Music – Instruments – Electrical musical tone generation
Reexamination Certificate
2000-04-21
2001-06-05
Nappi, Robert E. (Department: 2837)
Music
Instruments
Electrical musical tone generation
C084S723000
Reexamination Certificate
active
06242683
ABSTRACT:
The present invention relates to a stringed musical instrument transducer and, in particular, an flexible unitary electret film undersaddle pickup for converting string vibrations into electric signals, and to a method for its fabrication. The transducer is especially applicable for use with a guitar.
PRIOR ART
Saddle transducers ie. pickups for acoustic guitars, designed to transform string vibrations into electric signals, are mounted under the saddle of the guitar. They have a transducer part of a length corresponding to that of the saddle and typically containing different layers of electromechanical transducer elements, dielectric material and electrically conductive electrode layers, and a connection cable part in which the signals are taken to a preamplifier inside the guitar via a small hole (diameter typically 3 mm) bored in the guitar's resonance box under the saddle. Saddle transducers may typically have a one or more transducer element layers.
As electromechanical transducer elements, piezoelectric crystals or piezoelectric sheet (e.g. polyvinylidene fluoride PVDF) are prior art. In the commonest transducer structures, the connecting cable part is implemented using screened coaxial cable, which is connected to the electrode layers of the transducer part by soldering. Such a transducer is presented e.g. in U.S. Pat. No. 5,319,153. A drawback with this type of structures is the difficulty of fabrication of the transducer and relatively high manufacturing costs, because much of the work has to be done manually. Moreover, the connections to the preamplifier generally have to be made by soldering, because no connectors of sufficiently small size to go through the hole provided under the saddle are available for coaxial cables and because the connection between the transducer itself and the cable makes it impossible to mount the transducer from below. In addition, piezoelectric crystals and sheets are associated with a certain characteristic sound that is not quite in keeping with the guitar's own acoustic sound. Further, the prior art saddle transducers structures comprise many material types, which affects to the sound produced by the saddle transducer.
The electret field, or the permanent electric charge, is achieved by injecting charges into dielectric material.
A dielectric porous electret film and manufacturing process for same, applicable for use as electromechanical material for a stringed musical instrument transducer, is described in U.S. Pat. No. 4,654,546, said dielectric film comprising permanently charged, biaxially oriented, foamed, usually homogenous film layer containing flat lens-like, shredded or cavitated gas bubbles which can also be called as voids or cells. The term “dielectric cellular electret film” is used here to refer to generally porous type electromechanical films having a permanent electric charge injected into material.
WO-publication 96/06718 presents a procedure for pressure inflation of a pre-foamed plastic film, that makes it possible to manufacture strongly foamed film products, involving a high foaming degree and allowing the thickness of the product to be increased without increasing the amount of plastic material. The term “dielectric swelled cellular electret film” is used herein to refer to a foamed film-like plastic product as described in that WO-publication and having a permanent electric charge injected into material.
SUMMARY OF THE INVENTION
The object of the present invention is to eliminate the drawbacks of prior art and achieve an improved transducer of a completely new type for a stringed musical instrument, in which a dielectric swelled cellular electret film is used to transform the string vibrations into electric signals instead of piezoelectric films or crystals. Flat lens-like gas bubbles in the electret film effectively limit the mobility of electret charges in the dielectric material, because the gases have an electric resistance five decades better than the best solid insulating materials have. At the same time, compared to hard structure of piezoelectric materials, they act as an elastic soft layer during the conversion of string vibrations into electric signals allowing pressure variations caused vibrations to cause microscopic changes in its thickness. The change in thickness causes change in capacitance and produces an electrical output voltage in proportion to the sound source.
A further object of the invention is to produce a new type of stringed musical instrument transducer which, due to its elastic cellular structure, is capable of converting string vibrations into electric signals which, when converted into sound, compared to prior art piezoelectric saddle transducers, better correspond to the instrument's own, acoustic sound and allows playing at high volumes before feedback. Because of the elastic porous structure, the young's modulus of the material is significantly lower and thus the impedance matching with wood is better than with piezoelectric materials. This results in natural sound similar to instruments own acoustic sound without any harshness or “quacking” as typically with piezoelectric materials.
Another object of the invention is to produce a stringed musical instrument transducer which is of a construction thin enough to permit installation without changing any parts of the instrument, e.g. making the saddle lower, and which, when installed, does not affect the instrument's own acoustic sound and is as easy to install as possible without soldering.
Another object of the invention is to produce a stringed musical instrument transducer capable of converting the vibration of each string separately into an electric signal.
A further object of the invention is to produce a stringed musical instrument transducer as simple as possible, having no separate transducer part and no separate conductor for connecting it to a signal processing device, but which has a unitary, flexible and laminated structure and in which the connections for connecting it to a preamplifier can be disposed sequentially or side by side and which in itself is able to produce a balanced signal (differential transducer) according to the attached claims.
This kind of transducers can be very economically fabricated for example by screen-printing the required electrodes with silver paste on sheets of dielectric film (e.g. polyester), placing several electrodes side by side on the same sheet. By laminating such sheets and dielectric cellular electret film, preferably swelled, on top of each other so that dielectric cellular electret film is only placed on a desired area at one end of the sheet while the other end is provided with a connector part with different electrode layers side by side, a laminate sheet is obtained from which the transducers can be cut out e.g. by punching. After that, it is only necessary to join a suitable connector to the electrodes at the connector end of the transducer by pressing mechanically.
With this method, it is possible to produce ultra thin and flexible transducers of desired length, design and width, in which the electrodes in the transducer part are continuous extending from the transducer part adjacent to the saddle, through the hole to the inside of the guitar and up to the preamplifier and which are unitary, flexible and thin laminate in construction. Fabrication is faster and more economic than with conventional methods.
The structure of the invention thus allows the application of an effective and economic production technique, especially when the transducer is of the same width over its entire length. In this case, the transducers can be arranged closely side by side, producing no material waste. The structure of the invention makes it possible to produce a transducer of the same width and therefore very economic for the commonest acoustic guitars, which have a saddle width of 2.4-3.2 mm. This width is still sufficient for the connector of a single electrode. The structure of the invention allows a maximum amount of transducers to be produced from the same materi
Raisanen Heikki
Raisanen Lasse
EMF Acoustics Oy Ltd.
Fletcher Marlon
Helfgott & Karas P.C.
Nappi Robert E.
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