surface fleet threats...for anti-ship nothing beats the sub.

for air control and economic blocade short of sinking ships, sure surface fleets are useful.

Indranil
Forum Moderator
Posts: 7865
Joined: 02 Apr 2010 01:21

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Indranil » 05 Jan 2017 15:26

It is probably in the best interest of both Russia and India to resurrect Yak-44.

Manish_P
BRF Oldie
Posts: 2024
Joined: 25 Mar 2010 17:34

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Manish_P » 05 Jan 2017 15:39

^ +1

Could have been explored during the buying of the Vik

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 05 Jan 2017 16:04

Cain Marko wrote:
Read the Ferguson paper link - it is 2009, not 1984 - the abstract says it all:
We show an array of acoustic sensors towed below the sea's surface can be used for the passive detection and localization of the aircraft


While it is referring to a P3C, the disturbance caused on the surface by a chopper could be greater, and consequently, the propagation of the "acoustic energy" would probably also be greater than that of a P3C (this is my guess, but that sound frequencies make it through the water is not).
Be that as it may, it is quite clear that submerged sensors (as in a towed array) can passively listen to acoustic energy (sound?). Clear enough that the data can be used to actually pin-point the location of the emitting platform.

This is the same strawman that is repeatedly being thrown into this discussion. No one is disputing that some acoustic noise can be heard to some depth. Close to zero, but not zero. But what depth? What range? What angle with respect to the overflying aircraft? If one ignores physics and thinks questions are a nuisance then one is basically bullshitting.

Acoustic noise can be heard by sensors directly below the aircraft and by a sub if it happens to be just below the surface and just below the aircraft. What is the maximum depth of those sensors before they fail to pick up acoustic noise? No information has been provided. In fact no submarine sensor related information exists in any of the papers. How does one talk about what the submarine hears without that info. It must be guesswork .

But here is what is worse.

Rotor downwash agitating the water to create noise occurs with all helicopters and is not unique to the Kamov. In fact the heavier the helicopter the more the downwash. So what is unique about the loudness of the Kamov? What information is there that subs are detecting Kamovs more than other helicopters. What has been stated with great annoyance and indignation about Kamov noise is simply guesswork and personal opinions.

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 05 Jan 2017 16:16

For target designation, the submarine crew needs some of the following data from the target: its bearing, its range, its speed or its course. When submerged the crew will use acoustic detection to acquire this data. If faced with a helicopter with dipping sonar, the crew will get its bearing, its speed (virtually 0) and its approximate altitude which is enough aquire the target with accuracy. If faced with an MPA, the crew will know a threat is in the area. If the submarine is detected it will employ tactics to go to periscope depth and acquire the MPA with its optronics systems.
.

I am totally unable to see the name "Kamov" in this quote, with or without glasses. Why was the Kamov singled out for being noisy for a generic helicopter noise problem? No answer for that - but plenty of sidestepping of the issue

Subs are great platforms and difficult to destroy, but if they are going to be firing anti aircraft missiles it should be fairly easy to trap them using two helos. The moment a missile or a periscope emerges the sub has given away its location, and the sub can be attacked with every asset in the area.

A sub's strength is stealth. Breaking stealth to fight helicopters is not going to happen for trivial reasons. In fact every reference posted speaks of subs going silent when their sonars pick up any sign of tracking. It is hardly likely that the captain will say - lets go up and shoot the helo down. That would be the diametric opposite of stealth - like waving one's d__k in the air.

Singha
BRF Oldie
Posts: 66601
Joined: 13 Aug 2004 19:42
Location: the grasshopper lies heavy

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Singha » 05 Jan 2017 16:28

Indranil wrote:It is probably in the best interest of both Russia and India to resurrect Yak-44.


the kuznetsov has not been able to mount even 1 strike mission off syria and is scheduled to limp back end of month, tail between its legs. a few AD missions with AAMs have been shown. its flankers are either in hymenim or in the hangar.

one thing is proven - the liaoning is a paper tiger in strike carrier terms and is a large air defence ship in SCS, which is not so useful right now as PLANAF covers it from land. its merely a stepping stone until the cheen can steal/clone/invent a CTOL system and its steam generation plant.

the whole ski jump drama has reached its logical end for IN. the vikky and ADS will also cart around Mig29K for fleet air defence only. all talk of bombarding gwader and punishing karachi should be curtailed.

the debate on ADS2 is strictly over EMALS vs steam now. dont even think of ski ramp or I will ban you for 12 years.

vina
BRF Oldie
Posts: 6046
Joined: 11 May 2005 06:56
Location: Doing Nijikaran, Udharikaran and Baazarikaran to Commies and Assorted Leftists

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby vina » 05 Jan 2017 17:00

tsarkar wrote:If Issac Newton was alive, he would have been so proud of you!

That by every class in vibrations and acoustics that you sat through, you not only picked up every pebble & shell on the beach but you also swam across the vast ocean of knowledge to the other side and said, "bunkum" to others. Well done, boy!


While, I could have started with a retort of " Old Man", I am not going to. Why ? None of what I write here or read here is personal, nor do I care if it is personal against me. I care two hoots. I don't know anyone here , haven't met anyone neither does anyone here know who I am or have met me. That is the way it will remain as well as I have no plans to meet anyone. And no, nor do I intend to insult you or show you in poor light as you seem to assume, though I do admit to be part of at least two of the "cliques" you suggest seem to exist and possibly could apply for more :lol:

That said, what I DO care about is what is presented here , especially if it concerns areas I know pretty well about . Now the trouble as always is for anyone to accept, nay even contemplate ,especially in areas where one considers themselves as experts/authority figures that there might be others who actually have the background and knowledge, more so in a board like this,( in fields ranging from science, math,engg of nearly every specialisation, comp sci, medicine, finance, management, business, and of course, defense , foreign policy, public policy.. you name it) to be challenged when you extrapolate what you know and is correct and true, into areas which you don't based on a set of assumptions and could possibly be very wrong.

Surely, the Science of Physics exceeds beyond Impedance and the Science of Acoustics exceeds beyond-

That despite impedance, the low frequency signal remains, propagates in multiple ways as Urick discovered and can be detected..


Let us address the more substantive points. Impedance is fundamental to many branches of Engg. For electrical enggs and communication enggs, it is vital. Most Elec engg guys would instinctively know about impedance matching and transfer functions. You cant be an Elec Engg without knowing voltage, current and resistance. So for someone studying acoustics without knowing impedance is like someone studying electricity without knowing what resistance is.

While you may have possibly heard of Acoustic Impedance for the first time , lots of people from different backgrounds know about it. Enggs, doctors, physicists to name a few. I happen to know it too. So let me explain to you the physics behind it and why your assumptions, experiences (self reinforced by the papers you Googled up) lead you astray.

1. Air- Water boundary is characterized by huge acoustic impedance mismatch and that boundary is a nearly (99.9%) perfect reflector. (yes, that applies to sea water too, we are not talking electrical impedance here). So, nearly all the incident sound energy is reflected and a very small % goes into the water.

2. I know you think I am probably B.S ing when I put out all the analogies, fish, and earlier whale etc. But no, I am deadly serious. The speeds of sound in air and water are different, so knowing that, you can calculate the air to water refractive index for sound and from that, you can calculate the critical angle for sound beyond which there is Total Internal Reflection and the sound wont penetrate. This for the air/water boundary is a cone with a 13 deg half angle.

3. So really, for all the huge sound that the helicopter puts out, only a minute fraction (ie. 0.11% in the max case) and that too within a very small cone of 13 deg half angle can penetrate.

4. Now, you dig up the Ulrik and Brar_W posted IEEE reports and say, okay, I can detect that, and I know sound waves propagate across large distances, so I can hear it from afar. You also seem to understand that , it really is low frequency sound that propagates substantially and you in turn seem irritated when I write that high frequency sound dies down very quickly and dont talk about the low frequency sound necessary to confirm your assumptions.

5. Of all the ways sound can propagate , the directly refracted sound is the strongest signal. With the already very low signal strength, actually hearing direct sound is the best bet. If it is high frequency, the range is very limited. The sonar has to be directly under the sonic cone

6. What of low freqeuncy ? Why not talk about that as the papers discuss those are the ones that really propagate? Trouble is in the impedance mismatch, it is the highest frequency that penetrates best. The low frequency ones are lost.
When you are in a plane and your ears pop, what you lose MOST is the low frequency noises. That happens because of impedance mismatch due to pressure differences.

Check out this Audiology Text Book on middle ear, under the section Eustatian Tube and Pressure equalisation.

the Eustachian tube, which connects the middle ear cavity to the nasopharynx, normally opens and closes periodically thereby insuring that the static pressure in the middle ear will remain the same as atmospheric pressure. When the Eustachian tube fails to open, a negative pressure immediately begins to build in the middle ear cavity due to absorption of the trapped air by the middle ear mucosa. There is an increased stiffness in the middle ear mechanical transmission system. The transmission loss is greater for low frequencies and has been observed to be of the order of 20 dB for frequencies below 1000 Hz


In case you dont believe that this is the way impedance mismatch would behave when sound bounces off water, check the
wiki entry for underwater acoustics in Sound Reflection and Scattering in the section "Speed of Sound, Density, Impedance"
The large impedance contrast between air and water (the ratio is about 3600) and the scale of surface roughness means that the sea surface behaves as an almost perfect reflector of sound at frequencies below 1 kHz


7. So what that means, is that out of the sound energy that penetrates (the 0.11%), the bulk of the energy is concentrated , in the higher frequencies, which unfortunately die out very quickly. So the actual low frequency spectrum energy is a very small portion of the already small (ie. fraction of 0.11%) signal that the helicopter /aircraft radiates.

8. Together 6 and 7 mean that the best chance for a hydrophone to register aircraft noise is when it is directly overhead. Transmission is highly improbable.

9. Therefore the already highly improbable task of hearing sounds underwater is very nearly impossible.

10. Same reason why machinery, men peeing, flourscen lights etc cant transmit to water via air be detected. Practically non existant signal strength.

So let me do the menial work and email Dr Rao and get more illumination on Hudimac, Urick & Ferguson's work, and how good a student you were in Structural and Acoustic Engineering, and whether Newton's Ocean of Knowledge ends at Vina & Impedance.


I would never consider any sort task associated with learning "menial", after all Gurukul in earlier days was being fully menial in pursuit of knowledge .Please do whatever it takes to satisfy your curiosity, maybe being it from just picking up a standard acoustics text book ( by Ulrick himself, his underwater acoustics is one of the best) , talking to your fellow service folks who probably are specialized in this, and of course with any one else ,including Dr Rao. In case you are writing to him, please do mention that one of his former students fondly remembers him , his classes , wishes him very well and that is how you came to know of him.

rohitvats
BR Mainsite Crew
Posts: 7711
Joined: 08 Sep 2005 18:24
Location: Jatland

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby rohitvats » 05 Jan 2017 18:38

Indranil wrote: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.


With respect to the bold part - don't you think that (a) you're asking for something which is unlikely to exist in the first place or not available in public domain (b) Because it is not there or we're unlikely to get our hands on it, you're passing judgement on the relevance of the point raised? Why? Simply because the point raised does not fit into the neat little arguments we can think of?

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 05 Jan 2017 19:38

rohitvats wrote:
Indranil wrote: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.


With respect to the bold part - don't you think that (a) you're asking for something which is unlikely to exist in the first place or not available in public domain (b) Because it is not there or we're unlikely to get our hands on it, you're passing judgement on the relevance of the point raised? Why?

I am not Indranil but this is an easy question to answer:

If we accept (as you have stated) that:
1.The Kamov signature as detectable is unavailable in public
and
2. Because it is unavailable we are unlikely to get our hands on it

We are in a situation where anyone who claims that the Kamov is more easily detectable because of its noise is
a. Either revealing classified information
or
b. privy to open source information that he has quoted
or
c. bluffing

If classified information has been posted on BRF, it should not have been posted in the first place and since it has been posted, it is now no longer classified. Under the circumstances, every forum member has a right to ask how previously unavailable information has been posted on the forum. If it is open source public. We would like to see it. Or else it is an opinion/bluff

The person who posts such information must either admit that he has access to classified information which he has posted in error, or provide the source of his information or he can admit that he was bluffing.

May I repost what tsarkar posted?
posting.php?mode=quote&f=3&p=2096195
tsarkar wrote:
Aditya G wrote:Ka-226 is ideal for IN, given the long pedigree and history of Kamov helicopters in naval service.

Not quite, the Kamov is noisy and can be heard afar and detectable by submarine sonars, short ranged and maintenance intensive (two sets of rotors). The only USP was Ka-28 & Ka-31 were the only ASW & AEW assets in the post Pokhran years.

Sid
BRFite
Posts: 1640
Joined: 19 Mar 2006 13:26

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Sid » 05 Jan 2017 19:42

++^ I was just about to say the same thing.

I think we are too close to discussing actual operational details as with each page we are going deeper and deeper. I hope just to prove a point we don't end up spilling some info which is not available in public domain.

Manish_P
BRF Oldie
Posts: 2024
Joined: 25 Mar 2010 17:34

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Manish_P » 05 Jan 2017 19:48

Shiv Ji, no specific details (re the Kamov or our Submarines) have been divulged.
But Rohitvats and your point/concern is very valid.
If it is getting too close to details we all must respectfully desist from diving further

Picklu
BRF Oldie
Posts: 2073
Joined: 25 Feb 2004 12:31

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Picklu » 05 Jan 2017 21:01

^^ I think tsarkar is a pretty good judge of what is classified and what is not being from service himself.

None here are discussing classified details on Kamov or Sea King and the only characteristic that were discussed were "more noisy". This is general knowledge that twin rotors are more noisy than the single main rotor counterparts.

Rest was science discussion about propagation of sound in water discussed with lots of links to paper. Don't really see where the problem is.

Yes, the question by Indranil should not be answered in specific details (numbers) but otherwise I find this discussion fascinating while well within bounds
Last edited by Picklu on 05 Jan 2017 21:03, edited 1 time in total.

Rakesh
Forum Moderator
Posts: 8039
Joined: 15 Jan 2004 12:31
Location: Planet Earth
Contact:

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Rakesh » 05 Jan 2017 21:03

Can we all just stop this? Let us agree to disagree and leave it that.

Admins, please take over.

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 05 Jan 2017 21:09

Picklu wrote:
Rest was science discussion about propagation of sound in water discussed with lots of links to paper. Don't really see where the problem is.

It is a science problem onlee

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 05 Jan 2017 21:13

Sid wrote:I think we are too close to discussing actual operational details as with each page we are going deeper and deeper. I hope just to prove a point we don't end up spilling some info which is not available in public domain.

The science is totally in the public domain. The only point that was posted on this forum that is not available in public domain is that the Kamov is so noisy that it cane be detected by submarines and that quieter helicopters should be there.

From the science viewpoint this idea appears to be wrong. But if it is true I would like to see a source that says that the Kamov can be detected by submarines but not some other helo - say the Sea King. if the source is classified - well its too late now. it is no longer classified.

ShauryaT
BRF Oldie
Posts: 5240
Joined: 31 Oct 2005 06:06

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby ShauryaT » 05 Jan 2017 21:16

Mods: Please remove this post if not fit.

I asked the question to a very senior retd. IN officer, who was directly a pilot of the KA 28.

I must say that your query is very interesting, more so because I flew the Ka28 extensively in my flying days . Anti Submarine operation is still an enigma and submarines enjoy a definite advantage.

The Ka 28 has a dunking sonar and in a passive mode, the rotor sound is not transmitted through the sonar, however, in the active mode, there is certain amount of disturbance, but only at close quarters. The rotor wash at a dunk height of about 25 metres, causes certain disturbances. A fixed wing aircraft has no effect in transmitting sound through the water.
I hope that answers your question

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 05 Jan 2017 21:19

ShauryaT wrote:Mods: Please remove this post if not fit.

I asked the question to a very senior retd. IN officer, who was directly a pilot of the KA 28.

I must say that your query is very interesting, more so because I flew the Ka28 extensively in my flying days . Anti Submarine operation is still an enigma and submarines enjoy a definite advantage.

The Ka 28 has a dunking sonar and in a passive mode, the rotor sound is not transmitted through the sonar, however, in the active mode, there is certain amount of disturbance, but only at close quarters. The rotor wash at a dunk height of about 25 metres, causes certain disturbances. A fixed wing aircraft has no effect in transmitting sound through the water.
I hope that answers your question

That is the most appropriate and relevant answer to the question I asked. No sidestepping or fudging.

Picklu
BRF Oldie
Posts: 2073
Joined: 25 Feb 2004 12:31

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Picklu » 05 Jan 2017 21:35

The dunking sonars are tethered to the helo by a cable. That cable will carry some amount of helo noise to water. How much? I don't know.

However the bigger question is, why confine the sound propagated by dunking sonar only?

In case some sound is propagated in sea water from air directly that can be picked up by submarine, then a noiser helo on its way to action station, would have a bigger chance to be detected by a submarine at longer distance and hence the submarine can take evasive action much earlier. By the time the helo reaches action station and start operating its dunking sonar, the alert submarine would have gone to hiding. In that case, IN would definitely be right to ask for a quieter helo.

So, for the sake of the discussion, we should come back to the the basic science question without getting into the specific of dunking operation.

Lalmohan
BRF Oldie
Posts: 13262
Joined: 30 Dec 2005 18:28

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Lalmohan » 05 Jan 2017 21:49

soon as the sonar pings it is detected no? the helo vibes are a secondary issue?

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 05 Jan 2017 21:50

Picklu wrote:The dunking sonars are tethered to the helo by a cable. That cable will carry some amount of helo noise to water. How much? I don't know.

However the bigger question is, why confine the sound propagated by dunking sonar only?

In case some sound is propagated in sea water from air directly that can be picked up by submarine, then a noiser helo on its way to action station, would have a bigger chance to be detected by a submarine at longer distance and hence the submarine can take evasive action much earlier. By the time the helo reaches action station and start operating its dunking sonar, the alert submarine would have gone to hiding. In that case, IN would definitely be right to ask for a quieter helo.

So, for the sake of the discussion, we should come back to the the basic science question without getting into the specific of dunking operation.

Well all this has been comprehensively, doubly and triply addressed in previous posts. The answers are all there - but now there is so much information that the same answered questions are coming up again. No more

sudeepj
BRFite
Posts: 1702
Joined: 27 Nov 2008 11:25

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby sudeepj » 05 Jan 2017 22:16

ShauryaT wrote:Mods: Please remove this post if not fit.

I asked the question to a very senior retd. IN officer, who was directly a pilot of the KA 28.

I must say that your query is very interesting, more so because I flew the Ka28 extensively in my flying days . Anti Submarine operation is still an enigma and submarines enjoy a definite advantage.

The Ka 28 has a dunking sonar and in a passive mode, the rotor sound is not transmitted through the sonar, however, in the active mode, there is certain amount of disturbance, but only at close quarters. The rotor wash at a dunk height of about 25 metres, causes certain disturbances. A fixed wing aircraft has no effect in transmitting sound through the water.
I hope that answers your question


Perhaps the wash from the Kamov is different in from the wash from a Sea King, making it more vulnerable to detection. The specific frequencies in the wash will determine how fast the radiated sound dies. Lower frequencies will travel farther.

A related paper...

https://pure.strath.ac.uk/portal/files/ ... Apr_08.pdf

"Acoustic analysis shows that the principal contribution to noise radiated by both the coaxial and equivalent
single rotor systems is at the fundamental blade passage frequency, but that the coaxial rotor generates
higher sound pressure levels (by 10 dB for the evaluated configurations) than the equivalent single rotor
at all flight speeds. The sources of blade vortex interaction (BVI) noise are investigated and the principal
BVI events are identified. For the coaxial rotor, the most intense impulsive noise is seen to be generated
by the inter-rotor BVI on the advancing side of the lower rotor. The impulsive noise that is generated by
blade vortex interactions for the equivalent single rotor reduces in amplitude as the strength of BVI events
on the rotor decreases with forward speed. Conversely, the BVI noise of the coaxial rotor intensifies with
increasing flight speed due to the increasing strength of the interaction between the wake of the upper rotor
and the blades of the lower rotor. The impulsive noise due to BVI for the coaxial rotor is found to be higher
by 20–35 dB compared to the equivalent single rotor
."

There are many interesting figures in the paper, one specifically showing the 'iso-sound-pressure plots' under the chopper (much higher for the coax and in a much narrower cone below the chopper). I think T-Sirkar jis point that a Ka could be much more noisy over specific frequencies is valid. This would have been a very interesting topic but some members are getting into ego contests, needless sarcasm and personal attacks.

anjan
BRFite
Posts: 448
Joined: 08 Jan 2010 02:42

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby anjan » 05 Jan 2017 22:25

ShauryaT wrote:The Ka 28 has a dunking sonar and in a passive mode, the rotor sound is not transmitted through the sonar, however, in the active mode, there is certain amount of disturbance, but only at close quarters. The rotor wash at a dunk height of about 25 metres, causes certain disturbances. A fixed wing aircraft has no effect in transmitting sound through the water.
I hope that answers your question
Why should the active/passive mode matter to the transmittance of helicopter vibration? Either there is a physical connection or not. If the sonar is dunked there is one. Active pinging will make it's own noise but I don't see the connection to transmitting ambient noise/vibration.

Indranil
Forum Moderator
Posts: 7865
Joined: 02 Apr 2010 01:21

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Indranil » 06 Jan 2017 00:05

Rohit, I don't think that we are anywhere close to revealing classified information. If the posters knew the exact information, the discussion would not have been two pages long. They are guessing based on each other's "knowledge". The problem is that it has become too personal. People on both factions have to have the last word.

nachiket
Forum Moderator
Posts: 7229
Joined: 02 Dec 2008 10:49

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby nachiket » 06 Jan 2017 00:31

Lalmohan wrote:soon as the sonar pings it is detected no? the helo vibes are a secondary issue?

It was mentioned by someone (Tsarkar?) that even the dunking sonar operates in passive mode most of the time to avoid alerting the submarine of the helicopter's presence. If there are other friendly ships in the area, a ping from the dunking sonar would illuminate them too, making it easier for the submarine to avoid or attack them.

ramana
Forum Moderator
Posts: 53814
Joined: 01 Jan 1970 05:30

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby ramana » 06 Jan 2017 00:42

sudeepj, Good find.
The biggest contributor is the fundamental blade passing frequency.
This is dependent on the number of blades in the rotors.
So Kamov with coaxial rotor would have higher frequency than single rotor helicopter.
Next biggest contributor would be the fact that a coaxial rotor radiated 10dB more than the single rotor.

In effect Kamov rotors would have higher freq (?) and more power.

https://en.wikipedia.org/wiki/Kamov_Ka-27

SIx blades. So freq is a multiple of 6.

As higher freq travels less but with more power and thus it would be detected earlier.

So same conclusion as you did.

We can now close this once tsarkar also gets a chance to look at the paper.

nachiket
Forum Moderator
Posts: 7229
Joined: 02 Dec 2008 10:49

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby nachiket » 06 Jan 2017 00:49

^^Co-axial rotor helos don't have a tail rotor though unlike conventional ones. And the tail rotor makes noise of its own.

ramana
Forum Moderator
Posts: 53814
Joined: 01 Jan 1970 05:30

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby ramana » 06 Jan 2017 01:04

nachiket wrote:^^Co-axial rotor helos don't have a tail rotor though unlike conventional ones. And the tail rotor makes noise of its own.


Right but doesn't radiate that much power. Could be 10% of main rotors?

Cain Marko
BRF Oldie
Posts: 3904
Joined: 26 Jun 2005 10:26

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Cain Marko » 06 Jan 2017 02:59

shiv wrote:But here is what is worse.

Rotor downwash agitating the water to create noise occurs with all helicopters and is not unique to the Kamov. In fact the heavier the helicopter the more the downwash. So what is unique about the loudness of the Kamov? What information is there that subs are detecting Kamovs more than other helicopters. What has been stated with great annoyance and indignation about Kamov noise is simply guesswork and personal opinions.


Aah but you were initially saying that you had a problem with helos in general making sounds loud enough to be picked up by subs because sound can't propogate into water - now that this is clear, you come to another part. BTW, we are not talking about much downwash here - we are talking of an MPAA flying @ 300mts and 270kts (not much downwash) but still emitting enough acoustic energy to be picked up 20m below the surface.

Sure, we don't have any "public" sources that Ka-28s are louder than Sea Kings, but here I think TSarkarji knows his domain well enough to put forth an educated opinion - others are free to reject or accept. Problem was - he was being shut down based on the very premise that acoustic signatures can be picked under water. And I think this has been very nicely and clearly settled until of course you make this argument:


This is the same strawman that is repeatedly being thrown into this discussion. No one is disputing that some acoustic noise can be heard to some depth. Close to zero, but not zero. But what depth? What range? What angle with respect to the overflying aircraft? If one ignores physics and thinks questions are a nuisance then one is basically bullshitting.
Acoustic noise can be heard by sensors directly below the aircraft and by a sub if it happens to be just below the surface and just below the aircraft. What is the maximum depth of those sensors before they fail to pick up acoustic noise? No information has been provided. In fact no submarine sensor related information exists in any of the papers. How does one talk about what the submarine hears without that info. It must be guesswork .


Riiight! So what you are essentially saying is that studies that are published in peer reviewed journals that consist of experiments conducted within certain controlled conditions simply are not good enough to constitute as proof in your opinion. I suppose that is fine for an individual opinion. I'll again put down what was clear to mango man who does not have claims to IIT or elite leagues/cliques, from that paper (answers some of your questions too):

Acoustic signature of a P3C flying @ 305m is detected at long ranges by towed array @ 20m depth. Energy signatures not only come directly from overhead, but can also be detected bouncing off the bottom of the damned ocean ~ 4500+mts!

Hitesh
BRFite
Posts: 793
Joined: 04 Jul 1999 11:31

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby Hitesh » 06 Jan 2017 03:30

shiv wrote:
For target designation, the submarine crew needs some of the following data from the target: its bearing, its range, its speed or its course. When submerged the crew will use acoustic detection to acquire this data. If faced with a helicopter with dipping sonar, the crew will get its bearing, its speed (virtually 0) and its approximate altitude which is enough aquire the target with accuracy. If faced with an MPA, the crew will know a threat is in the area. If the submarine is detected it will employ tactics to go to periscope depth and acquire the MPA with its optronics systems.
.

I am totally unable to see the name "Kamov" in this quote, with or without glasses. Why was the Kamov singled out for being noisy for a generic helicopter noise problem? No answer for that - but plenty of sidestepping of the issue


Because aside from Merlin and Kamov, no other helicopters exist for being noisy wrt to sonar operations. USN primarily rely on P-8 and P-3 planes and NATO relies on P-3 or Atlantiques. And the British only uses the Merlin in conjunction with its battlegroup where they don't have to worry about being attacked because they already receive air protection.

Try to understand Tsarkar's point of view and give the benefit of doubt to him since he had some naval experience whereas you don't and act primarily as an armchair general.

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 06 Jan 2017 07:55

Cain Marko wrote:Riiight! So what you are essentially saying is that studies that are published in peer reviewed journals that consist of experiments conducted within certain controlled conditions simply are not good enough to constitute as proof in your opinion. I suppose that is fine for an individual opinion

In fact a random peer reviewed medical paper quoted would have been as relevant for the topic as the ones that were posted. I am not asking for what is heard by towed arrays at 20 meters. None of the references spoke of what was detectable in a submarine so none of them answered my question. If you are dissatisfied with my response it's not my problem. The information I asked for is either unavailable or not posted. Except for one forum statement that insisted that Kamovs are heard in submarines louder than other helos. I am asking for a reference and not a personal opinion.
Last edited by shiv on 06 Jan 2017 08:02, edited 1 time in total.

shiv
BRF Oldie
Posts: 34982
Joined: 01 Jan 1970 05:30
Location: Pindliyon ka Gooda

Re: Indian Navy News & Discussion - 15 Dec 2016

Postby shiv » 06 Jan 2017 07:58

Hitesh wrote:Try to understand Tsarkar's point of view and give the benefit of doubt to him

Are you giving me instructions? I don't suppose you would like it if I gave you instructions about what to do with your instructions. You state your views. I state mine. Pompous patronizing advice from you has a place reserved for it and will be directed there.


Return to “Trash Can Archive”

Who is online

Users browsing this forum: No registered users and 5 guests