Sound board for stringed instrument
[451 Apr. 29, 1975 3,664,911 5/1972 Takabayashi............,........ 84/291 X 3,699,836 10/1972 Glasser 84/291 3,724,312 4/1973 Yamada et 84/193 SOUND BOARD FOR STRINGED INSTRUMENT [76] Inventor: Charles H. Kaman, Prattling Pond Rd., Farmington, Conn 06032 Oct. 16, 1974 Primary E.\&#39;aminerLawrence R. Franklin Attorney, Agent, or Firm-McCormick, Paulding &amp; Huber [22] Filed:  
 [21] Appl. No.: 515,365  
 [57] ABSTRACT A multiple ply sound board for the top of the hollow 84/291 gig/ body portion of a guitar has inner and outer plies of 554/791 oriented graphite fiber material, and one or more medial plies of wood. The high stiffness to weight ratio of the graphite fiber plies permits the overall thickness of [51] Int. Cl. [58] Field of Search 294, 193, 192  
 [56] References Cited UNITED STATES PATENTS the sound board to be reduced in thickness compared to a conventional wood Wound board.  
 10 Claims, 5 Drawing Figures 5 X 7 23 4 00/ .4 8 Wm k u w a MK 1 67 99 H 1 52 4 9 78 2 33 l SOUND BOARD FOR STRINGED INSTRUMENT CROSS REFERENCE TO RELATED APPLICATION The sound board disclosed herein is especially suited for use in a guitar constructed in accordance with that described in my copending application, Ser. No. 515,364 entitled &#34;Guitar Construction&#34; filed concurrently herewith.  
 BACKGROUND OF THE INVENTION This invention relates generally to sound boards for stringed musical instruments, and deals more particularly with a multiple ply sound board of laminated construction wherein the inner and outer plies are fabricated from graphite fiber material and hence lend the sound board a relatively high ratio of stiffness to weight thereby enhancing the tonal qualities of the resulting guitar.  
  Stringed musical instruments such as guitars, violins. and lutes or the like characteristically include a hollow body portion with the strings extending from one end of the neck to a bridge or the like supported on the front face or top of the hollow body portion. The vibration of the strings is transmitted to the face or sound board of the hollow body for purposes of amplification and enhancement of the tone of the instrument. Further. the sound board&#39;s vibrations are transmitted to the quantity of air inside the body. and the resulting sound heard is an integration of all these vibrations. It will be apparent therefore. that the physical characteristics of the sound board are quite significant in determining the ultimate tonal qualities of the instrument.  
  The influence of the sound board is such that its stiffness to weight ratio should be kept as high as practical.  
 consistent with providing the necessary structure for reacting to the forces exerted by the strings on this portion of the guitar. Some internal bracing is usually provided on the underside of the sound board for this purpose. but such bracing must not interfere with the vibration patterns associated with the various notes and chords which are to be played upon the instrument. In an all wood sound board for a conventional guitar. for example. a thickness of the order of 0. 10 to 0.12 inches has been found to yield satisfactory tonal qualities. at least when the wood used is seasoned spruce or the like. Somewhat thicker sound boards have been proposed with a laminated wood construction utilizing stiffer woods in the outer plies and less stiff, but lighter woods. in the inner ply. This does not yield higher ratios of stiffness to weight than is true of the one piece relatively thin sound boards found in present day guitars. US. Pat. No. 2.674.912 issued to Petek in I954 illustrates a laminated sound board for a violin. However. the thickness required in the violin disclosed in this patent results in a sound board of twice the thickness characteristic of present day guitars. This thickness results in excessive damping of the harmonic response characteristics associated with the sound of a conventional guitar, and hence the laminated wood structure suggested in this patent has not found application to guitars generally.  
  In a guitar. the sound board must exhibit a wide frequency response. especially to accommodate the harmonics of the various sounds produced by its strings. This result cannot be accomplished with a relatively thick wood sound board of the type disclosed in Petek In order to provide a guitar with a frequency response broad enough to provide a suitable vehicle for these harmonic vibration modes the sound board must be made as thin as possible. a thickness on the order of 0.10 inches being common in present day wood guitar sound boards. Wood. as a material. exhibits peculiar internal hysteresis or damping characteristics which affect the rate of decay of sound vibrations in a wood sound board. Therefore, it follows that a guitar with a wood sound board thinner than the present day 0.10 inch thickness will result in a more brilliant tonal quality than has been achieved with prior art wood sound boards.  
  One aim of the present invention is to provide a sound board fabricated primarily of wood. or of an equivalent material having similar sound transmitting qualities, but which is thinner than prior art sound boards. and hence capable of producing these brilliant tonal qualities. and without sacrifice to the desired degree of damping characteristic of wood.  
  Another prior art guitar construction is illustrated in US Pat. No. 3.641862 issued to Rendell in I972. The guitar construction disclosed in the Rendell patent utilizes alternating layers of wood and aluminum foil all of which layers are glued together to provide a laminated product which is stated to be suitable for use as a sound board, but which is somewhat stiffer than the one piece wood sound board. However. such a sound board has been found to be somewhat heavier. and hence not responsive to the upper harmonics referred to previously. In short. the use of metal foil in reinforcing a wood laminate sound board has the effect of decreasing the stiffness to weight ratio. but the more desirable situation is to increase this ratio preferably in a wood guitar sound board thinner than has been possible heretofore.  
  Another object of the present invention then is to provide a guitar sound board having a stiffness to weight ratio greater than that possible from prior art laminated designs, and to achieve this result without sacrifice to the sound boards ability to exhibit the sound decay rates associated with present day all wood sound boards which have a thickness on the order of 0.10 to 0.12 inches.  
 SUMMARY OF THE INVENTION This invention resides in a sound board for a musical instrument of the guitar type, that is a stringed instrument wherein the vibrating strings are adapted to transmit sound energy to the sound board. which board comprises the front or top portion ofa generally hollow body of the type adapted to amplify and enhance the sounds originating from vibrating guitar strings. The preferred sound board is quite thin. preferably less than the thickness of conventional guitar sound boards of wood or the like. and has at least one medial ply or core of wood sandwiched between inner and outer plies of oriented graphite fibers embedded in a resin matrix. These graphite fibers provide inner and outer plies having a combined thickness which is only one-half to onefourth that of the overall thickness of the sound board. but nevertheless permit a sound board to be constructed primarily of wood with the result that the tonal qualities of the instrument are not adversely effected. but are instead rendered more brilliant than has been possible with prior art sound board constructions.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a guitar having a sound board constructed in accordance with the present invention.  
  FIG. 2 is a side elevational view of the FIG. 1 guitar illustrating the fundamental mode of vibration of the sound board in schematic fashion. The higher order harmonic modes, though not shown. are also necessarily present.  
  FIG. 3 is an enlarged fragmentary view showing the multiple ply construction of the sound board.  
 FIG. 4 is a view similar to FIG. 3 but showing an alternative embodiment.  
  FIG. 5 is a sectional view through the sound board. and also illustrates a bracing member of improved construction.  
 DETAILED DESCRIPTION Referring now more specifically to the drawing. a guitar incorporating the present invention will be described with reference to the several views in order to provide a background for the detailed description of the sound board itself. The guitar illustrated has a body portion which includes a top or sound board 10, a bowl 12 which is preferably made of fiberglass and resin molded to the three dimensionally curved shape. The .&#34;sound board to be described more fully hereinafter is attached to the periphery of the curved bowl 12 by a suitable epoxy resin or comparable adhesive, which material may be of the same type utilized in fabricating the bowl. and a plastic molding 14 further comprises the means for securing the sound board to the bowl. A neck 16 is attached to the upper end of the body and defines a fret board 18. A singular circular opening is located adjacent the lower end of the fret board. but two openings might be provided in accordance with the teaching of my above-identified copending application.  
 In said application two clef openings are located adjacent the periphery of the sound board to either side of the lower end portion of the finger board 18.  
  The guitar neck 16 extends upwardly from the top edge of the sound board, and is suitably attached to the curved bowl. with the upper end of the neck defining apparatus for tensioning the strings. Such apparatus is conventional. and need not be described in detail herein. However. it is noted that the strings extend downwardly across the upper surface of the fret board. and a plurality of frets permit the strings to be pressed against the frets while the instrument is played. The lower ends of the strings are attached to a bridge structure 24 directly mounted to the sound board at some distance below the lower edge of the sound opening, and preferably in the area of the middle of the sound board. As so constructed and arranged the vibrating strings transmit motion to the sound board thereby amplifying and resonating the sound produced by such strings as suggested in FIG. 2. In addition, a guitar of this type utilizes the volume of air provided in the bowl in a manner not unlike that of an organ pipe to pulse this quantity of air and to further amplify and enhance the sound produced by the instrument. .The actual sound produced by such a guitar is an integration of the contributions made over the entire audio spectrum by the guitar strings, the sound board, and the air volume defined by the guitar bowl portion, and the sound opening.  
  Turning next to a detailed description of the sound board 10, a preferred form for this element of the combination is depicted in FIG. 3, and comprises a composite wood and graphite fiber board consisting of a wood core 20 which preferably includes a single ply as shown in FIG. 3, but which may include one or more wood plies as shown in FIG. 4 at 2014 and 20b. The term medial ply&#34; as used in the appended claims is intended to cover one or more of such wood plies, and it is also noted that a composite material having properties comparable to wood might also be suitable. However, in order to achieve the results outlined hereinbelow. such a composite material would at least have to exhibit a strength and density similar to that of wood. This medial ply is not just a filler material. or spacer, which must contribute to the strength of the instrument, but this medial ply must also have the proper damping and high frequency response characteristics. Wood, though presently preferred. may be replaceable by material, not now known, which will yield such a balance. The advantages of wood. however, are well illustrated by the following observations. The thicker the wood sound board. the more damping is achieved. The converse of this statement leads to the corollary that the thinner the wood sound board. the greater the sustain&#34; of the instrument. Such sustain is sometimes desirable, as in a folk type guitar, whereas damping is necessary in a Flamenco&#39; type guitar. The harder the wood chosen. the more brilliant the tone. The orientation of the wood grain is also important in that the higher frequency response is favored as the grain direction gets closer to a lateral or perpendicular orientation with respect to the longitudinal axis of symmetry of the instrument.  
  In accordance with the present invention. and as shown in both FIG. 3 and FIG. 4, the sound boards 10 and I011 are both characterized by an inner and outer ply preferably of oriented graphite fibers embedded in a resinous matrix. In laying up these plies an epoxy resin may be coated on the surface of the wood core. or the resin matrix in the graphite fiber plies may be utilized to bond these plies to one another under appropriate conditions of temperature, pressure and time so as to set the epoxy resin and thereby permanently bond the plies to one another.  
  These inner and outer graphite fiber plies are of equal thickness, and each preferably has a thickness comprising approximately one-fourth the total thickness of the sound board. In its presently preferred form, each such graphite fiber ply 22 has a thickness of 0.005 inches, and the single ply wood .core of FIG. 3 a thickness of approximately 0.020 0.030 inches. Somewhat thinner graphite fiber ply thickness can be utilized, but the ratio of graphite fiber plies thickness to total thickness of the composite sound board is preferably characterized by a single-ply wood core thickness of at least two-thirds the total combined thickness of the composite board, and preferably is approximately threefourths thereof. In FIG. 4, the graphite fiber plies have a combined thickness of 0.010 inches. and the double ply wood core has a thickness of approximately 0.040  
 to 0.050 inches. Thus. the double ply wood core of FIG. 4 has a total thickness of four-fifths to five-sixths of the total combined thickness of all plies in the composite board.  
  The orientation of the fibers in the graphite plies is preferably such that they are parallel to the longitudinally extending guitar strings. This configuration has been found to improve the frequency response characteristics of the composite sound board. The importance attached to use of oriented graphite fiber plies with a wood core in a composite board for a guitar cannot be overemphasized. Such a guitar has superior sound qualities which may be due to the fact that the transmissibility of graphite plies is vastly superior to other materials. such as fiberglass for example. Thus. in spite of the fact that fiberglass has been proposed heretofore for use in guitar sound boards, even if the glass fibers are oriented longitudinally, the resulting instrument would not exhibit the same tonal brilliance made possible for the first time in a guitar with the composite graphite/wood sound board as disclosed herein.  
  Turning next to a more detailed description of the fabrication for the wood core, the preferred wood uti lized in this core comprises a birch veneer of approximately 0.020 0.030 inches in thickness, at least with a single such layer or veneer as used in the FIG. 3 version. It is important to the lateral stiffness of the sound board that the grain in this birch ply 20 run transversely as shown or that it at least run at a bias to the longitudinal direction followed by the graphite fibers or filaments. In larger instruments a plurality of plies 20a and 2012 can be used as shown in the FIG. 4 version. However, the grain should still be run generally transversely, or at least on a bias with respect to the longitudinal orientation mentioned in the previous paragraph for the graphite fibers in the inner and outer layers.  
  Thus. when a single such birch veneer ply, or layer 20 is utilized in the wood core. its grain may run generally perpendicularly with respect to such longitudinal direction, or at an acute angle with respect thereto. It should perhaps be noted that an angle of has been found to yield satisfactory results with a single ply wood core on the order of 0.020 inches in thickness. On the other hand, when two such layers of birch veneer are utilized in a wood core their respective grain orientations should he at acute angles to one another in the wood core, but both wood veneer plies 20 and 2012 in FIG. 4 are preferably oriented generally obliquely with respect to the longitudinal orientation for the graphite fibers in the inner and outer layers. The term generally obliquely&#34; has been adopted in the appended claims as a limitation to the bias of the wood grain in the sound boards core, that is. as meaning that this grain be oriented at an angle in the range between 90 and 30 with respect to the longitudinal direction defined previously.  
  It is noted that birch generally has a stiffness modulus of approximately l.5 X 10 pounds per the square inch, and a density on the order of 0.03 pounds per cubic inch. For purposes of comparison, the ratio of stiffness modulus to density provides a comparative figure of 53 X l0 for birch. It is noted that in a conventional guitar, that is in a sound board fabricated entirely of wood, this ratio dictates a conventional thickness for present day wood sound boards of approximately 0.10 to 0. l 2 inches. This thickness has been found to yield satisfactory results from the point of view of developing proper tonal qualities in a guitar which is rugged enough to be strung with conventional tension forces in its strings and thereby creating moments which tend to dish the sound board in the area of the sound opening. Still by way of comparison, the stiffness modulus for oriented graphite fiber material embedded in a suitable matrix is approximately 25 X 10 pounds per square inch. Since the average density of such material is approximately 0.057 pounds per cubic inch, the ratio of such stiffness modulus to density for such graphite fiber material provides a comparative number of over 400 X 10. as compared to the 53 X 10 characteristic of birch. As a consequence of the greater stiffness achieved in a composite graphite/wood sound board fabricated and constructed in accordance with the present invention it has been found that conventional strings can be provided with the usual tension and that the sound board described herein may be much thinner than the all wood board referred to previously and still react to the moments imposed on it. The thinner sound board permitted for the first time by practicing the present invention provides a guitar in which higher order harmonics are readily accommodated, and in which the sustain&#34; of the instrument can be significantly extended.  
  The advantages of adopting graphite fiber material in the construction of a guitar are not limited exclusively to use of this material on the sound board itself, and FIG. 5 shows a related adaption whereby this material is used on the exposed side of a wooden brace 30, which brace is cemented to the underside of the sound board itself, and more particularly to one of the oriented graphite fiber plies 22. The brace 30 may comprise either a longitudinally extending stringer, or a cross brace, or be oriented at some angle to the longitudinal direction. Most significantly. however. the brace 30 has a depth which is minimumized as a result of the layer 22a of graphite fiber material on its exposed side. As so constructed the brace 30, like the sound board itself. has a wood core. and has a reinforcing layer of graphite fibers on both its inside and its outside surface. As a result, considerably less wood can be used in the guitar bracing generally, a design advantage which also contributes to the basic result desired in this disclosure. namely to provide a sound board with a relatively high ratio of stiffness to weight. and hence one with more brilliant tonal qualities than has been possible with prior art sound board constructions.  
 I claim:  
  1. In a stringed musical instrument of the type having a hollow body portion and a neck with strings extending from the outer end of the neck to a bridge located on the body. the improvement comprising:  
 a. a sound board for said body portion. and supporting said bridge thereon,  
 b. said sound board including inner and outer plies which are made from graphite fibers embedded in a resin matrix, and at least one medial ply between said inner and outer plies made from a material having a modulus of elasticity and a density generally similar to that of wood.  
  2. A sound board of the type defined in claim 1 wherein the combined thickness of said inner and outer graphite fiber plies is no more than 0.02 inches.  
  3. A sound board of the type defined in claim 1 further characterized by at least one bracing member cemented to the inner graphite fiber ply. and a reinforcing layer of graphite fiber material adhered to at least the exposed side of said bracing member.  
  4. A sound board of the type defined in claim 1 wherein a majority of said graphite fibers are oriented in generally parallel relationship with respect to the instruments strings.  
  5. A sound board of the type defined in claim 4 wherein said inner and outer graphite fiber plies have a combined thickness which is less than one-half that of the combined thickness of all plies in the sounding board.  
  6. A sound board of the type defined in claim wherein said at least one medial ply is fabricated from a material with a modulus of approximately 1.5 X l0 pounds per square inch, said inner and outer graphite fiber plies having a modulus of at least times that of said medial ply.  
  7. A sound board of the type defined in claim 3 wherein said combined thickness of said plies in said sound board is approximately 0.05 inches. and wherein said combined thickness of said inner and outer graphite fiber plies is approximately 0.01 inches.  
 8. A sound board of the type defined in claim 7 .wherein said inner and outer plies are bonded to said LII LII  
 medial ply by a thin film of epoxy adhesive provided on both sides of said medial ply.  
 9. A sound board of the type defined in claim 7- bers in said inner and outer plies.