At 12:20 PM 7/21/2010 +0000, you wrote:
>Am I right in thinking that hydrophones are waterproof contact
>microphones? If so, it should be possible to hook a contact mic up to
>pick up surface vibrations on an object outside of water, right?
>I tried attaching a Dolphin Ear hydrophone to a vibrating surface and was=
>underwhelmed by the result. I'm sure the mic is fine but, rather, my
>technique is flawed: I believe I may not be making a good enough contact -=
>I wonder if the air between the hydrophone and the solid is muffling the
>sound. Can any of you share tips on getting an optimal contact between a=
>hydrophone and a solid substrate - a tree, for example? 'Blu tack', an
>elastic band to hold it close, gaffer tape, conducting jelly, Joe's Sticky=
>Stuff? Thanks.
Having retired from a career in underwater acoustics and hydrophone design,=
I never thought of a hydrophone as contact microphone however I guess this=
title could be applied...
I think the issue you are facing is very similar to a problem common in
hydrophone deployment and that is getting good coupling from the surface of=
the hydrophone, i.e. sound window to the water. The issue is air being a
much lower density than water by a significant amount is a perfect mirror
to acoustic sound waves. When a hydrophone is first imersed in water you
will see a shiny sort of coating on the surface of it which are minute air=
bubbles and performance of the hydrophone will be dramatically reduced
until this air coating is absorbed into the water. Common practice when
testing hydrophones is to place them in water and let them soak for 24
hours before testing commences. There is a shortcut when needed to
expedite the testing, and that is common dishwashing liquid soap in a
squeeze bottle. When needing a quick look at how a hydrophone is working,
to wet your fingers in water, a drop of soap on your finger ti[ps and rub
it on the face of the hydrophone and you have instant wetting when the
hydrophone is placed in water... Another issue with coupling hydrophones
and water is fingerprints or any other type of greasy substance such as
O-ring lube on the face of the hydrophone, a clean oil/grease free surface=
is essential...
Now, applying this logic to what you are attempting to do, yes some sort of=
coupling medium is needed to exclude the air... I've not tried it myself
however I have heard of petroleum jelly, (Vasolene) being used, zinc based=
heatsink compound would probably work, or if this is for a short test, soap=
will work.
The ultimate solution would be epoxy if this is to be a permanent
installation. Epoxy glue once cured is a very good conductor of sound...
--
Thanks,
Mitch & Shadow...
http://www.4shared.com/dir/UTASxktL/wildlife.html
Shadow's area: http://www.4shared.com/dir/ecfWjyZb/Shadow.html
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