I specify a custom cable to be used in our hydrophones. Before creating
the spec for our current coaxial cable, I bought several samples of
stock cables for evaluation. Among them was a Mogami coaxial mic cable
with the conductive PVC layer that Philip mentions. I have to say that
I was really impressed with the quality of construction! And the
conductive layer outside the shield was really effective at reducing
this microphonic effect (technically referred to as triboelectric
noise). The Mogami cable was the best in that category out of a few
that I tried. First impressions made by just connecting an open-ended
ten-foot length of cable to a preamp and listening left me thinking that
this was absolutely essential. But after making prototypes, the
difference was more subtle between the cheapest balanced line with a
twisted shield and this Mogami coax that I'd deemed best. I did not
hear a difference between good-quality cables with tight, high-angle
braided shields--with and without the conductive layer. My conclusion
was that the parallel capacitance of the piezo devices used in the
transducer was several orders of magnitude higher than the capacitance
of the affected area of the cable and that the piezo element effectively
dampened triboelectric noise to the point that it was insignificant
compared to the mechanical noise transferred into the pickup though the
cable.
So certainly, if you're using a really low-cost cable with internal
voids and twisted shields, or if you're using a high-impedance,
low-capacitance pickup element, an improvement in cable can make a
pretty significant difference. But with hydrophones, where they are
typically used while hanging from their cable, the mechanical vibrations
transmitted though the cable are likely to overwhelm the electrical noise.
Philip Tyler wrote:
>
>
> This info from Mogami.
>
> Microphonic noise is caused by the minute voltage generated when a
> cable is flexed, stepped on, etc. Some cables are designed to prevent
> this with a conductive PVC layer placed under the shield conductor to
> drain away this voltage. This is usually associated with high
> impedance circuits driven by voltage with very little current flow.
>
> So it may be a function of this that is causing your problems, if your
> unbalanced cable does not have this conductive layer you could try a
> length of some that does and see if that helps.
>
> Phil
>
> ________________________________
> From: Grant Finlay <
> <grantfinlay%40clear.net.nz>>
> To:
> <naturerecordists%40yahoogroups.com>
> Sent: Wed, 27 January, 2010 23:15:39
> Subject: Re: [Nature Recordists] extension cables
>
> Slightly O/T but related..
>
> I've recently brought some hydrophones and tested them out a few days
> ago. Very happy with the quality of the underwater recordings BUT have
> noticed they seem to be microphonic, as in pick up audio from above
> the water at a very low level but just audible.
> info:
>
> Approx 10m cable, unbalanced jack to XLR input plug, into my SQN mixer.
>
> Is the simple solution to shorten the unbalanced cable? (But I still
> want some length on the cable though)
> Sorry if it seems like a dumb question but I only ever use
> professional "balanced" equipment and have never come across this
> issue before...
>
> Grant
> http://naturesounds .co.nz
>
>
>
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