On 5/10/2012 6:06 PM, Robb Nichols wrote:
> 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.
>
>
There is another factor about setting hydrophone depth and that is wave
motion. Sound is attenuated by air/gas bubbles in the water and on days
with lots of wave activity, the layer of water near the surface is
saturated with small bubbles which will act as a strong attenuator. One
reasons depth sounder hydrophones on boats won't work well the faster a
boat moves is due to the hull entrapping air in the water underneath the
hull. Boat propeller wash, water from a pump are all subject to
increasing the amount of air in water and conditions to avoid...
As to strumming cables, there is a simple solution. I'm not sure how to
best to describe it but the concept is a length of material attached to
a cable such as a strip of canvas or thin rubber 4" to 6" wide with
slits every inch or so ty-wrapped by one edge along the underwater part
of the cable so it extends behind the cable in the current, will stop a
cable from strumming.
--
--
Mitch Hill
(Sent from HP DV6T)
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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