Balanced Piezo Contact Microphones

At the illutron art ship, we are using a lot of piezo elements to make contact microphones and hydrophones (underwater microphones)

Piezo elements can be a noise nightmare. And if you are using the piezos for hydrophones, to capture underwater sound, the noise can be even more challenging, unless you get everything very well sealed.

We came up with a trick, to get a balanced signal from piezos:

Simply using two piezo elements sandwiched, back to back, you can generate a balanced signal, that will be completely noise free.

Step 1: How to Make It

You need

• 2 normal piezo elements
• Microphone cable
• XLR plug

Connecting

• Sandwich the two piezos "belly-to-belly" (crystal and wires in, metal out)
  • We just use a wad of normal silicone sealant in-between the piezos (Think Oreos)
• Join black wire from Piezo A to red wire from Piezo B and connect to Gnd on your xlr / microphone-cable
• Connect the red wire from piezo A to + (hot) and the black from piezo B to - (cold) on your xlr / microphone-cable
• Connect a XLR-plug to the other end of your cable

Step 2: Try Your Noise Free Contact Microphones

Plug the xlr-connector into the balanced input of a mixer or recorder, and enjoy the crisp clear noise free sound from your piezos.

This vimeo video have sound recordings using the balanced contact microphones: https://vimeo.com/472428276

Headphones are recommeneded - Enjoy! :-)

More about the singing strings here : christian.liljedahl.dk/strings-singing-in-a-stream/

Experience so far

• The sound from the piezos are very clear and noise free
• The oscilloscope have verified, that the output truely is balanced
• The piezo microphone is very sensitive and responsive.

As hydrophones

I tried this piezo-design out as hydrophones. I could just stick them in the water and get sound. I didn't even care to waterproof them, and they worked perfectly and noiselessly under water too.

So, for quick and dirty underwater microphones, just sandwich two piezos like this and dump them in the pond.

Comments

[Knallberto]

Thanks for sharing! I habe two questions: How exactly did you make the black shell? and could you explain the "wad of normal silicone sealant" part, what exactly is normal silicone sealant?

[chrlilje]

Hi - The black shell was on the piezos I have from the manufacturer. They don't have any other function than giving a bit more solidity and durability.

The silicone sealant I refer to are just normal bathroom sealant. Sikaflex 591 is the expensive and very durable solution.

[Knallberto]

thank you

[se7108626]

First of all thank you very much!!
I built it for my violin and it sounds fantastic!!
I currently connect it to xlr directly to the mixer.
I wanted to ask. Is there any way to convert the balanced signal to an unbalanced signal.
I have a wireless guitar system with a TRS mono plug. And if I connect it to the pickup the signal is no longer balanced.
Is there a way to kepp the pickup in a balanced signal and connect it to an unbalanced female plug?
I tried to look for information at google about this and I heard about some electronic component called a BALUN but I'm still not sure exactly how to do it can you help me?

[chrlilje]

If you want to convert a balanced signal to unbalanced, but keep the benefit of the noise cancellation of the balancing, the easiest way (to me) is through a small mixer.

I like to use the small Behringer Xenyx 302USB - It runs of USB 5V power so it can be powered from a normal powerbank in the field.
It even works as a handy sound interface for a computer.
You can pick it up from about 60 usd - A very handy little mixer for that price.

If you don't care about the noise cancellation or it isn't a problem for you, just use any XLR to TRS plug.

I hope this helps you - Don't hesitate to ask further :-)

[se7108626]

Thank you for the quick reply.
But a mixer won't help me.
As I mentioned, I use a wireless guitar system. And his plug is unbalanced. so i want to be "wireless" when i perform on the stage
My question is, if there is another way to convert a balanced signal to unbalanced without losing the benefits of noise cancellation.
i mean, what makes the XLR balanced?
what happens to the third cable (ground) inside a mixer? (or the second one the cold) it has to be grounded somewhere.... so if i will understand the idea behind that, maybe i will be able to built some small circuit that will do it without a mixer..
have you heard about the BALUN
i mentioned? or any idea?
thank you again!!!

[chrlilje]

Ah, sorry, I didn't understand how portable you wanted it :-)

You could use the balun transformed method - I remember a friend of mine did some piezo experiments with baluns, but he never really got great functionality from it.

Another off-the-shelve solution could be a small battery operated microphone preamp.
Like this one (havn't tried it, but looks promising) :
https://www.ikmultimedia.com/products/irigpre2/

[Xiqils]

Does this need phantom power? I will use this with my Zoom F3 recorder and the Zoom provides 24V or 48V phantom power

[ChristianL4]

No phantom power needed. I have used them often with my own Zoom recorder.

[Xiqils]

Thank you. I will definitely try them soon. I was wondering if the same principle could work for a PIP recorder (sony A10) where the TRS jack is soldered as shows in the attached picture

[chrlilje]

The Sony PCM-A10 has a stereo input jack. Tip and ring are left and right channel I believe.
You can absolutely record with the setup you have sketched, but you will not get the benefit of the noise cancellation of a true balanced input (XLR)

But since you get two channels in (left / right) you could probably gain some noise cancelling in post by subtracting the two channels.

[Xiqils]

Oh, I see. I will try it with XLR. Thanks again for sharing valuable information. Have a nice day

[deekai]

Hi! Great project! What is the output impedance and resonance frequency of the piezo disks you used? Apparently, impedance varies from kOhm to mOhm, resonance can go up to several kHz. I guess for mic pre a impedance in the kOhm range would be better? Or did you use the instrument input with mOhm input impedance? Any help very welcome. Thanks in advance, D

[chrlilje]

I must admit, that I have no idea about the impedance of the disks we used. The piezos came from a large batch we got from somewhere in Asia. They were quite cheap.
I never tried to measure the exact impedance.
We usually just plug them into the microphone (xlr) input of any mixer and crank up the gain to something that works :-)

[sam.bronstein]

This is great! Can you share the sample sounds some other way? Link isn’t working :)

[chrlilje]

Ah, thanks - Looks like my friend have deleted it from Soundcloud. I will ask him tomorrow if he can dig it out from somewhere :-)

[nortok00]

Hello. I came across this project while searching for diy contact mics and really want to try this. I'm new so forgive the newbie question.

The only part I don't understand is wiring the cables (XLR and microphone). Are we choosing to wire just one of the cables (so choose the XLR cable or choose the microphone cable depending on the needs) or are we wiring the piezo and both of the cables together? I understand your wiring schematic for one cable but not wiring both. Reading the instructions hasn't clarified and none of your pictures show the cables in detail. Can you clarify?

[chrlilje]

Hi nortok00 - I am not sure I fully understand your doubts, but i'll try to clarify.
I use XLR cable and microphone cable in a confusing way I think.
A microphone cable usually have an male XLR plug in the end.
In the other end of the microphone cable, we attach our two piezo disks.

The XLR plug goes into a mixer, typically, or directly into an active speaker sometimes.

You will use 2 piezo disks for 1 microphone cable with 1 male XLR plug in the end.

I have attached an image of the finished piezo, hopefully this will also clarify.

Do let me know if you have further questions, and I will try to answer as good at I can.

[nortok00]

Thank you! Now I understand (or hope I do). So it's just one cable I'm buying and not two? That would make sense. In your list of things needed I thought it was a microphone cable and an XLR cable and that had me confused as to how these would be wired to the piezo.

[chrlilje]

You just need 1 cable. The microphone cable has 3 conductors inside. A shield (ground wire), a red and a white wire. Those are the ones I refer to in the drawing :-)