A Piezo Tab Instrument

by Bart on March 29, 2012

Life gets busy and I rarely get around to posting anything here on the Experimental Musical Instruments news page these days. The lack of posts is certainly not because there’s no news; there are always good things happening in the world of inventive instrument making, and usually at least a few interesting things happening here in the Experimental Musical Instruments office as well.

But I’m back at the keyboard for the moment, and I’ll take the opportunity to write about an instrument I’ve recently made with one of the products we sell. 

The product is the small, inexpensive piezo films, the one-inch ones sometimes called piezo tabs, manufactured by Measurement Specialties Inc.  These little films have a thin layer of piezo material sandwiched between two layers of protective plastic. They’re typically used as contact pickups: attach one of them to a soundboard or other vibrating body with adhesive or double-sided tape and it will convert the board’s vibration into an analogous electronic signal which can be sent to an amplification system. 

When you first wire up one of these little films, before attaching it to an instrument, it’s quite natural to give the film a little flick of the finger just to make sure it’s wired properly and the signal is coming through. I’ve noticed that the resulting sound is kind of cool.  It’s a quick, percussive thump, but with a penetrating sort of resonance.  And it makes a difference how you hold the film while flicking: hold it near the end with just a bit of film extending out freely and it will produce a higher tone; hold it near the base with a longer overhang and the tone is lower.

Naturally this suggests a musical instrument. I designed a system for holding a row of twelve of the film tabs in a one-tier keyboard-like configuration in such a way that the overhanging length of each tab is easily adjustable. The outputs go to a little onboard mixer/preamp velcro’d to the body of the instrument, and from there to a regular musical instrument amplifier.

That’s really all there is to it, but a few more words about the preamp are in order. Two facts about the piezo films come into play here. One is that they produce an extremely high-impedance signal that doesn’t do well with long cable runs, tending to lose strength and fidelity. So, if the playing situation may call for a long distance between the instrument and the main mixer or amp, it helps to have the onboard mixer/preamp which takes the original signal and converts it to lower impedance before sending it on its journey. The other fact is that the piezos suffer a loss of signal strength if you wire too many individual piezos together. With the little onboard mixer, instead of joining the output from all twelve together I was able to join them in smaller subgroups and send them to different mixer inputs.  The mixer buffers the groups from one another and solves too-many-together problem.  The mixer I used (a model called MM-141 made by Nady) has four inputs, so I was able to group the twelve piezos in threes. It also conveniently allows for local volume control and balancing between the subgroups. Little mixers such as this have become fairly affordable and widely available. Within the subgroups, the piezos are best wired together in parallel, not in series.

I’ve now fooled around with the new instrument a bit and even played it once in public. The variously pitched popping sounds make a distinctive addition to any musical stew; standing out without crowding.

I recall that I came up with a good name for the instrument immediately after making it, and now I can’t remember what it was.  Let me know if you have any ideas.

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