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Help with DIY Piezo Hydrophone
Old 13th October 2014
  #1
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1 Review written
🎧 10 years
Help with DIY Piezo Hydrophone

Hello,
I've recently been dipping my toes into the world of DIY contact mics. I've had decent initial success with a cheap piezo disc element that I soldered to a T/S jack (so I could use it with different cable lengths into my recorder) and waterproofed with plastic dip for water use, however, the unbalanced signal is subject to a lot of unwanted hum. So I did some digging and found a guy that had "glued" two piezo elements together (crystal side in and facing each other) and used it successfully as a cheap hydrophone with a nice balanced signal, eliminating the hum. He connected the ground wire from each piezo element together and then soldered them to the braided sleeve of the XLR cable. Then he connected the red center wire from one piezo element to the hot pin of the XLR cable and the other red center wire from the other piezo element to the cold.

I tried to do the same by sandwiching two piezo elements together with a thin layer of silicon caulking and have run into some trouble. I'm getting a signal into my recorder when I have the black ground wires of the sandwiched piezo elements soldered to the ground sleeve of the XLR cable and the center wire from just one of the piezo soldered to the hot pin wire of the XLR cable. But as soon as I touch the center wire from the other piezo element to the cold pin wire of the XLR cable it kills the signal completely. It works exactly the same if I solder a red center wire from one of the piezo elements to the cold pin wire of the XLR cable and then touch the red center wire from the other piezo element to the hot pin wire of the xlr cable. This is my first foray into working with XLR cables so please excuse my ignorance. I've been doing some reading on it and I seem to be missing/misunderstanding at least one key concept here. Any suggestions or insight would be greatly appreciated.

Here is the link to the website I used:

http://www.instructables.com/id/Bala...t-microphones/
Old 13th October 2014
  #2
Lives for gear
 
🎧 10 years
Being cheap piezo's, maybe they aren't consistently wired with the same polarity? Try wiring one red and one black to ground, and the other red and black to pins 2 and 3, and see if you get an output that way.

I have to say, though, that if that was the case, I'd be surprised that the two elements would supply a signal so identical that you'd get complete cancellation.
Old 13th October 2014
  #3
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Thanks for your thoughts on this. I think the piezo discs are wired correctly as the black wire on each disc is soldered to the brass rim and the red wire is soldered to the white crystal layer (like the ones pictured in the stock image above). I was wondering if there had to be a physical connection between the two discs sandwiched together and perhaps I had prevented that by using two much silicon caulking between them.

I just made another "piezo sandwich" and will try out it once the caulk has had a chance to set. I used only a small dollop of caulking this time to see if that makes a difference. I guess I could have also tested out each piezo disc separately using an unbalanced connection to make sure the leads were soldered properly. The piezo discs came with leads already soldered on.

I will try other wiring combinations as you suggested. Is there any way I can damage my recorder by improperly wiring the piezo to the XLR cable? I'm using a SD 702, and I don't want to fry it's mic input.
Old 13th October 2014
  #4
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🎧 10 years
Read the post from trumq again. He is right.
Cheap piezos can have different polarities, depending wich way up the ceramic part was glued onto the brass disk in the factory.
I sort them with a DMM, set to milivolts DC.
I put the piezo onto e.g. a rubber O-ring, so it can flex. Ceramic part up.
Connect the brass part to the multimeter's black lead.
Then gently and slowly press the center of the ceramic with the red multimeter lead.
You will have a positive reading when pressed, and a negative when released.
Or the other way round.
A 100n capacitor clipped to the multimeter leads helps slowing things down, so reading becomes easier.
Normal (2Kohm) mic. inputs are not really suited for piezo pickups.
Piezo needs a high impedance input. At least 1Megohm, like an instrument/guitar input.
You can find many DIY FET impedance buffers on the net.
Leo..
Old 13th October 2014 | Show parent
  #5
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Quote:
Originally Posted by LeeYoo ➑️
Cheap piezos can have different polarities, depending wich way up the ceramic part was glued onto the brass disk in the factory.
Leo..
I had no idea that was possible. I wrongly assumed one wire to the brass, second wire to the ceramic part and all was good. I have a DMM and will give what you suggested a try.

By way of update, my second attempt with a smaller dollop of caulking also failed. It exhibited the same behavior as the first.

I then tried making one more, but first adding a single layer of electrical tape over the ceramic part of each piezo, covering the entire ceramic part, including the solder points for the black and red leads. I thought maybe the solder points of each disc were touching when I sandwiched them and causing a polarity problem. I sandwiched them together with a bit of caulk and voila! I have a working balanced piezo microphone. Of course now that I have read your response, it might just be the result of me getting lucky when I plucked another pair of peizo discs out of the bag they came in. I will test the polarities as you suggested of the remaining discs.

Thanks to both of you for all your help! I appreciate it.
Old 14th October 2014
  #6
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I've been kinda interested in toying with a hydrophone. Does it pick up the water movement?

I love the idea of water samples.
Old 14th October 2014
  #7
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I'm not sure yet. I dipped the working one in plasti-dip to waterproof it. The coating is still drying. I'll try it out tomorrow in water and let you know.
Old 14th October 2014
  #8
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🎧 10 years
I tried something like that ages ago.
The piezo (brass) needs hard contact with water on one side (e.g. perspex), and something soft (air/gel) on the other side, to be able to flex to the soundwaves.
e.g. the brass part epoxied (thin!) to a piece of thin perspex (CD case?).
And a second piece of perspex sandwiched to the back with an O-ring in between.
Google "DIY piezo hydrophone" for examples.
AFAIK real hydrophones use a tubular piezos, where the inside of the tube is the "hot" connector and the outside is the ground.
The whole thing, with or without impedance converter, is potted in some sort of urethane gel with the same properties as water.
I also have seen people using earbuds, fully sealed in silicone glue.
Maybe some other hydro dude can chime in.
Leo..
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