Abstract:
A transducer for a stringed musical instrument utilizes a coaxial structure. A thin layer of a piezoelectric polymer material is extruded about an inner, electrically conductive core. An outer conductor is formed about the piezoelectric polymer material. Polarization of the piezoelectric polymer material is accomplished in conjunction with the extrusion process. The piezoelectric polymer material has an optimized thickness for consistent responsiveness across a desired range of input stimuli, and is capable of maintaining the integrity of the transducer over time. The transducer configured for placement underneath the saddle in a bridge of a stringed musical instrument.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Patent. Application Ser. No. 60/091,742, filed Jul. 6, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to a musical instrument transducer. More particularly, it relates to a piezoelectric transducer used with a stringed musical instrument such as a guitar. 
     The prior art shows a variety of electromechanical transducers employed with musical instruments, particularly guitars. Many of these transducers are not completely effective in faithfully converting mechanical movements or vibrations into electrical output signals which precisely correspond to the character of the input vibrations. This lack of fidelity is primarily due to the nature of the mechanical coupling between the driving vibrating member (i.e. a string) and the piezoelectric material of the transducer. Some of the prior art structures, such as those shown in U.S. Pat. Nos. 4,491,051 and 4,975,616, are also quite complex in construction and become quite expensive to fabricate. Furthermore, a transducer using a piezoelectric material requires a conductive layer, a ground layer, and some form of shielding to prevent electrical interference. These multiple layers not only increase the complexity of the transducer, but interfere with the ability to attach leads to the transducer as it is made smaller to operate in a musical instrument. 
     Differently shaped transducers have been produced for musical instruments. Generally, transducers for stringed instruments have a flat, elongated shape. The piezoelectric layer for such transducers can also be elongated, or can be individual crystals between electrodes. Alternatively, one prior art transducer was coaxially arranged, with a center electrode, surrounding piezoelectric layer, and outer electrode, as illustrated in U.S. Pat. No. 4,378,721. 
     Each shape offers unique difficulties in construction and varying degrees of quality in operation and performance. For good performance, the piezoelectric layer needs to respond to small string movements at a variety of frequencies. With a thicker layer of piezoelectric material, the material needs to be more flexible; if made too thick, the piezoelectric layer may be too brittle for the intended use, and may not provide satisfactory response characteristics across of range of input stimuli including the smallest string movements. To achieve sufficient resilience in a coaxial arrangement, U.S. Pat. No. 4,378,721 discloses a material formed from a rubber material mixed with a powdered piezoelectric ceramic and a vulcanizing or cross-linking agent. Piezoelectric ceramic is typically brittle and inflexible. This reference relies upon a rubber matrix to bind together the powdered ceramic material The use of a rubber material results in a significantly thicker piezoelectric material layer, which is inconsistently responsive across a variety of input frequencies; the rubber matrix tends to damp input stimuli, resulting in degraded response. A thicker piezoelectric layer, even if comprised of rubber, becomes more difficult to physically accommodate, to bend or to otherwise manipulate. Over time, it has been found that the composite piezoelectric layer such as described in this reference tends to deform in response to compression such as is typical in a stringed instrument application. 
     A further disadvantage of the coaxial transducer as described in U.S. Pat. No. 4,378,721 relates to its formation through a casting or molding process, such that the length of the resulting transducer is dependent on the size of the molds available. Other manufacturing processes are not suitable for the composite piezoelectric material due to a low degree of cohesiveness. 
     Additionally, the polarization of the piezoelectric material so of this reference must be performed after completion of the casting procedure. Two opposing, plate-like electrodes, on either side of the transducer, are used to initialize the magnetic domains of the piezoelectric material, thereby complicating and extending the manufacturing process of such a transducer. Therefore, a need exists for an accurate, responsive transducer with a thin, relatively stiff piezoelectric layer which can be economically formed into a coaxial arrangement. 
     BRIEF SUMMARY OF THE INVENTION 
     The deficiencies of the prior art are substantially overcome by the transducer according to the present invention, which includes a coaxial structure having a central conductor, a piezoelectric polymer layer, and an outer conductor. The central conductor may be formed of a wire bundle or a solid wire. A piezoelectric cylinder of either a piezoelectric copolymer or a monopolymer is formed about the central conductor. The piezoelectric material may be substantially thinner than that of the prior art, thus providing significantly improved response characteristics for the output signal, while providing a desired degree of flexibility and resistance to deformation over time. 
     The outer conductor can be formed as a braided sheath or simply as a conductive paint on the outside of the piezoelectric material. Other embodiments include the use of conductive foil, conductive shrink tubing, or any other flexible, conductive material which has a minimal impact on the flexibility of the overall transducer and on the response characteristics of the piezoelectric material. An additional mechanically shielding layer may also be provided, though this layer must not significantly interfere with the responsiveness of the transducer. Leads are attached to the central and outer conductors in order to complete the transducer. The coaxial transducer may be provided with a length sufficient to fit within the saddle of a guitar, underneath the strings. Other embodiments may be configured for use with other stringed musical instruments. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     This invention is pointed out with particularity in the appended claims. The above and further advantages may be more fully understood by referring to the following description and accompanying drawings, of which: 
     FIG. 1 is a perspective view of a stringed musical instrument, in particular guitar, that has incorporated therein the transducer of the present invention; 
     FIG. 2 is a cross-sectional view taken along by  2 — 2  of FIG. 1; 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG. 1; 
     FIG. 4 is a cut-away view of the structure of the transducer according to the present invention; and 
     FIG. 5 illustrates a procedure for fabricating a transducer according to the present disclosure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a guitar that is comprised of a guitar body  110  having a neck  112  and supporting a plurality of strings  114 . In the embodiment disclosed herein, as illustrated in FIG. 3, there are six strings  114 . The strings  114  are supported at the neck end of the instrument (not shown). At the body end of the strings, the support is provided by a bridge  116 . The bridge  116  includes a mechanism, such as illustrated in FIG. 2, for securing the end  117  of each of the strings  114 . The bridge  116  is slotted, such as illustrated in FIG. 2, in order to receive a saddle at  118 . The strings  114  are received in notches in the saddle  118  at the top surface. 
     FIGS. 2 and 3 illustrate cross-sectional views of the bridge and saddle with the positioning of the transducer of the present invention. The transducer  120  is positioned within the bridge underneath the saddle. As illustrated in FIG. 3, the transducer extends below the entire saddle underneath each of the strings of the instrument. In one embodiment, a portion of the transducer, when fully installed under the saddle, is bent towards and into the interior of the instrument, where conductive leads are attached for communicating the output signal to appropriate signal conditioning and/or amplifying circuitry (not shown). In this embodiment, installation of the transducer is achieved by feeding a free end of the transducer, opposite the conductive leads, into an opening in the interior of the guitar, beneath the bridge, until the transducer extends under the length of the saddle. 
     The structure of the transducer is illustrated in FIG.  4 . The transducer of the present invention is formed of an inner conductor  210 , a piezoelectric polymer layer  220 , and outer conductive layer  230 . The inner conductor in the illustrated embodiment is formed of a conductive material having cylindrical or substantially cylindrical shape. It may be a single wire (not shown) or a twisted bundle of a plurality of individual wires  211 . Such a bundle may further include non-conductive elements (not shown) useful for increasing the volume or rigidity of the inner conductive core  210 ; while it is preferable that the transducer of the present invention be sufficiently flexible that it can easily conform to irregular surfaces under the saddle and can be bent for facilitating installation within a bridge, it may also be useful for the transducer to exhibit a degree of mechanical rigidity as well. According to one embodiment, the inner conductor  210  has a diameter of approximately 0.075 to 0.080 inches. 
     A layer of a piezoelectric polymer material  220  is formed about the inner conductor  210 . In one embodiment, the piezoelectric material is formed to have a thickness less than the diameter of the central conductor. In particular, a further embodiment provides the piezoelectric material having a thickness less than half the diameter of the inner conductor. According to a specific variant of this embodiment, the piezoelectric material has a thickness between approximately 0.010 and 0.015 inches. However, in other embodiments, central conductors are employed which are of such dimensions that the piezoelectric layer is as large as or larger than that of the central conductor. 
     The piezoelectric material is more accurately termed a piezoelectric polymer. The material is an amorphous structure containing many thousand individual crystals, which is constructed by combining different polymeric elements and subjecting them to high temperatures. This forms a fused material containing thousands of crystals. The piezoelectric polymer used in this invention may be a polyvinylidene fluoride (PVDF) copolymer. Alternatively, it may be a PVDF homopolymer. PVDF homopolymers are described in U.S. Pat. No. 4,975,616. PVDF copolymers can include, but are not limited to, vinylidene/tetrafluorethylene and vinylidene/trifluoroethylene polymers. The use of a thin layer of a piezoelectric polymer with a stiffer conductor provides the desired resilience for acceptable outputs from the transducer in a musical instrument and a desired, even responsiveness to a broad range of input frequencies without mechanical loss due to damping. The piezoelectric polymer is sufficiently resilient to offer the desired flexibility without the need for a rubberized matrix, and is resistant to compressive forces over time, such that the original transducer shape is maintained. Polymer materials as used in the presently disclosed transducers also tend to resist becoming brittle over time. 
     Around the piezoelectric polymer material, an outer conductive layer  230  is formed. The outer conductor  230  may be a braided sheath of wires. Alternatively, the outer conductor may simply be a conductive paint applied to the outer surface of the piezoelectric material. Further embodiments include the use of other flexible, conductive materials, including conductive foil, conductive shrink tubing, or other similar materials. The outer conductor  230  also forms a shield about the transducer. Conductive leads (not shown) are attached to the inner conductor  210  and the outer conductor  230  for providing signals from the transducer. The manner of attaching these leads can be according to state of the art practices with respect to coaxial cables outside the field of transducers. The conductive leads are preferably shielded to avoid the introduction of noise. 
     With reference to FIG. 5, a transducer according to one embodiment of the present disclosure is fabricated according to the following procedure. An electrically conductive central core is provided. Extrusion tools as known to one skilled in the art are employed in forming the piezoelectric polymer material layer about the central core. As part of the same process, the outer conductive layer is formed about the piezoelectric layer. The exact process for application of the outer layer depends upon the material chosen: conductive paint may be sprayed; conductive foil may be wrapped; conductive mesh may be woven. 
     As part of the extrusion process for this transducer, electrodes may be provided to polarize the piezoelectric polymer material as it is extruded. For instance, exposure to a DC field results in substantial alignment of the magnetic domains within the piezoelectric material. Once so aligned, the piezoelectric material is capable of generating a detectable potential when subject to the stresses to be monitored, in this case, the vibration of strings on a guitar or other musical instrument. Thus, a transducer according to the present disclosure may be fabricated to any length desired and simultaneously polarized, eliminating waste and simplifying the manufacturing process. The exact order of the steps of FIG. 5 may be rearranged in order to accommodate preferred manufacturing practices. 
     In alternative embodiments of the present disclosure, the cross-section of the resulting transducer is not perfectly round, but may be symmetrically or asymmetrically ovoid. Further, one or more sides of the transducer cross-section may be flat. For instance, the transducer assembly may have a rectangular cross-section. The choice of cross-sectional configuration may depend upon the environment into which the transducer is to be installed and any apertures through which the transducer must pass in order to reach its operating position. It is preferred in one embodiment that the central conductor have a diameter or thickness which is greater than the maximum thickness of the surrounding piezoelectric layer, regardless of cross-sectional configuration. Appropriate extrusion tooling is employed for these various configurations. Flexibility in determining transducer length through an extrusion process is maintained. 
     Further layers may be incorporated into the transducer as presently disclosed. For instance, it may be desirable to incorporate a mechanical shielding layer over the outer conductive layer. However, care must be exercised in selecting a shield material which protects the outer conductor without compromising the responsiveness of the piezoelectric material. 
     Having described at least one embodiment, it should now be apparent to those skilled in the art that numerous other modifications and changes can apply to this invention. Specifically, variations in the dimensions listed herein are contemplated. Additionally, while a transducer according to the present invention has been described for use with an acoustic guitar, the transducer may be utilized with other stringed instruments such as, without limitation, violas, pianos, or electric guitars. Such modifications and changes are contemplated as falling within the scope of the invention, which is limited solely by the pending claims.