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Originally Posted by Cohaagen
1480, mate, it doesn't mimic cavitation, although that is an interesting concept for a decoy. All Bold/Pillenwerfer does is create a fine bubble cloud to create a false return for the poor Hostilities Only ASDIC operator to bash away at fruitlessly.
Assuming your principle, even a novice would notice that the cavitation source wouldn't be producing any "chop-chop" wash noise, neither would there be the otherwise expected sound of electric engines, or turbulence from water passing over openings and protusions in the casing usually associated with a submarine.
There was a German decoy (forget the name) which could actively emit noises and, within limits, move at slow speed, but it sadly isn't modelled in the game.
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Cohaagen, you make some valid points but I want to break down my observations down to the basest level and explain why I have come up with the theory that: the BOLD or Bolde decoy, employed by the Ubootwaffen in the early 1940's mimic's a noise produced by submerged submarines. The noise being cavitation:
1. Cavitation is the noise produced by a properly functioning prop. The prop in and of itself, does not produce a "chop" sound in a
submerged submarine.
"Propeller sounds are of two general kinds- (1)
singing, due to vibrations of the
propeller blades, and (2)
cavitation. Cavitation sounds are the most important of all submarine sounds. Vibrations of the propeller blades may be due to faulty design or manufacture and are generally not difficult to eliminate."
2. Cavitation is a distinct sound because it is the disruption of a medium.
"Cavitation results when the propellers turn so rapidly that the water does not close in behind the blades. Thus,
a stream of bubbles resembling those in a boiling kettle is formed. These bubbles may be caused by reduced pressures on the backs of the propeller blades or by vortices at the tips of the propeller blades."
3. Other mechanical noises a submarine produces and the actual noise the water makes as it is disturbed by a moving submarine are of a secondary indicator to an experienced hydrophone operator.
"Besides these two main sources of submarine sounds, there are some minor sources, such as splashing of water at the bow and in the wake when the submarine is at the surface; when submerged, the fittings of the vessel, such as handrails, may be set into vibration by the turbulent flow of water past them.
These sounds are considered to be of small significance compared with those due to cavitation. "
4. A passive listening device is only a tool. The actual interpretation of the noise it picks up depends on the person listening.
"Any listening system must consist of (1) a hydrophone, (2) an electronic receiver, (3) a bearing indicator, and (4) a speaker or headphones. The sound-listening problem for the operator consists primarily of learning to distinguish between (1)
sounds emitted by another ship's machinery through the hull and from the propeller and (2) the multitude of other sounds that exist in the ocean."
5. When the BOLD was 1st employed, it worked because hydrophone operators were attracted to the similar noise that a submarine made.
" ...sea water would enter the canister and react with the calcium hydroxide, releasing
a dense volume of hydrogen bubbles. The result would give a sonar 'signature' very similar to the U-boat itself." Gordon Williamson pg 85,
WOLF PACK.
So I will take a great leap in logic and still contend that the BOLD in it's original form mimics the sound of cavitation. Cavitation = noise made by bubbles. BOLD= noise made by bubbles. Yes, as the operators got better with experience, so did the decoy as danurve and UScpl point out.
The other cited source is the NAVPERS 10884 circa 1953.