A Comprehensive Guide To Contact Microphones/Piezos [enhanced reality]
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A Comprehensive Guide To Contact Microphones/Piezos

This page attepts to be a conprehensive guide to conact microphones. Information on the web is plentiful and highly redundant, the precious bits are hidden in between. So I have compiled this guide from from various ressources on the web and my own research and experiments. If you find errors, omissions or have any other comment, please by all means contact me.

This is a work in progress, I have put up the references first so you can at least use what I have found valuable resources.

Piezo discs and Fluoropolymer Piezo Film Tabs

Contact Microphones come in two flavours, either piezoelectric elements or piezo(sensors or beepers, doesn't make a difference]] or, lately also Fluoropolymer Piezo Film Tabs (short PVDF).

piezo discs film tab

PVDF don't have self-resonance because they don't have a metal disc, and they're flexible, so you can attach them to a curved surface (use epoxy or silicone), and they're more robust, piezo discs are rather brittle.

Piezos come as discs in various sizes (I've seen them with diameters between 1 and 5 cm) and are extremely cheap, depending on size and quantity ordered you can get them for $0.20 apiece. PVDF are about $3 for a piece of 2.5x1cm.

You can order both off ebay, piezo discs are also available at every store that sells electronic parts.

Readymade vs. DIY

So everything out there sold as transducer, drum trigger or contact microphone is either a piezo or a PVDF. There's nothing you can do to the transducers themselves that makes them sound different than piezos or PVDFs, you can only make them more robust.

That meeans you can build your contact microphones yourself if you're on a budget, or order them redymade if you are lazy. You can buy piezo discs with wires attached or you can solder wires to the discs yourself, it's easy. Don't worry about breaking them, they do tolearate quire a lot of heat. A good soldering iron is recommended, though.

Piezo Preamps

I used to believe piezos have a bad frequency response when it comes to bass. Richard Mudhar points out in this article and this post on his webpage that simpy plugging a piezo into a line input with 50k impedance or a microphone input with 7k impedance effectively creates a high pass filter at around 1kHz. I have to read up on impedance one day, but for now I just believe him that you at least need a simple fet buffer or a this low noise preamp. Follow the links and you'll find the theory and schematics. You should not connect piezos directly to your equipment, because a piezo dropped or tapped hard can output more than 30 Volts, your mixer or preamp might not like that. The buffer and the low noise preamp use diodes to prevent the piezos from overloading the circuit.

Another source for piezo preamp schematics and application tips is Richard Lermann's site. It uses a TL0721) and no diodes for protection. I have built this opamp and used it for some time. I have yet to compare it to Richard Mudhar's preamp.

Yet another preamp was designed by Alex Rice, this one uses two FETs (or a single dual fet). Some people think that FETs are better than integrated opamps for preamps. One day I'm going to resarch that more. Alex's circuit is also not protected by diodes, but simple enough that you can fit it into an XLR plug if you use SMD components. Nifty.

Housing for Piezo discs

Since piezo discs are rather brittle, you may want to protect them from shock, especially if you intend to use them on different instruments or things.

One idea is to coat them in rubber (Picture shows a harp pickup from Ridgeview Audio, $18):

Pickup from Ridgeview Audio, $18

Another is to glue them into a bottle cap (picture shows Creme Dementia through GetLoFi, $15):

Or put them inside a film bottle with a magnet behind it ($15 from GetLoFi):

These come in small metal housing (Belcat EGT-202,about $18, theres also the ETG-101 with just one pickup):

Richard Mudhar shows this picture of a piezo glued to the back of the actuator magnet of an old hard disk. This piezo has a fet buffer on top.

How to attach piezos

  • just tape them to a surface
  • glue them with
    • epoxy
    • hot-glue
    • silicone
    • or superglue
  • glue them to a magnet

Use Plasti Dip to coat them in rubber, you just dip them into this and let it dry. A can of 0.5 liters is about $8, so it pays quickly. I assume coating piezos in rubber does not only protect the disc and the soldering but also reduces high frequency response a bit, but that's only a guess.

If you want to build hydrophones (see the excellent guide by leafcuutterjohn) you might consider using Plasti Dip instead of copious amounts of hot-glue to seal the electronics from water.

References

General Usage and Schematics

ready-made contact microphones

1) Richard Mudhar recommends a NE5534 for lower noise
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misc/contact-microphones/home.txt · Last modified: 2009/08/11 09:37 by beb