Mark,
We have settled on piezo elements that are 27mm in diameter as suggested by a study of this type of "dipping" hydrophone...I find mine on e-bay under "piezo" and I purchase them in groups of 25 for around $10.00 plus a small shipping fee. The leads are still in place and this makes it easy to attach these to much larger interface cabling.
Going extremes...I have used them as large as 50mm and it still worked well...the overall frequency responce was slightly lower (I hardly noticed) This has to do with the wavelength of the sound and the outside diameter of this piezo element.
The reason for this constant testing and investigation, is so that a hydrophone can be produced that will work as well as we can produce one. We hope to concider all possible angles and leave no stone unturned.
In an earlier testing session, Jon felt the performance of several hydrophones of various sizes were the same when detecting a small transmitter locator from across a small pond. In the end, the overall size of this mounting surface may have no effect.
This unit was produced to ensure this is indeed the case.
David Bartsch
Date: Sat, 21 Jun 2008 20:51:15 -0700
From: mark@Harbortronics.com
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] Larger area hydrophone
All hydrophones operate by converting mechanical strains into electrical signals. The strain is caused by the sound pressure wave stressing the material. With a bender hydrophone, you are wanting the center of the diaphram to deflect relative to the circumference - make a dish shape back and forth.
I like to think about these things at the extremes... it's often useful.
In this case for instance, imagine the surface area increased hugely... think of a trampoline! If you glued a small bender in the middle of the trampoline, then the actual strain in the diaphram of the bender is really tiny... the sound pressure wave is creating a deflection in the trampoline, but the stresses are distributed over the whole surface, so the bender only 'receives' a very small amount of the sound energy received by the whole surface.
Turn it around... make the hydrophone diameter as small as the diaphram of the bender. Now, all of the stresses received by the hydrophone work directly on the bender.
The larger the surface area of the hydrophone, the more signal it will receive, but just increasing the hydrophone area itself won't help put more stress on the bender, and obviously screws up the depth capability.
I believe that an optimum bender hydrophone would consist of piezo ceramic going right to the edge of the metal disc, and for the best depth capability, you should actually mount the bender so that when the hydrophone surface dishes due to pressure, the ceramic is under compression, and not tension.
I'm curious what benders you guys are actually using
My $.03,
Mark
David Bartsch wrote:
Mark,
Jon asked if enlarging the flat surface upon which the piezo element was mounted would aid the unit in picking up incomming sound...so I made the following testing hydrophone to answer this question.
It is designed to simply "dip" deep enough to render results and will never find deployment on any vessel. In short, it is a very large version of the "dolphin ear" dipping hydrophone.
uld improve the ability of the hydrophone to pick up incomming sound energy.
You will note the extreme use of rubber shock mounting used to best isolate the two wafers from one another.
Also, as I had mentioned earlier, with the lack of an 'o' ring retainer ring, the use of this unit in deeper water is very limited. It is a shallow water testing platform only.
David Bartsch
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