The Bellman
08-04-2005, 10:57 AM
Markshot, over at Subsim Com posed some interesting questions and the reply from Timmyg00 of NCHQ is instructive :-
A couple of excerpts from the TACMAN:
Quote:
MISCONCEPTION: Thermal layers will always hide you from an enemy that is on the other side of the layer.
TRUTH: The thermal layer is not the “glass ceiling” or “magic shield” that many players believe it to be.
This misconception springs primarily from the behavior of early PC submarine simulation games, in which the acoustic environment was not as accurately modeled as it is in Sub Command. Thermal layers in early sims seemed to behave as a “glass ceiling”; signal strengths would drop significantly as the thermal layer was crossed.
This caused many players to believe that this behavior is normal.
Quote:
definitions: · isothermal: little or no temperature change with increasing depth; leads to little or no sound-speed change with depth
· positive gradient: speed of sound increases with depth
· negative gradient: speed of sound decreases with depth
· boundary: the depth at which two gradients meet, also known as the thermal layer, boundary layer, sonic layer
From what I had read in FAS - used as a reference source for the TACMAN - wavefronts are refracted UP toward the shallows in a positive gradient, and DOWN toward the depths in a negative gradient. This indicates (and was reported by FAS) that the wavefronts are refracted towards water with a slower sound speed.
As to the original questions in this post:
MarkShot wrote: (1) Sound waves are bent towards the slower SSP.
Correct.
MarkShot wrote:
(2) Because of this bending behavior sound tends to travel into slower SSP water from faster SSP water and sound tends to not travel into faster SSP water from slower SSP water.
Also correct.
MarkShot wrote:
(3) Thus, if there is no layer or two subs are on the same side of a layer, then the sub in the slower SSP water has the better chance to detect the sub in the faster SSP water at some appropriate distance. (assuming equivalent subs)Correct, although this should be qualified by citing the surface noise mentioned above; in a positive gradient, the sound waves are refracted up toward the shallows, where surface noise is greater. This indicates to me that there would be less benefit in lingering near the surface in a positive gradient environment in an effort to gain contact on a quiet submarine. On the other hand, in a negative gradient environment, there would be more benefit in going deeper to take advantage of the downward refraction.
MarkShot wrote:
(4) If there is a layer such that the water below the layer has the slowest SP compared to water above the layer, then it is possible for a sub below the layer to hear sounds generated above the layer, but for sonar above the layer to be unable to hear sounds generated below the layer. This one is tricky. The environment in question would have a negative gradient on BOTH sides of the layer, with one gradient being sharper than the other (probably the one on the top). Not impossible, but I don't recall ever seeing it in SC or DW (not that it never happens, but that I just don't remember seeing it ) However, if this environment existed, I'd say that this is largely correct - except that I'd replace "unable" with "less able".
MarkShot wrote:
(5) If the layer itself should manifest the slowest SSP, then sounds which travel into the layer could be conveyed over an extended distance. Correct. This is called a "sound channel", and is usually only seen at the boundary
with the thermocline (not to be confused with thermal layer).
MarkShot wrote:
(6) If the layer itself should have the highest SSP, then this leads to a shadow zone where a hiding sub cannot be heard. Correct again; this is called a "surface duct" environment. However, in such a surface duct environment,one that produces a shadow zone where the wavefronts diverge, only very sharp gradients on either side of the layer will produce a shadow zone that acts like the "glass ceiling" mentioned above.
MarkShot wrote:
(7) Sound refracts in water in accordance to the SSP. Sound also reflects off the surface and the bottom. (This reflection is impacted by sea state and bottom type.) Correct. Rocky bottoms product the best reflections; reflection ("bounce") quality is less for a mud bottom, and worst for a sand bottom. As for surface conditions, I am guessing, but a calm surface should produce better reflections (as well as low noise), while a choppy or wavy surface produces scattered reflection and higher noise.
TG
_________________
NavalCommand-HQ Forum
A couple of excerpts from the TACMAN:
Quote:
MISCONCEPTION: Thermal layers will always hide you from an enemy that is on the other side of the layer.
TRUTH: The thermal layer is not the “glass ceiling” or “magic shield” that many players believe it to be.
This misconception springs primarily from the behavior of early PC submarine simulation games, in which the acoustic environment was not as accurately modeled as it is in Sub Command. Thermal layers in early sims seemed to behave as a “glass ceiling”; signal strengths would drop significantly as the thermal layer was crossed.
This caused many players to believe that this behavior is normal.
Quote:
definitions: · isothermal: little or no temperature change with increasing depth; leads to little or no sound-speed change with depth
· positive gradient: speed of sound increases with depth
· negative gradient: speed of sound decreases with depth
· boundary: the depth at which two gradients meet, also known as the thermal layer, boundary layer, sonic layer
From what I had read in FAS - used as a reference source for the TACMAN - wavefronts are refracted UP toward the shallows in a positive gradient, and DOWN toward the depths in a negative gradient. This indicates (and was reported by FAS) that the wavefronts are refracted towards water with a slower sound speed.
As to the original questions in this post:
MarkShot wrote: (1) Sound waves are bent towards the slower SSP.
Correct.
MarkShot wrote:
(2) Because of this bending behavior sound tends to travel into slower SSP water from faster SSP water and sound tends to not travel into faster SSP water from slower SSP water.
Also correct.
MarkShot wrote:
(3) Thus, if there is no layer or two subs are on the same side of a layer, then the sub in the slower SSP water has the better chance to detect the sub in the faster SSP water at some appropriate distance. (assuming equivalent subs)Correct, although this should be qualified by citing the surface noise mentioned above; in a positive gradient, the sound waves are refracted up toward the shallows, where surface noise is greater. This indicates to me that there would be less benefit in lingering near the surface in a positive gradient environment in an effort to gain contact on a quiet submarine. On the other hand, in a negative gradient environment, there would be more benefit in going deeper to take advantage of the downward refraction.
MarkShot wrote:
(4) If there is a layer such that the water below the layer has the slowest SP compared to water above the layer, then it is possible for a sub below the layer to hear sounds generated above the layer, but for sonar above the layer to be unable to hear sounds generated below the layer. This one is tricky. The environment in question would have a negative gradient on BOTH sides of the layer, with one gradient being sharper than the other (probably the one on the top). Not impossible, but I don't recall ever seeing it in SC or DW (not that it never happens, but that I just don't remember seeing it ) However, if this environment existed, I'd say that this is largely correct - except that I'd replace "unable" with "less able".
MarkShot wrote:
(5) If the layer itself should manifest the slowest SSP, then sounds which travel into the layer could be conveyed over an extended distance. Correct. This is called a "sound channel", and is usually only seen at the boundary
with the thermocline (not to be confused with thermal layer).
MarkShot wrote:
(6) If the layer itself should have the highest SSP, then this leads to a shadow zone where a hiding sub cannot be heard. Correct again; this is called a "surface duct" environment. However, in such a surface duct environment,one that produces a shadow zone where the wavefronts diverge, only very sharp gradients on either side of the layer will produce a shadow zone that acts like the "glass ceiling" mentioned above.
MarkShot wrote:
(7) Sound refracts in water in accordance to the SSP. Sound also reflects off the surface and the bottom. (This reflection is impacted by sea state and bottom type.) Correct. Rocky bottoms product the best reflections; reflection ("bounce") quality is less for a mud bottom, and worst for a sand bottom. As for surface conditions, I am guessing, but a calm surface should produce better reflections (as well as low noise), while a choppy or wavy surface produces scattered reflection and higher noise.
TG
_________________
NavalCommand-HQ Forum