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Originally Posted by Amizaur
The singing whale is a VERY loud event, on good sonars hearable form THOUSANDS miles :-). We have data about how loud a whale sings, it almost could kill a man if it was close to it :-). But of course at first whale sing is probably not a very broadband event (although I feel it would show up on BB), and more importantly - whales don't sing all the time :-P
So the sound of whales should rather reflect their normal activity, which is much quieter I believe :-). Shrimp is quite noisy thing too (I don't remember if we had data on that), even single animal can be heard on sonar quite far, but again maybe not that loud. We'll take a look at this. Submariner's and sonarman's opinion is welcomed how far could a shrimp be heard on sphere (BB) sonar ?
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The main thing to take into affect is of course the SSP. Give me a few moments to research and jog my memory...
Take into consideration that it would be close to seeing a noisy trawler or supertanker though. With this new computer, I don't have the dB comparisons you provided.
A few equations to work up...
FOMpassive SL + DI - NL - DT
This is just an example since I don't have the dB of biologics. Something else to consider is we don't know the exact species of whale or shrimp as this will tell the dB value produced.
The big point is that the environment pays a major role in the ranges observed. A Victor III in the Norwegian Sea (relatively quiet sea and deep) at 12 knots may be detected at several miles. The same submarine in the Med (relatively shallow and very noisy) may be detected at a 1000 yards. At flank speed (27 knots), the Victor III may be detected at 20 miles direct path, 25-40 miles bottom bounce, and possibly to 3 or more CZ's (convergence zones) at 30-33 miles, 60-66 miles and 90-99 miles in the Norwegian Sea, by ship based sensors and sonobouys, and for literally thousands of miles by SOSUS.
Source Level (SL) expressed in decibels (dB). Sound pressure level of individual noise sources of the target, i.e. propellors, drive shafts, reduction gears, steam turbines, electrical generators, reactor coolant pumps, diesel engines, main propulsion motors, other pumps and motors, speed related components (hull resonance's occurring at different speeds). ASW tacticians and operators will use the most detectable steady state noise sources for a given target as their primary detection, classification, and tracking frequencies.
Ambient Noise (AN) expressed in decibels at a given frequency(dB)(which includes sea state, rain, biologics, distant shipping noise, underwater geologic disturbances, etc.) i.e., anything not target related.
Recognition Differential (RD) expressed in dB. The sensitivity of the equipment and operator proficiency (i.e. ability to detect and classify a target unalerted). Tends to be a subjective number.
Directivity Index (DI) in dB. The improved sensitivity of directional sonar systems, where the receivers can be focused on a given sector.
Propagation Loss (PL) in dB at a given freq.. Sound energy is attenuated by spreading losses, absorption (sound energy converted to heat energy), reflection, refraction, etc. Prop loss varies directly with frequency.
Self Noise (SN) in dB. Primarily flow noise over the sensor array, but can also include system noise, artifacts (caused by electrical interference within the equipment-- a design limitation, also affects RD).
Target Strength (TS) in dB. The "sonar cross section" of a target. Amount of sound energy reflected from a target.
Signal Excess (SE) in dB. How much signal is left after accounting for all the variables mentioned above.
These variables are what make up the passive and active sonar equations. The passive sonar equation is as follows:
SE = SL - PL - AN - RD + DI
Propagation loss is usually calculated and displayed on a graph, to which we apply a Figure Of merit (FOM), calculated from a version of the passive equation:
FOM = SL - AN - RD + DI
Using this graph we can determine the expected detection range for a given frequency, including the usability of various transmission paths... direct path, bottom bounce, convergence zones.