FR2598843A1 - COMPOSITE STRUCTURE FOR STRINGED INSTRUMENT HARMONY TABLE AND ITS MANUFACTURING PROCESS - Google Patents

Abstract

THE INVENTION RELATES TO A COMPOSITE STRUCTURE FOR A HARMONY TABLE OF STRINGED INSTRUMENTS WHICH INCLUDES A SKELETON 1 CONSISTING OF A THIN WALL OF LONG FIBERS BOUND BY A POLYMERIZED RESIN BY A 2 OR TWO 2, 3 LAYERS OF A TRIM MATERIAL WITHOUT MECHANICAL RESISTANCE FUNCTION, BONDING TO SKELETON 1 BY POLYMERIZED RESIN.

Description

The present invention relates to a soundboard material for

stringed instruments (plucked, rubbed

or minted) and its manufacturing process.

The emission of a sound by means of a vibrating string comes from the vibrations of a plate (soundboard) which constitutes an amplifier associated with a resonance chamber, from the vibrations of the string which are transmitted to it by a link mechanical (such as for example the bridge) which also enriches the vibration of a large number of harmonics. 10 The quality of the sound thus obtained therefore depends essentially on the material used for the manufacture of this resonance table as well as on its installation on the instrument with the

various reinforcements necessary for its good mechanical strength.

In violin making, this material is made up of certain types of wood resulting from an experience of several centuries during which a natural selection and an optimization of the shapes, thicknesses, densities, ...

all the characteristics specific to the emission of a sound possessing the qualities sought by the luthier have been empirically effected.

Despite the experience accumulated and transmitted over time, the making of a high-end stringed instrument calls on all the know-how of luthiers who, from a selected species and roughly defined shapes, 25 are to even to intervene specifically to take into account

hazards due to the lack of consistency in the characteristics of the base material, wood.

The present invention is the result of several years of work and experimentation with the aim of determining and developing a substitute for violin wood which has the same properties as this wood from the point of view of the production of sounds. with as a secondary concern to present the same aspect as the wood and approximately the same aptitudes to be worked by a luthier to allow him to "sign" the instrument by giving it the timbre which characterizes it, while overcoming the disadvantages of wood which are essentially the absence of constancy in its properties from one piece to another and the variation in its characteristics over time or as a function in particular of the humidity and temperature of the surrounding atmosphere. This research to imitate as closely as possible the lutherie wood first consisted of an in-depth study of certain lutherie woods (the spruce of the mountainous regions of Europe and the red cedar of Canada-western red cedar-) to determine the determining parameters for the timbre and establish relationships between the measured properties and the

acoustic result.

All the results of the measurements carried out, on the mechanical level (directional moduli of elasticities 15 and corresponding damping coefficients) on the level of the structure of the material (density, heterogeneity, dimensions of the main elements - wood tracheids - ) in terms of acoustics (spectral analysis of a sound produced by an impact on the material) as well as with regard to the speed of propagation of sounds in the three directions, has been treated by the method of factorial correspondence analysis. From this work, relations between the different parameters for each of the species studied were identified, which made it possible to understand certain behaviors of wood. We were finally able to establish links between these parameters

and luthiers' criteria such as these are expressed subjectively to characterize the timbre of an instrument.

The merit of this research was to reveal a certain number of objective characteristics that a material must exhibit in order to have the expected acoustic qualities, and even to predict the timbre of an instrument which has been possible.

determine the significant criteria.

The invention is therefore an extension of this research consisting in creating a material having the criteria sought to verify that from an acoustic point of view the qualities of the material were indeed comparable to those.

of natural wood with the same criteria.

To overcome the drawbacks of wood, some instrument manufacturers have proposed various means ranging from the treatment of the wood itself to the creation of a completely synthetic material. Thus, document DE-C-2 055 164 describes a process for impregnating

wood tracheids by synthetic polymers.

Document US-A-4 348 933 relates to a composite material in which a central layer of wood is covered on both sides with a layer of layers of fibers.

of carbon. The direction of the fibers is crossed. Finally, the document CH-A-652 229 relates to a composite material consisting of a central metal core (steel) covered by two single or multi-layered wood plates.

It is certain that none of these materials could be suitable because each one of them does not have all the characteristics which lets hope for a behavior close to that of a wood of violin making. A material similar to that according to the American document has been placed on the market in the form of a guitar soundboard and has not met with any commercial success in the field of classical guitar, so much the

sound obtained was different from the traditional sound.

The present invention therefore relates to a composite structure for a soundboard of stringed instruments which comprises a skeleton formed by at least one thin wall of oriented long mineral or synthetic fibers giving the structure its mechanical properties, covered over at least one of its faces of a lining material having a density significantly lower than that of the thin fiber wall, and having no characteristics

own mechanics other than those necessary for its cohesion.

This structure has a first advantage in that it has a mascroscopic heterogeneity like that existing for wood. Indeed, wood is an oriented fiber material which therefore has a longitudinal modulus of elasticity very different from the transverse modulus of elasticity. However, these two moduli of elasticity are parameters that the aforementioned research has made it possible to identify as having a significant impact on the acoustic behavior of wood. The skeleton produced by one or more layers of long fibers allows an approach of the mechanical behavior

wood.

The presence, in the structure according to the invention, of a filling material also offers the advantage of obtaining a product of overall density comparable to that of wood, correcting for too high a density of the skeleton, another influencing factor. acoustic behavior. In other words, by varying the relative thickness of the backbone and of the packing material, the density of the composite structure can be adjusted to values.

which we know that they correspond to the desired acoustic effect.

The third advantage of the structure according to the invention lies in the possibility of locally affecting the thickness of the packing material by machining. Thus, from a rough draft, a luthier can make alterations to the soundboard according to his know-how and the

stamp he intends to obtain.

In addition to these advantages, the structure of the invention has a perfectly reproducible character which lends itself to industrialization of high-end violin making as well as to artisanal violin making. It also retains great stability over time by not being at all

sensitive to temperature and humidity conditions to which the instrument is subjected.

In the case of thin soundboards of low mass (100 to 150 grams) such as they are found in the lute or the guitar, we will choose a carbon fiber skeleton in the form of layers preimpregnated with polymerizable resin. Taking into account the thickness of a web available on the market, it is possible to juxtapose several layers to produce this skeleton to take into account the

mechanical resistance that the table must offer.

It may be advantageous to cross one or the other of these plies with respect to the remainder in order to play on the transverse or radial modulus of elasticity of the skeleton and to improve its resistance in this direction. It has in fact often been observed that lute tops split along the wood fibers under the effect in particular of variations in the degree of dryness of the atmosphere and in temperature. A skeleton with layers of crossed fibers overcomes this drawback. The crossing angle will be determined according to the ratio of the two moduli of elasticity that should be obtained.

for the mechanical and acoustic characteristics of the material.

For thicker soundboards, the question of mass being less acute, other types of fibers can be chosen (glass, rock, metal, or even synthetic aramid, polyamide or natural polyesters of the type. cotton or the like. It may also be advantageous to constitute the skeleton as being itself composite, that is to say comprising two thin walls made of layers of oriented fibers between which a low density filling material has been placed. mechanical characteristics of such a "core" are, like a box, comparable to those of a solid structure in a single material

with however a much lower mass.

In most cases, the skeleton will be sandwiched between two plates of low density packing material, but one can imagine that some sounds

support a single plate of trim on the skeleton.

This material will preferably be veneer, obtained by peeling, which has the advantage of giving the composite structure of the invention the same appearance as the instruments from a traditional violin making, while allowing easy working of the thicknesses. . It is not, however, departing from the scope of the invention to envisage a filling material other than synthetic, based on a mat of short fibers embedded in a resin or an adhesive, provided that the density of this material remains compatible with the one that the structure must ultimately present. The method of manufacturing such a structure is relatively simple and easy to master since it consists of superposing the different layers and causing the resin polymerization of the preimpregnated fiber webs which not only ensures the cohesion of the backbone, but also bonding the trim layers to the backbone. This process is called "co-firing" in the case of a thermosetting resin and is carried out between the two parts of a mold closed under pressure on the product with the addition of heat. However, it may be possible to use thermoplastic resins, the process then being dictated by the polymerization or curing cycle of

these resins.

Of course, the construction of the structure can be shaped, that is to say that the different layers of product are cut to the outline of the table to be produced and are linked together in a mold having the desired curve. Finally, it is possible, on the one hand, to create a prestressing 25 of the skeleton by acting on the tension of the fibers which constitute it and on the other hand to put in place at least some of the bracing of the soundboard in the form of inserts added in the mold and connected to the surface layers

of the structure by draping a prepreg sheet.

The invention will be better understood during the

description given below by way of purely indicative and non-limiting example which will make it possible to identify the advantages and secondary characteristics thereof.

Reference will be made to the accompanying drawings in which: - Figure 1 schematically illustrates an embodiment of the structure according to the invention, - Figures 2 and 3 are diagrams of variant embodiments of Figure 1, - Figure 4 partially illustrates a mold

incorporating dams associated with the structure according to the invention.

Referring to Figure 1, we see a composite structure according to the invention comprising a central skeleton 1 made of carbon fibers coated with an epoxy bonding resin and all oriented parallel to a longitudinal direction L of the structure. This central layer is plated on either side of a light material, for example unrolled wooden sheets 2 and 3. The assembly of these three constituent parts was carried out by co-firing, that is to say heat and pressure polymerization of the coating resin of the fibers forming the bond between the fibers and the surface layers to

the middle layer.

We can cite, as an example, the realization of a sample intended for a lute table which, assembled and tested by musicians, blind, was confused, from the point of view

acoustic with a traditional spruce top soundboard.

The central skeleton 1 consisted of two superposed webs of pre-impregnated commercial carbon fibers 0.25 millimeters thick (each web having a thickness of 0.125 millimeters). The orientation L of the fibers, the top mounted on the lute, is parallel to the neck of the instrument. Layers 2 and 3 are formed by sheets of wood (spruce) unrolled with a thickness of 0.6 millimeters,

the total thickness of the table therefore being 1.45 millimeters.

The measurements of the physical and mechanical characteristics of the material revealed: - a longitudinal modulus of elasticity (in the direction L 35 of the fibers) equal to 11.6 GPa, - a transverse modulus of elasticity (in the direction perpendicular to the fibers) ) equal to 0.94 GPa,

- an average density of 0.56.

As an indication, it will be noted that the spruce has the respective values of these parameters: 14.2 GPa - 1.1 GPa - 0.423 In accordance with the conclusions of the theoretical study recalled in the preamble above, the composite material of The invention was to exhibit acoustic qualities similar to those of spruce, all other things being equal, which was confirmed by the musical test of the experimental lute. In Figure 2 there is shown a skeleton 4 of the structure according to the invention in which the layers of fibers 5 and 6 are crossed, at an angle A determined experimentally as a function of the desired ratio between the two moduli of longitudinal elasticity and transverse. It has in fact been observed that for certain instruments such as the violin, the soundboard has greater transverse stiffness than that of a lute or guitar top. The crossing of the layers makes it possible to take this observation into account and to provide a composite structure adapted to this or these types of instrument (violins, cellos, violas, double basses, etc.). FIG. 3 is a diagram showing that the skeleton 7 of the structure can itself be composite and formed of two thin walls 8, 9 in layers of fibers, crossed or not, between which is interposed a layer 7a of low density material (wood for example). The outer layers 10 and 11 of trim, also made of wood, play the same role as those described above. Thus the skeleton, placed in neutral fiber of the structure, can have sufficient stiffness to withstand the mechanical forces to which it is subjected without however reaching a mass which would be prohibitive for obtaining a low overall density. This type of material could be suitable for harpsichord or piano soundboards. It is also possible to provide a greater alternation of layers for the composition of the skeleton in order to satisfy the mechanical requirements submitted to the table while respecting the requirements.

mass constraints.

Finally, the diagram of FIG. 4 shows a two-part mold 12 and 13 whose indentations 12a and 13a define either two planes or two surfaces whose shape corresponds to the desired shape of the soundboard. It will also be noted that the indentation 13a corresponding to the internal face of the table, for an instrument with a closed case of reasoning (lute, guitar, violin, etc.), comprises housings 13b for the installation of stiffening elements. 14 (called dams) and 15 a peripheral draping 15, in the form of a sheet of fibers preimpregnated with resin, binding the dam to the table itself during the polymerization of the resin. Depending on the nature of the resin used in the structure of the invention with its polymerization cycle, the parameters, pressure, heat and time, under which the mold will be kept closed to obtain the final product will be determined. It is also possible to provide for placing in the mold the constituent elements of the material or of the table under a certain constraint in order to locally modify in the free state the shape or the elastic and vibratory characteristics of the table, or to take into account. takes into account the stresses and strains that will be

imposed during assembly on other parts of the instrument.

The composite structure according to the invention can thus constitute a substitute for violin wood having similar acoustic characteristics while having very significantly improved durability and stability. This product can be worked by artisan luthiers in a manner equivalent to that of wood and these

instrument makers can apply all their knowledge

to do. It advantageously lends itself to a much more advanced industrialization of the manufacture of instruments since by its very design and its production principle, a soundboard can be completely produced at the same time as the structure itself. Finally, it allows the marketing of products with new controlled acoustic characteristics which are not found in any of the known natural products, due to the numerous variations in shape, constant thicknesses or not, size and constitution of which it can. be the object and thus generate the creation of new

sound stamps or even new instruments.

The invention finds an interesting application

in the field of string instrument making.

Claims (13)

1. Composite structure for a soundboard of stringed instruments, characterized in that it comprises a skeleton (1,4,7) formed by at least one thin wall (1,4,8,9) of long fibers oriented giving the structure its mechanical properties, covered on at least one of its faces with a lining material (2, 3, 10, 11) with a density significantly lower than that of the thin fiber wall, and having no specific mechanical characteristics other than those necessary for its cohesion. 2. Structure according to claim 1 characterized in that the skeleton (1) consists of a sheet of fibers (1) long parallel connected to each other by a polymerized resin. 3. Structure according to claim 1 characterized in that the skeleton (4) is formed by the superposition of at least two layers (5,6) of long parallel fibers, connected together by a polymerized resin, the two layers being adjacent crossed with respect to each other at a predetermined angle (A) and joined by polymerization of the resin coating the fibers. 4. Structure according to claim 1 characterized in that the skeleton is constituted by at least two layers (8,9) of parallel long fibers, connected to each other by a polymerized resin and separated by an intermediate layer (7a) of a material. of lower density than that fibers. 5. Structure according to claim 4 characterized in that the layers (8, 9) are crossed with each other. 6. Structure according to any one of the preceding claims, characterized in that the skeleton (1,7) is covered on both sides by a lining material. 7. Structure according to any one of claims 1 to 5 characterized in that the skeleton (1,7) is covered on one of its faces intended to form the face. outside of the instrument, with a packing material. 8. Structure according to any one of the preceding claims characterized in that the trim material (2,3,10,11) is a sheet of wood obtained by unwinding. 9. A method of producing a composite structure according to any one of the preceding claims, characterized in that it consists in superposing the various layers which constitute it, oriented in an appropriate manner, in a mold (12,13), layers of fibers being coated of resin not yet polymerized, and to proceed with the polymerization of the resin by closing the mold under pressure with a possible addition of heat, the resin coating the fibers constituting the binder of the various layers between them. 15 10. A method according to claim 9 characterized in that, prior to the preceding phases, each layer is cut along a contour corresponding to the contour of the soundboard to achieve. 11. The method of claim 10 characterized in that one uses a mold (12,13) curved to impart the desired shape at the soundboard. 12. A method according to any one of claims 10 and 11 characterized in that the layers of fibers are placed under prestressing in the mold. 13. A method according to any one of claims 10 to 12 characterized in that one places in the mold (12,13) of the inserts (14) joined to the structure by draping a sheet (15) of binding impregnated fibers to constitute the dams of the soundboard. 30