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more... Acoustic SoundboardPremier BlogAcousticApril 2014

Acoustic Soundboard: One Word—Plastics

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The introduction of polyvinylidene fluoride polymers to the piezo-pickup design arena was a true game changer.

In my last four columns, we’ve taken a detailed look at a variety of under-saddle piezo pickups. In all the cited examples, the sensing element was made of relatively hard PZT ceramic material—the only material available at the time of their designs. Also common to these pickups was the use of an individual sensing element for each string.

Starting in the mid ’80s, however, a revolution was occurring with the development of new materials that exhibited piezoelectric activity. This new generation of materials was made from polyvinylidene fluoride polymers (PVDF) and PVDF copolymers P(VDF-TrFE). Using such processing techniques as stretching, heating, and polarizing with very high DC-voltage fields, these polymers could be transformed from ordinary plastic into fully piezo-active films/sheets.

From a transducer designer’s point of view, these new materials were like a gift from the gods, because they were thin, flexible, and easy to cut with a simple shear or die-cutting press. Unlike the hard PZT ceramics (which were too brittle and fragile to use in long, thin strips), the polymers could traverse the entire length of a saddle. With PZT-based designs, the individual elements had to be placed precisely under each string, which meant that many variations of a given design were necessary to accommodate the differing string spacing from guitar model to guitar model.

With these new materials, it was no longer necessary to test and match individual elements for capacitance and thickness: The polymer sheets were being manufactured using an extrusion process that produced dimensionally precise and electrically homogeneous material.

Add to that the fact that polymers are low-Q materials. This means they do not have strong resonances—they’re perfect transducer material for amplifying a musical instrument. Also, the softness of the materials provided new designs with a slightly slower response that many players felt was more natural and less “jumpy” than the PZT-based designs.

The first polymer-based system I’m aware of was the S.O.R.S system that appeared in Gibson acoustics around 1988. This design involved wrapping the lower half of the saddle with piezoelectric film before it was inserted into the slot. When set up properly, the system sounded great. But the critical tolerances required for fitting the saddle into the bridge slot, as well as the inherent fragility of attaching electrodes to the output cable, were detrimental to maintaining the consistency and quality of the output signal. The design was deemed impractical and discontinued after a couple of years.

From a transducer designer’s point of view, these new materials were like a gift from the gods because they were thin, flexible, and easy to cut with a simple shear or die-cutting press.

In 1990, my company released the Acoustic Matrix line of pickups and EMG released their AT pickups. L.R. Baggs followed with their Ribbon Transducer, Highlander released the iP-1 system, and B-Band released the 22R pickup. In the early 2000s, my company released the Sonicore transducer to the OEM market, and then L.R. Baggs introduced their Element transducer.

All of these designs are functionally similar: They employ battery-powered onboard preamps and involve placing a piezoelectric polymer under the saddle of an acoustic guitar. The polymer is deformed by the changing force generated by the saddle reacting to the vibration of the strings. The opposing surfaces of the polymer are in contact with conductive electrodes that accumulate the varying charge, which is then passed along to the powered preamp via the attached lead wires. Each design also incorporates a grounded conductive shield that surrounds the transducer to block any environmentally produced electromagnetic interference.

There are three basic design categories here: The Fishman Matrix and EMG AT are what I call “constructed designs.” They are constructed from core materials, including the piezo polymers, and are wrapped with a thin and non-resilient shield. The B-Band 22R and L.R. Baggs Ribbon Transducer are laminates of conductive materials and piezo polymers. (In the case of the L.R. Baggs Ribbon Transducer, a resilient rubber layer is added.) Finally, the Fishman Sonicore and Highlander iP-1 are coaxial designs with a braided-wire shield, and the L.R. Baggs Element is a semi-coaxial design with a braided-wire shield.

These transducers all work well, but each has an upside and a downside. I feel that constructed transducers respond with the most accuracy and detail, and have the least effect on the pure performance of an instrument when played acoustically. This is because they don’t contain spongy or resilient materials that tend to dampen an instrument’s responsiveness. The downside to not having the resilient materials is that the installation must be precise, with flat and parallel surfaces on the bottom of the saddle and the bottom of the saddle slot.

It’s my opinion that pickups with a braided-wire shield tend to dampen the acoustic response of an instrument and have a less-detailed sound due to the presence of the braided shield that’s pliable and soft. The upside here is that they are somewhat more forgiving with respect to installation, and have a more pronounced bass response because the soft materials can benefit bass-deficient instruments.

Until next time, I’ll leave you with this: In the 1967 film The Graduate, Mr. McGuire told Ben Braddock, “There’s a great future in plastics.” Indeed, plastics did help shape the future of under-saddle transducer design.

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