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Originally Posted by Mau
hi all,
Learned today that there are frequencies lower than 50 Hz.
Those ones are mainly (7-12Hz) for Shaft and blades. So I am surprised that we don't touch them in DW for classification.
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Well the frequency will change with propeller rotation speed :-).
The propeller generated noise was thought to be main way of detecting quiet russian subs in the 80' (IIRC), by very low frequency sound waves created by rotating propellers, but later Russians become using (after Toshiba scandal?) propellers with skewed blades (from Akula up) and detection ranges were signifantly reduced... anyway with today's much longer and more sensitive towed arrays it's probably again important method of very long range detections in deep water. You can skewe the blades or use shrouded propulsor but probably can't eliminate the propeller generated frequencies completly, and in deep water they are traveling very long ranges with little loss... Don't know what in case of a pump-jet - if they are generating detectable freqs too ? Probably yes, only different - lower amplitude (quieter), higher frequency and masked from the sides...
(edit: for those who don't know what we are talking about - just like a rotating fan in your computer or on your desk creates noise, every rotating propeller creates acoustic "noise" in the water, only on very low frequencies. Much lower than 50Hz. This noise is very hard to eliminate, but can be reduced in similar way like quiet air fans for computers are done - so more and highly skewed blades, shrouded propulsors ect.
from the LoBlo's link - it's Miasnikov's article:
'Discrete lines in the 0.1-10 Hz band are caused by rotation of the propellers. This noise is difficult to suppress. Moreover, the noise from the propeller may be heard in the ocean at a distance of up to several thousand kilometers since absorption by ocean waters at this low frequency is negligible. The method of narrow-band filtration of the spectrum's discrete line at frequencies up to several Hz is the basic working principle for sonar systems for long-distance detection." )
P.S. What I have remembered was from:
http://www.chinfo.navy.mil/navpalib/...d-war-asw.html
The Third Battle: Innovation
in the U.S. Navy's Silent
Cold War Struggle
with Soviet Submarines
"Phase IV of the Third Battle: ASW and Acoustic Parity: 1980-1990
The Victor III was a harbinger of the Akula, the first Soviet submarine which approached or achieved acoustic parity with its American contemporaries. Though first deployed in 1978, it was in 1981 that the full significance of the Victor III's quieting sank in.(82) From public testimony it is possible to describe broadly the quieting steps finally taken in this class. In a 1984 reference to operations against Victor IIIs, CNO Admiral Watkins testified:
"…What we also learned was that where we had the towed array that covers the low frequency band it was effective every time. The lesson is…we need to get the low frequency end developed and accelerate its introduction into the fleet. Now we are working on that. We have put extra dollars into the low frequency end so that we can go after the propeller blade rates and the other things we have to get on a quiet submarine."(83)
The significance of this statement is in its reference to the importance of propeller blade rate tonals for detecting Victor IIIs, which indicates that other, higher frequency, narrowband tonals like those generated by a ship's service turbo-generators had been reduced."