If you feel that you have a lot of time in your hands or would like to experiment here's how to build you own transducer (with an intro)...
Piezo Materials and Instrument Pickups
A piezo transducer is a pickup that uses some sort of piezoelectric material to convert sound (pressure) energy into electrical energy. Simply put, a piezo material is any material that will generate an electrical charge when mechanically deformed.
There are four types of piezo materials used in the manufacture of instrument transducers - lead zirconate titanate (PZT) ceramic chips, PZT ceramic bender discs, polyvinylidene fluoride (PVDF) plastic film, and PVDF coaxial (coax) cable. PZT chips find their way into what are now old style under saddle guitar transducers, transducers for various bowed instruments, and manufactured arched top guitar and mandolin bridges. PZT discs consist of PZT material bonded to thin brass disks, and are commonly used for soundboard pickups for flat top guitars and for bridge-mounted pickups for upright basses. PVDF film may be found in all sorts of transducers from under saddle guitar transducers to under-bridge-foot transducers for bass viols. PVDF coax cable is manufactured just like the single conductor shielded cable used to make instrument cables, except that instead of an insulating material between the center conductor and the outer shielding braid, PVDF material is used there. It is used in manufactured under saddle pickups for acoustic guitars and is the material that will be used to construct a transducer in this project.
All of these piezo materials have physical and electrical characteristics which make them suitable for extremely high fidelity transducers for musical instruments, a fact which is belied by the rather humble applications to which these materials are often applied. PZT ceramic chips are used to make the igniters for gas barbecue grills and camping lanterns. PZT disks are probably most frequently used as cheap buzzers in electronic devices and as microphones and speakers in telephones. And the piezo coax cable used in the transducer in this article is most often found buried in the road to let automated traffic lights know when a car is approaching. Again, although used for a wide variety of applications, these materials are no less suitable for the high fidelity application of musical instrument pickup.
Pickup Placement
To make a pickup for a musical instrument, two wires are connected to a piezo material. The resulting element is mounted so that sound from the instrument causes the material to deform, producing an electrical signal which varies in proportion to the deforming vibrations. Pickup placement is critical to good sound reproduction and always represents at least one tradeoff. The fact is there is no perfect place to locate a piezo transducer, as all locations color the sound in one way or another. In guitars, piezo transducers are most often located on the underside of the soundboard or in the bridge. Soundboard placement is problematic in that the soundboard vibrates in a modal fashion related to frequency. This means that a transducer mounted on the soundboard will be more sensitive to certain frequencies and less sensitive to others, those frequencies depending on where on the soundboard the transducer is located. Soundboard transducers are also prone to feedback even at low levels of amplification.
These days the most popular place to locate a piezo transducer on a guitar is in the bridge, and for flat top guitars the most popular place in the bridge is directly under the saddle. Here, string tension variations caused by the vibration of the string translate into pressure variations on the saddle, which rests right on top of the transducer. This placement is less prone to feedback than soundboard placement and also produces a more balanced frequency spectrum. On the downside, under saddle placement reads more of the sound of the strings themselves, and less of the sound of the entire instrument. Again, there is always a tradeoff with transducer placement, but this is one that works out well in practice.
I’ve chosen piezo coax cable for the pickup in this article. Piezo coax cable is probably the easiest material to use without special tools and jigging. All piezo electric materials mentioned present very high electrical impedances and as such must be thoroughly shielded to prevent the pickup of electrical noise and hum. (IN EARLIER POSTS, THIS IS THE REASON WHY YOU HAVE THE LINE LEVEL VS XLR (BALANCED LINE) OR NEED YOUR DI BOX TO TRANSFORM HI TO LO Z) As the piezo coax cable is well shielded as manufactured, no additional effort is necessary to shield the transducer element itself. This material is also very simple to cut-- it can be cut to length using wire cutters. Finally, piezo coax cable is very easy to wire up.
With the discussion of piezo materials and instrument transducer design and placement accomplished, let’s look at the construction of an under saddle transducer.
Parts List
Approximately 5” of piezo coax cable, approximately 0.125” in diameter (Measurement Specialties Inc. PVDF piezo coax cable or equivalent).
Approximately 18” of high Z, shielded, single conductor microphone cable, 0.125” or less in diameter. (Mogami 2368 or equivalent).
An assortment of diameters of heat shrink tubing.
Copper shielding foil with conductive adhesive.
Cyanoacrylate glue.
See the Sources section at the end for information on where to get these supplies.
Tools
Soldering iron (pencil type, approx. 35W) and solder.
Wire strippers.
Pliers.
TO FOLLOW IN NEXT POST.........Construction
