PVDF

Polyvinylidene Fluoride (PVDF) is a piezoelectric plastic. If you purchase a piezo pickup made of "film," it is made of PVDF. PVDF is said to generally have a broader, more balanced frequency response than ceramic piezo discs. If one or both of my readers remember, in my January post On Film I recorded a steel-stringed acoustic guitar with a commercially-produced piece of piezo film taped by the bridge. The frequency response was excellent but the signal was very weak, resulting in having to crank up my preamplifier and getting some hiss.

Hoping to resolve this problem, I purchased some PVDF film from McMaster-Carr in various thicknesses (.003", .005" and .01"), planning to make my own, louder, piezoelectric film pickups. Guidance on doing this is not readily available. There is a fairly good guide on using PVDF from Measurement Specialties, although it's not focused on musical instrument pickups. I also found some good information on making connections to piezo film from Trout Cove Lutheries.

Three things I have learned are: 1) you have to attach electrodes to the film, preferably each side, for only between these electrodes will mechanical stress be translated into voltage; 2) PVDF has some problems with hum; and 3) the dimensions of the film affect the low frequency cutoff (the lowest frequency for which the output of the circuit deviates less than 3 dB from the nominal value). So far I've focused on making the electrodes.

I cut a small piece of the .003" thick PVDF into a 1" by 4" strip, slightly larger than my commercial piezo film strip. For electrodes I stuck strips of conductive copper shielding tape on each side. I placed the stripped end of a wire under each and stapled them (unfortunately backwards) for a more secure connection. The result:


I taped this to a nylon-stringed guitar, ran it through my homemade mint-box preamp (see my first post) and recorded. The result was not stellar, but it did record audio and didn't sound awful, although not worth posting. I'm going to experiment with conductive glue painted on each side and possibly with silver ink from pens sold for circuit board repair (most commercial films have electrodes made from silver ink painted on surface). I'm thinking that different sizes and thicknesses of film will affect frequency response and signal strength .

Wooden Chips (wrapped in heat shrink, almost free)

I built two new pickups today, both using the same new design. I've become frustrated with dipping piezo discs in Plasti-Dip. I can't get the dip to dry evenly so that the surface of disc will be parallel with the layer of Plasti-Dip and thus flush with the soundboard of the guitar. I also worry about breathing in the allegedly carcinogenic fumes.

For my new design, I've mounted two discs on small squares of wood: one of balsa and one of basswood. I had intended to buy only balsa. But, at the hobby store, they sold large rectangles of basswood along with balsa for only $2.50, so I thought I would try it as well. I've learned since that basswood is from the linden tree (see above), used for electric guitar bodies, and supposedly has good acoustic qualities.

I soldered some lightweight, unbalanced Mogami lavalier microphone cable ( #W2368) on to two small unidentified piezo discs. I then glued each disc to a square of balsa and basswood, respectively. I tried to glue them so that the entire surface of the disc would contact the wood. These small piezo discs are made to buzz, rather than pick up sound, and are designed to resonate at a certain frequency, the resonance emphasized if a only specific point on the the surface of the disc is contacted. By having the entire surface contact the wood, the resonance is dampened and the frequency response broadened.


I then put each disc/wood combination into a tube of heat shrink and heated. I soldered 1/4 inch female jacks to the other end of each cable and wrapped the cables in shielding foil. I then placed each jack it heat shrink (clear the only kind I had that was large enough) and ended up with an respectable-looking pickup.


Both pickups sound good when taped behind the bridge on my nylon string guitar. The balsa pickup, maybe since balsa is lighter and not as dense, sounds and little hollow, emphasizing the midrange. The basswood pickup sounds great I think. Although it lacks a some bass (despite its name), the midrange and high-end sound very clear, i.e. it doesn't produce the slightly muffled "underwater" sound like other pickups I've built with piezo discs. It's also loud, but the wood seems to dampen some of the frequencies that otherwise peak to the point of distortion with my other disc pickups. I'm hoping that if I move the pickup nearer the sound hole, I'll get some more low-end. I think this basswood pickup is the best disc pickup yet. It doesn't have quite as broad a the frequency response as the film pickup, but its signal strength and clarity are excellent.

Some samples with me noodling around on a nylon string guitar:

Balsa Pickup

Basswood Pickup