Frequencies lower than 50Hz

[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. the 50Hz-60Hz are mainly for Auxiliary machinery (pumps).

Am I right Compressioncut?

Thanks
Mau

[LoBlo]

http://www.fas.org/spp/eprint/snf03221.htm

[Amizaur]

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.

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."

[Henson]

VLF tracking is an important tool. As stated before, low frequency tonals travel farther.

Unfortunately, the bearing scatter is atrocious. The sooner we receive a higher frequency tonal, we will immediately use the more reliable information and all but discard the VLF.

[Mau]

So what you are saying is that it is hard to have a good (none distoted) signal from that range of frequency and that is why we don't use them. We are not using them in DW (anyway I don't think so when we are looking at the chart for the frequencies, unless it is process but we don't see it really in the narrowband ones). Is that the same thing in RL?

[Henson]

I imagine the reason we don't see anything less than 50 Hz in DW is closely related to the reason EVERY contact has ONLY 5 tonals...

You are right though. Low frequency tonals are not as good as higher frequencies because of bearing scatter. It makes sense if you think about it; higher frequencies produce a tighter, more stable sine wave.

In reality, contacts have a plethora of tonals you can track, and where it gets even more complicated is that they can change depending on the target's aspect. A contact sounds different when he's pointing you than he does when he's showing his port quarter.

Don't put a lot of stock in DW narrowband being too close to reality (though the TACTASS gets closer than anything else in-game). It's good enough to play with, but bringing NB into real life parameters would cause a several year learning curve. There's a reason it takes so long to qualify Sup.

[Amizaur]

Originally Posted by Henson

You are right though. Low frequency tonals are not as good as higher frequencies because of bearing scatter.

What is bearing scatter ? Has it something to do with sound wave scattering and arriving to receiver from different bearings or is it something freq related ? And second (related?) question, what is angular resolution of a towed array for such low freqs (1-10Hz), can you get exact bearing of such signal - like for higher freqs, or maybe only it's general direction +/- 30 deg for example ? I'm not sure how precisely can be measured bearing of signal that has wave lenght comparable to towed array size... :hmm: Even if it arrives from the side...

[sonar732]

Originally Posted by Henson

In reality, contacts have a plethora of tonals you can track, and where it gets even more complicated is that they can change depending on the target's aspect. A contact sounds different when he's pointing you than he does when he's showing his port quarter.

Ahh...the classic picture of a train whistle coming at you, being beside you, and lastly, moving away from you.

A School flashbacks....

Where's my coffee maker, two cans of coffee...one for money, the other with coffee, and a box of hot chocolate! :rotfl:

[LoBlo]

Originally Posted by Amizaur

http://www.chinfo.navy.mil/navpalib/...d-war-asw.html

This is one of the best ASW reads I've read in a long time.