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Originally Posted by ridgewayranger
Hi All,
The 'Ping' beloved by movie makers is the sound which the operator hears and also comes out of the speakers. Because most normal hearing cannot detect those frequencies the incoming sound is mixed with another frequency 1k/c lower or higher than that transmitted, this results in a 1 k/c note still bearing all the characteristics of the original, pitch, doppler etc. The process is known as heterodyning. The sound heard in the submarine when the beam struck the hull was sort of a click/squeak. The reducing interval between these indicated they were in contact and of course the decreasing range, giving a good skipper some idea when to take avoiding action. Because of this Hunter/Killer groups devised a creeping attack whereby one ship held contact but used constant transmission interval while the others formed a line and virtually carpet bombed the target, hence their high rate of success.
RR
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Exactly. What someone hears in the submarine is not the carrier frequency (it is the information-HF) but the transient frequency (which brings the wave to the target and back-LF). The compilation is called modulation. Also the doppler (if the submarine shows movement along the bearing) increases or decreases the LF.
Modern sonars, when they are operated like WW2 era, sound like a hammer hit agaist a steel block BUT NOT as a hit against the pressure hall. If the rest water mass is quiet and the contact approaches, then it sounds very clear and is less absorped and vice versa. Some times, it seems to me like a huge bubble colapsing in sound!
The alternation of the interval is an indication but not a certain one.