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Permalink Reply by Doc Oakroot on April 24, 2009 at 4:06am

Microphonics is really a specific failure in an electromagnetic pickup. In a properly built EM pickup, the coil and the magnet are locked in place with respect to each other and, consequently, the pickup is completely immune to ambient sound. Properly installed, it can only pick up movement of the strings.

When the potting of the pickup fails... or when some other mechanical failure occurs, so that the coil can move with respect to the magnet, then anything that shakes the pickup (like vibration of the top of the guitar) is picked up as sound and the pickup is said to be microphonic.

A piezo, mounted to the top of your guitar will always be microphone - in fact, I've used a cigar box guitar with a piezo as a microphone for a special effect.

On a production guitar with a piezo, the piezo is mounted in the bridge and mostly isolated from the top, acts less like a microphone. So if you tap on the top of most production piezo guitars, the tap is not picked up (this is actually a problem for some players and they pay extra to get a microphone built into their guitars to get around it).

You can build your piezo into the bridge and isolate the bridge to avoid microphonics, but 1) it's hard to do it well with disk piezos most of us use, and 2) it will degrade the acoustic sound of the instrument in proportion to how effectively you isolate the bridge.

I find it's adequate to just mount the piezo on the bottom of the top right under the bridge. It is microphonic, but no feedback.

Silicone is available at the hardware store (probably in the paint section). A thin layer between the piezo and the top will soften the harsh highs quite a bit. Encasing the piezo in silicone is unnecessary, since the piezo can't really pick up sounds from the air in the box on its own, but won't hurt anything.

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Permalink Reply by Mark Werner on April 24, 2009 at 4:18am

I've used the silicone method on my last couple of builds, and it seems to work well. "Microphonic" is likely not the best word, but usually describes the effect of the piezo picking up every contact noise with the instrument.
Any finger brush, even brushes up against clothing and such.
It's not, as Doc says, feedback; it's just a very sensitive device. The silicone does seem to filter out some of this....

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Permalink Reply by Ben on April 24, 2009 at 4:19am

Thanks Doc, Very Informative. I guess the confusion came from applying the word microphonics to piezo. I haven't potted my handwound pickups yet. Just electrical tape to make sure they're not going anywhere but I've been thinking about it. Thanks for the info.

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