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 .