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Saltwater Hydrophone

use the surrounding saltwater as the transducer
 
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While browsing for something unrelated, I came upon this notion in an "ask the experts"- type archive, see [link]. Said expert's answer was "I don't know." - which set my halfbaked idea detector a-ringing!

The 'bakery's only explorations into hydrophones seem to have been of the ever-popular piezoelectric kind. [link2]

A "solid state" hydrophone could be constructed of insulating material, with a thin section allowing entry of the saltwater to serve as conducting media between two electrodes. This could have strong advantage should large arrays of hydrophones be required (as for underwater submarine detection, etc.)

As noted in [link1], the excitation current should be a/c and of fairly high frequency to avoid changing the salinity in question.

Does salt water (not very compressible) change salinity with change in pressure sufficiently? The hydrophone need not have extraordinary signal/noise ratio to be useful...

csea, Nov 22 2004

saltwater hydrophone question http://www.madsci.o...062077177.Ph.q.html
can you sense a sound wave in (salinated) water by electrical resistance [csea, Nov 22 2004]

Drowning alarm http://www.halfbake...ea/Drowning_20alarm
nice reference to u/w radio propagation [csea, Nov 22 2004]

Soundpressure level calculator http://www.sengpiel...ator-soundlevel.htm
ref1 [Ling, Nov 23 2004]

properties of seawater (conductivity) http://web.deu.edu....toprak/ani4081.html
ref2 [Ling, Nov 23 2004]

[link]






       [UB],   

       I see that have not been sufficiently clear. It's not a static electrolyte concentration I'm trying to measure, clearly that does change with depth and a host of other variables.   

       What I'm after is an instantaneous (well, maybe just up to a few kHz) increase in salinity as a measure of an acoustic waveform. It need not be large, as it can be effectively amplified electronically.
csea, Nov 22 2004
  

       >I'm having trouble understanding why the hydrophone needs to work on electrolyte concentration rather than pressure waves.   

       In a word: simplicity. Conventional hydrophones have to go to great lengths to waterproof and protect sensitive innards. This one would use the surrounding saltwater itself as the transducer.
csea, Nov 22 2004
  

       Add one diver to that recipe....I'd better watch out for your boat.
normzone, Nov 22 2004
  

       <clipped> Conductivity increases by the same amount with a salinity increase of 0.01, a temperature increase of 0.01°C, and a depth (ie pressure) increase of 20 m.<unclipped>
If a sound wave has a pressure level of, let's say, 60dB then this is the equivalent of 0.000002m difference in depth. But 20m pressure change has the same effect as 0.01C, so 60dB SPL will change the conductivity of seawater the same amount as a 0.000000001C change in temperature.
In conclusion, I'm afraid the noise to signal ratio is impossibly high.
Ling, Nov 23 2004
  

       [ling] Thanks, just what I was looking for!   

       Oh, well, another idea bites the salt...
csea, Nov 23 2004
  

       Looks like all your statistics are now just "static". I appreciate your effort: it was interesting, for me at least.
Ling, Dec 16 2004
  

       Thanks, [Ling]. I'm used to blind ends in science, and am learning about similar ones in society!
csea, Dec 16 2004
  
      
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