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Offline Martin Colloms  
#41 Posted : 29 July 2015 07:19:24(UTC)
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sounds like ideal sub woofers to go with the Devialet Phantoms


Martin Colloms
Offline kengale  
#42 Posted : 29 July 2015 12:18:20(UTC)
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Originally Posted by: Togil Go to Quoted Post
"I wonder what you'd make of the system I'm working on at the moment:

Drivers (x2) 900mm dome, 100mm throw, mounted at opposite ends of a 1m tube enclosing all the electronics
PA (x2) 60kW, 25Hz - 250Hz class D
Active closed-loop system with feedback directly from the driver position.

Sound levels (underwater) - enormous! We have to ramp up the sound levels over 3/4hr to allow sea mammals time to flee the area. "



Are you looking for MH370 ?


No - far too low a frequency for sonar imaging. It's for geological surveying. An array of these transmitters 3 or 4 miles across is towed through the water and the transmissions travel to the bottom, into the ground, and reflections from discontinuities in the ground then travel back to the bottom and through the water to receiving arrays for analysis. Traditionally the transmissions use compressed air guns to produce high-level transients - this is the next stage to produce controllable waveform transmissions.
Sonar is a whole string of compromises: for good resolution you need high frequencies, but then you can't get as much power into the water because the transducers are smaller, and the losses through the water are higher. For seeing if things such as ships are out there you need transmissions in the mid-audio range, to see their shapes and details and pick up such things as small mines into the hundreds of kHz. For body imaging sonars ("ultrasound scanning") you need to go from low MHz upwards - I've worked on scanners for imaging liver disorders working at 6-8MHz, and some scanners go up to 50MHz and above.

So you see the problem for finding MH370: for enough detail to recognise bits of a crashed aircraft you need high frequencies, but then the transmission losses mean you can't get a good range, and you have to tow your transmit/receive assembly pretty near the sea bottom and scan over and over again.

Here's the sort of pictures you can get with commercial kit at 400kHz or so with suitable processing http://www.krakensonar.c...Pix.Brochure.Nov2012.pdf
You'd need this sort of resolution to recognise bits of a plane.

Offline malteser  
#43 Posted : 07 August 2015 11:22:39(UTC)
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Indeed, if you're allowed to share, what's the purpose of this? It sounds dreadful. Can you imagine if the noise was airbound? You'd have a restraining order put on you immediately.
Regards,
Frank.

All opinions are my own and do not reflect the opinion of any organisations I work for, except where this is stated explicitly.
Offline kengale  
#44 Posted : 07 August 2015 18:35:58(UTC)
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Originally Posted by: malteser Go to Quoted Post
Indeed, if you're allowed to share, what's the purpose of this? It sounds dreadful. Can you imagine if the noise was airbound? You'd have a restraining order put on you immediately.


http://www.acousticecolo...oceanairgunexecsumm.html gives a good idea of the purpose behind this sort of equipment, as well as outlining the ecological impact. Using electronics and transducers to generate the impulses instead of airguns means we can not only give a better waveform from the analysis point of view but restrict the bandwidth so that the higher frequency transients which can cause a hearing loss are not present.

Surprisingly it's almost exclusively sea mammals that suffer from these high level transmissions - fish don't seem worried or damaged by them.

Offline malteser  
#45 Posted : 12 August 2015 08:41:06(UTC)
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Very interesting Ken, thanks for that. I was unaware of these methods of surveying, though I was aware of the military sonar experiments. I guess the biggest problem here is horizontal propagation. If that could be controlled or filtered to dissipate more rapidly, then the environmental concern would be lower. Then again, I was surprised at the levels thrown out by whales!

As for fish being less susceptible, I think that's just 'not proven' at this point. There seems to be a paucity of understanding in this regard.

But very interesting indeed, thanks again.
Regards,
Frank.

All opinions are my own and do not reflect the opinion of any organisations I work for, except where this is stated explicitly.
Offline kengale  
#46 Posted : 12 August 2015 14:32:18(UTC)
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Originally Posted by: malteser Go to Quoted Post
Very interesting Ken, thanks for that. I was unaware of these methods of surveying, though I was aware of the military sonar experiments. I guess the biggest problem here is horizontal propagation. If that could be controlled or filtered to dissipate more rapidly, then the environmental concern would be lower. Then again, I was surprised at the levels thrown out by whales!

As for fish being less susceptible, I think that's just 'not proven' at this point. There seems to be a paucity of understanding in this regard.

But very interesting indeed, thanks again.


Here's an older doc about these sorts of sonars. https://www.cagc.ca/reso...mic/seismic_vs_sonar.pdf Note how even with the seismic systems it is a two-dimensional array of transmitters: this give a degree of control of the polar diagram of the transmissions, and by phasing the timings of the transmissions across the array one can give it directionality AND point it in a specific direction, not necessarily directly at right angles to the array.

Offline Togil  
#47 Posted : 31 October 2015 09:24:38(UTC)
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What do I see in Harrods ? Devialet Phantoms making the same noise as in Munich...

Also they are now selling KEF Blades and various Naim equipment although I didn't see Statement on display...
Hans
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