Questions about discretion

[Leitender]

Hi makman,

I see what you mean. But sound propagation in the water is non-directional. You can hear the propeller noise even if you are directly in front of a vessel. There may be a phase shift, and there may be a sound reduction if the propellers were covered by the hull, but the sound still reaches you even if you are at that position.

Compare it to sound propagation in the air: You can hear a car passing by e.g. even behind a wall. The wall cannot avoid the sound propagation. AND: It makes no difference, if the car passes from left to the right or if the car is closing or departing. Of course distance makes a difference, but it doesn´t matter if the car is heading towards you, departing from you or just passing by at a determined position (i.e. the sound propagation is independant of the AOB).

But there is a difference between air and water: Sound propagation in the water is much better than in the air because of the thickness of the medium. E.g. Propagation velocity in the air is 343 m/s in the water around 1400 m/s (4 times faster).

Historically, contacts were reported against single ships from u-boats at distances of up to 20km, against concoys up to 100km. The range of the GHG mounted in the german heavy cruiser "Prinz Eugen" was reported to be 50km. We know that whales are able to communicate with each other at distances of hundreds of kilometers (!) if they use special sound channels (and they know how to use them!). So if we talk about such dimensions, the angle on bow has hardly any influence concerning the discovering ability. So if we talk about the propeller noise of a fletcher class destroyer at a distance of 400m, there should be no difference, be AOB=0 or AOB=90, under consideration of the fact that such a sound transmitter could be heard at 20km distance.

Bigwalley

That may have answered your question? In reality there may be a gradually difference, but in the game I´ve never experienced a difference in getting listened earlier or later, depending on the destroyer´s position. makman´s tests confirm that experience. Did you made different experiences?

makman again

didn't know that ! and this is explaining why some parameters i saw here and there in s3d are not 'active' ! thank you Leitender

This not knowing either. I haven´t seen how the sh3.exe works. I only know from comparable problems, that the program needs input, be it necessary or not, else it may overflow . So if the exe is told to read 10 values from "sensor data", it may complain if there were only 9.

PS: Further replies soon, must urgently go to work

[BigWalleye]

Originally Posted by Leitender

Hi makman,

I see what you mean. But sound propagation in the water is non-directional. You can hear the propeller noise even if you are directly in front of a vessel. There may be a phase shift, and there may be a sound reduction if the propellers were covered by the hull, but the sound still reaches you even if you are at that position.

Compare it to sound propagation in the air: You can hear a car passing by e.g. even behind a wall. The wall cannot avoid the sound propagation. AND: It makes no difference, if the car passes from left to the right or if the car is closing or departing. Of course distance makes a difference, but it doesn´t matter if the car is heading towards you, departing from you or just passing by at a determined position (i.e. the sound propagation is independant of the AOB).

But there is a difference between air and water: Sound propagation in the water is much better than in the air because of the thickness of the medium. E.g. Propagation velocity in the air is 343 m/s in the water around 1400 m/s (4 times faster).

Historically, contacts were reported against single ships from u-boats at distances of up to 20km, against concoys up to 100km. The range of the GHG mounted in the german heavy cruiser "Prinz Eugen" was reported to be 50km. We know that whales are able to communicate with each other at distances of hundreds of kilometers (!) if they use special sound channels (and they know how to use them!). So if we talk about such dimensions, the angle on bow has hardly any influence concerning the discovering ability. So if we talk about the propeller noise of a fletcher class destroyer at a distance of 400m, there should be no difference, be AOB=0 or AOB=90, under consideration of the fact that such a sound transmitter could be heard at 20km distance.

Bigwalley

That may have answered your question? In reality there may be a gradually difference, but in the game I´ve never experienced a difference in getting listened earlier or later, depending on the destroyer´s position. makman´s tests confirm that experience. Did you made different experiences?

makman again

didn't know that ! and this is explaining why some parameters i saw here and there in s3d are not 'active' ! thank you Leitender

This not knowing either. I haven´t seen how the sh3.exe works. I only know from comparable problems, that the program needs input, be it necessary or not, else it may overflow . So if the exe is told to read 10 values from "sensor data", it may complain if there were only 9.

PS: Further replies soon, must urgently go to work

My "question" was not a question at all, but a request for documentation. You have given me your opinion. What I asked for was a reference to technical documentation which supports your statement. I asked because your statement conflicts with my analysis if the physics of the situation. I wanted to review the source of your statement before I presented my own opinion, which follows the same argument Makman presented. (His presentation was much less wordy!)

Sound propagation in a continuous medium is isotropic. It is the same in all directions. However, we are not talking about a continuous medium. We are talking about water in which sits a long steel tube filled with air. On one end of that tube is an acoustic generator - the screws, shafts and electric motors. If I am sitting dead ahead of that tube, and the generator is on the other end, then that cylinder of air, which is a poor conductor of sound, is between me and the radiator. There is propagation through the steel tube, but that is a very small cross-sectional area when compared to the cross-section of the air bubble. (Only the cross-section that is actual steel itself counts as steel, not the air-filler interior.)

In effect, we have a radiator on one end of a long narrow sound absorbing tube. When the sensor is directly in front of the absorber, most of the sound waves it receives must travel around the absorber. The actual area of the absorber transmits very little and so the sound level is lower. Moving the sensor to an abeam position, it becomes exposed to more of the direct transmission from the generator through the water, and the sound level rises. Directly aft, there is no occlusion by the absorber, and the sound level is highest. In fact, there may be some reflection off the steel hull in that direction, so sound level may be greater than the isotropic case.

IAC, the important point is that the sub is not an isotropic radiator and that we can expect a non-uniform, directional sound field with the strength in any direction being related to the aspect of the sub. The greatest sound level will be directly aft and the lowest level directly forward.

Your example of a car is not strictly analogous. The car has sound generators at front (engine), rear (exhaust), and all four corners (tires rolling on pavement). The sound field is not isotropic - a noisy exhaust will be more noticeable when the car is going away than when it is approaching. You can verify this for yourself. But, in general, the car produces a significant amount of noise in all directions. Moreover, the sound condictivity of the medium inside the car is about the same as the medium outside (air versus air, ignoring the metal structure and the seat cushions, etc.). The air-filled submarine is sitting in water, which has a sound conductivity (that's not the technical term, but it will do.) 3.5 times greater.

A better analogy to the sub (although in reverse) is a shotgun microphone. This is a device with a sensor at one end of a long tube of sound-absorbent material. The tube blocks sound coming from every direction except the direction the tube is pointed. This allows sounds from a long distance to be picked up, because the background is reduced. For example, you can listen to a conversation from across a crowded, noisy football stadium. The sub is the situation in reverse. The tube is sound-absorbent (relative to the surrounding water) and the sound source is at one end. When you try to listen through the tube, the sound level is significantly reduced.

A nuclear submarine has a more distributed sound source than an electric boat. The reactor and power plant are in response to occasional transient sound contacts would reach a point where they were at risk of a head-on collision! Check Peter Sasgen's Stalking the Red Bear for details of the doctrine and its use in practice. Now, a lot has changed from WW2 diesel-electrics to nuclear attack subs, but sound propagation has not. So I have to project back and say that a WW2 boat running submerged had an anisotropic sound field with the greatest sound level dead aft and the lowest dead ahead, just as modern boats do.

I can see thate your model of the sub as an isotropic radiator would be somewhat more valid in a far-field situation, where the sub is 10 km or more away from the sensor. But the anisotropy should be more significant at ranges around 1 km and under - tactical ranges for a WW2 submarine attack.

If anyone has either data or technical analysis to contradict my conclusion, I hope they will post it. I would far rather have hard technical information on this subject than duelling opinions.

As for the game, as opposed to RL, Makman has convincingly showed that its hydrophone detection model does not take target aspect into account. Whether it should is another question. I can say, without providing further detail, that more sophisticated models do.

EDIT - Some further thoughts about listening to the car behind the wall: The source is loud and the sensor (as Alexander Graham Bell discovered) is highly non-linear. If you put the wall between you and a marching band, you wouldn’t notice much of a difference! But we can refine your analogy by using an electric car instead of one with an IC engine. The electric car is quiet, but not truly silent. If we are on the sidewalk, we first hear it when it is fairly close, but we do hear it. Now go to the other side of the wall. We hardly hear the quiet electric car at all, even when it passes alongside. The wall makes much more apparent difference (to our nonlinear ears) in the case of the low-level sound. But that has much to do with the way our ears work and little to do with sound levels and attenuation.

[makman94]

seems very complicated the whole theme in real life.
if we consider that temperature of water is ,also, a great factor for sound's 'travelling' then i am thinking that creating 'sophisticated' hydrophones would have been a whole new simulator by itself.
one of the biggest lack in sh3 is ,indead, a not modeled hydrophone room.