There are a couple of practical things to consider: Low frequencies do
not seem to propagate well near the surface. I have to admit that this
is an observed phenomenon; I've never really studied it. I suspect that
it is relative to the depth of the hydrophone being at least half a
wavelength from the surface. But you can hear it easily as you lower a
hydrophone into the water, if your hydrophone is linear at low
frequencies. Mitch, do you know more about this?
Wave noise, and the noises coming from your boat are attenuated with
distance as you place the hydrophone into deeper water. This isn't
usually a factor except in ocean environments. Another thing that may
not be of concern to you unless you're working in deep water is cable
strum. This isn't strictly about depth in the water, but is related.
It is rare to get perfect days with no currents or winds. If you're
hydrophone is not hanging strait down--water is either flowing past it
or you're moving across the surface--the hydrophone cable will probably
strum like a guitar string under tension from the drag. That strumming
will be transferred to the hydrophone as acceleration noise and can be
huge, depending on the orientation and type of the sensor. A longer
cable will lower that frequency and make that noise easier to filter out
of your recordings.
So deeper is typically better. But conversely, if your hydrophone uses
a bender plate to amplify strain on the piezo material with an air void
behind it (basically the piezo discs that are common in low-cost and DIY
hydrophone designs), it is likely to have a pretty substantial reduction
in sensitivity as depth increases and the bender plate becomes more
loaded by static pressure. This probably isn't of significant concern
if you're talking about recording at a one-meter depth verses six or
eight meters, but if you have a longer cable, you might begin to hear
it. This is true of other designs as well, but is less significant.
Regards, Robb
On 5/10/2012 2:33 PM, soundings23 wrote:
>
> Fascinating, thanks Mitch. I like the idea of an aquatic audio "channel".
>
> --- In
> <naturerecordists%40yahoogroups.com>, Mitch Hill <>
> wrote:
> >
> > On 5/10/2012 3:45 PM, soundings23 wrote:
> > > Doing some more hydrophone recordings today. Just a thought ...
> does depth affect the sound? I'd kind of expect it too, at greater
> pressure I'd imagine sound to travel differently in what is a denser
> medium (if that's the right way to describe it)?
> > >
> > > Tony
> > >
> > >
> > Fundamentally, depth alone does not affect sound, however there are
> some
> > unique effects of depth that can alter the path sound takes to get to
> > the hydrophone, primarily what is refereed to in oceanography as the
> > "Thermocline", a level at which temperature of the surface waters and
> > temperature of waters of the deep ocean go through a transition or
> > inversion and act as a mirror to sound. This zone can be also used to
> > advantage as a channel that can carry underwater sound great distances.
> >
> > However the thermocline is at greater depths than most of us need be
> > concerned about it, typically below 600 feet.
> >
> > Another concern is the air/water interface which also acts as a mirror
> > to sound, as is a very flat clean bottom such as a sand flat which will
> > also act as a sound mirror.
> >
> > --
> > --
> > Mitch Hill
>
>
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