WO1999000787A1

WO1999000787A1 - Harmonic board in composite material

Info

Publication number: WO1999000787A1
Application number: PCT/IB1998/000946
Authority: WO
Inventors: Glauco Corbellini; Paolo Fazioli
Original assignee: Glauco Corbellini; Paolo Fazioli
Priority date: 1997-06-27
Filing date: 1998-06-18
Publication date: 1999-01-07
Also published as: IT1295156B1; ITUD970114A1; ITUD970114A0; AU7545598A

Abstract

Harmonic board in composite material used as a construction element in the production of resonance chambers for stringed musical instruments, structures for boats, furniture, etc. which are subjected to mechanical bending stresses, the board defining a substantially parallelepiped structure, or comparable thereto, consisting of an upper face on which the mechanical stress acts and which is subjected to compression, a lower face subjected to tension and an intermediate volume, the board consisting of a base panel (14) made of light material defining the shape of the board, the two counter-opposed faces of the panel (14) being connected by a plurality of transverse stiffening elements (16) housed in through apertures (13) uniformly distributed on a substantial part of the surface of the panel (14), the faces of the panel (14) being lined with a respective covering layer (18) made of a web of high resistance fibers impregnated with resin.

Description

"HARMONIC BOARD IN COMPOSITE MATERIAL"

* * * * *

FIELD OF APPLICATION This invention concerns a harmonic board made of composite material as set forth in the main claim.

The invention is applied, particularly though not exclusively, in the field of music, in order to make resonance chambers for stringed instruments so as to increase their sound efficiency and the effect of the sound obtained from the vibrating strings.

In the following description, for the sake of simplicity, we shall refer specifically to an application of the invention in the field of music, but the term harmonic board should be taken generally to mean a panel made of composite material constructed so as to incorporate the following characteristics :

- rigid and light,

- highly resistant to mechanical; heat and chemical stresses, - long-term stability,

- substantially unaffected by climatic variations.

Other uses of the board according to the invention, apart from the field of music for violins, guitars or particularly for pianos, include in aeronautical and naval construction, furnishing, componentry technology or otherwise.

STATE OF THE ART

When defining the characteristics of harmonic boards employed in the production of musical instruments, or generally in construction boards for example used to make boats, the most important characteristics to consider are rigidity, elasticity and lightness.

It is known that rigidity is a material's ability to resist specific mechanical stresses without changing its original shape, while elasticity is the material's ability to modify its original shape in order to resist a mechanical stress and then return to its original shape when the mechanical stress has ceased. The most important problems to solve when building a boat, a plane, a musical instrument with a soundbox, a piece of furniture, etc., are to ensure maximum resistance to bending stresses with the least possible weight, to ensure maximum long-term stability and finally to ensure that the device is substantially unaffected as outside climatic conditions vary.

In the case of a solid beam with a rectangular section suitable to bend at least partly, supported at the ends and subjected to an up-to-down bending stress at a central point, an intermediate plane is defined on the section of the beam wherein the reactions of the material are substantially nil.

The upper plane facing the stress is subjected to the maximum compression reaction, while the lower plane is subjected to the maximum tension reaction.

If the beam is made of a homogeneous material, the plane with the nil reaction is the median plane of the beam.

The beam's capacity to react to these stresses can be calculated with a formula which varies linearly with the length of the beam, or substantially with the distance between the supports, but is squared according to the thickness .

If, instead of a solid beam, we consider an equal volume with thin walls which is subjected to bending, the thickness of the lower plane is subjected to tension and, according to the elasticity of the material of which it is made, it can support a load substantially until the yield point is reached . The thickness of the upper plane is subjected to compression and therefore, if no connection is created between the two opposite planes of the beam, this compression generates a deformation of the beam. On the contrary, if we hypothesise to connect the upper plane and the lower plane by means of pins or struts, considering a homogeneous distribution of the pins along the length of the beam which is subjected to bending, we obtain a resistance moment which approaches that calculated for the beam with the homogeneous section, and is squared as the thickness varies , with a much lower weight .

This means that in a beam with a homogeneous section, a part of the material inside the thickness of the section does not give any contribution to the resistance moment, although it does contribute to the weight of the beam, with all the negative results which an increase in weight without any increase in resistance involves, especially in the applications mentioned above.

With reference to the specific field of harmonic chambers for stringed instruments, harmonic boards are used to transmit and propagate the sound produced by the vibration of the strings and to increase the sonority of the instrumen .

In pianos for example, the harmonic boards are formed by strips of wood glued one next to the other to achieve a continuous panel which, on the side opposite the one where the strings are applied, is reinforced and stiffened by transverse reinforcement elements known as chains.

On the face opposite the one where the chains are applied, shaped strips or staples are arranged; the strings, which are stretched between the sides of a frame, have a point of contact on these strips which is under tension; the frame is a separate structure from the harmonic board. The resting point of the strings on the staples is on a more outerly plane of the harmonic board than the plane containing the points attaching the ends of the strings to the frame; each string therefore exerts a considerable continuous pressure on the board even when it is not excited and made to vibrate, thus exerting on the board a constant bending moment balanced by the resistant moment of the material which the board is made of.

Harmonic boards in pianos behave substantially like a diaphragm made to vibrate by the strings when they are struck by the percussion hammers associated with the keys when the pianist plays.

The result of the action of the harmonic board, which conditions the effect and duration of the sound transmitted by the vibration of the strings, is substantially a function, given the same dimensions, of the modulus of elasticity, the moment of inertia and the weight of the section of the harmonic board.

If it is to have a good effect, the harmonic board must be light so that the string which makes it vibrate will use less energy, but it must also have a good rigidity so as to contain the constant bending stress generated by the strings which are or are not made to vibrate, since the thrusts exerted by the strings are capable of reaching values of around 250 kilograms.

Therefore, it is necessary to search for the optimum compromise between lightness and rigidity against bending, seeking embodiments and materials which can contribute over the whole section to the beam's resistant moment, in such a way as to obtain a greater rigidity with the same weight.

Another disadvantage which has to be overcome is that the harmonic board needs to be pre-loaded, and at present this can only be done by qualified and specialised personnel. Another disadvantage of harmonic boards known to the state of the art is that the sound characteristics they possess are closely connected to the material of which they are made; in the case of solid wood it is therefore not possible to produce harmonic boards which can be standardised or adapted to the specific application.

Moreover, materials used at present tend to vary their characteristics with the passing of time, and are highly influenced by outside climatic conditions, especially by temperature and humidity.

The present Applicants have designed and embodied this invention to overcome all these shortcomings and to obtain further advantages .

DISCLOSURE OF THE INVENTION The invention is set forth and characterised in the main claim, while the dependent claims describe variants of the idea of the main embodiment.

The purpose of the invention is to provide a harmonic board made of composite material which will solve in an optimum manner the compromise between lightness and resistance to mechanical, heat and chemical stresses, and will be suitable to maintain its desired characteristics in the long term.

Another purpose of the invention is to achieve a harmonic board wherein the mechanical characteristics - particularly rigidity against bending - can be varied, in a relatively rapid and easy way, from one point of the board to another, achieving specific zones or strips with different resistance according to the type of application. According to the invention, the harmonic board is substantially parallelepiped in shape, or can be compared thereto, and consists of a box-like structure whose wide faces are defined by two sheets, respectively upper and lower, which enclose an inner space.

According to a variant, the board consists of a single panel with a defined thickness .

According to a further variant, the board consists of two sheets enclosing a light filling material, for example expanded plastic.

According to the application, the sheets or panel can be substantially plane, or they may have a desired curve, for example like an arc, or in any case they can be pre-bendable so as to achieve a shape which is defined and stable in the long term.

The sheets or panel are made, according to the invention, of a material with a very low specific weight, less than 0.15 grams per cm³. The material used for the sheets or panel, in a first embodiment, is expanded and/or synthetic resin, possibly impregnated or treated.

According to a variant, a suitable light wood can be used, such as plywood or balsa or some other similar material. On at least part of the surface of the sheets or panel a plurality of through apertures are made, which pass through the inner space or volume orthogonally from one side to the other, and emerge on the surface of the sheets or panel.

The through apertures are advantageously arranged aligned so as to define a plurality of parallel rows arranged according to a preferential pattern.

The through apertures are used to insert mating hollow elements of structural stiffening, for example in the shape of tubes or similar, which function as pins connecting the two counter-opposed surfaces of the harmonic board.

The material and the density of the pins, that is to say, their reciprocal distance, are a function of the desired structural rigidity which the harmonic board is to be equipped with, and can vary from zone to zone in the same board.

In a piano, for example, the density will vary according to the type of spectrum of frequencies of the specific strings, whether they be the strings for high or low notes.

Inside the hollow stiffening elements, reinforcement filaments are inserted which are made to pass alternately from one face of the board to the other, connecting adjacent apertures with each other. According to a variant, the connections made with the reinforcement filaments can affect apertures which are at some distance from each other, according to the different level of rigidity to be obtained in the harmonic board; therefore every filament may skip one or more apertures according to the result to be obtained.

According to a variant, adjacent through apertures are connected with each other by grooves or channels made on the faces of the harmonic board, which serve to house the reinforcement filaments below the plane of the board. According to the invention, the faces of the harmonic board are then lined with a layer made of a web of fibers with high resistance and extremely low weight, suitable to make the structure solid and stiff; the fibers are impregnated with resin. One embodiment of the invention provides to use carbon fibers, which have the characteristic that they do not stretch, and therefore they do not consume energy even when subjected to bending or drawing stresses.

According to a variant, the layer of covering is a multi- layer consisting of layers of fibers, each of which has its own orientation or its own value of resistance.

The reinforcement filaments and the layers of fibers together create a stiff reticular structure which solidly connects the upper plane to the lower plane, imprisoning the base material which constitutes the two faces of the harmonic board and forcing it to collaborate in resisting the stresses. To be more exact, on the face opposite the face where the mechanical stresses are applied, that is to say, the face subjected to tension, the reinforcement filaments and the layer of fibers distributes the tension forces over the whole surface which is therefore affected in its entirety by the board's resistance moment.

Moreover, the dimensional stability of the most stressed surfaces is guaranteed due to the inclusion of the multi- connections consisting of the transverse stiffening pins. According to the application, that is to say, the parameters of rigidity and lightness required, it is possible to act on the thickness of the planes of the light base material, on the height and number of the stiffening pins, on the density of the reinforcement filaments inserted for each of the pins, on the density and thickness of the covering layers.

All this gives extreme versatility and flexibility in designing the harmonic board according to the invention, ensuring in every situation that it is adaptable to the requirements and that the required parameters will be obtained.

These characteristics can also be repeated, and therefore it is possible to plan and make boards which are perfectly standardised, something which is not at all possible with traditional wooden boards . ILLUSTRATION OF THE DRAWINGS

The attached Figures are given as a non-restrictive example and show a preferential embodiment of the invention as follows: Fig. 1 shows an exploded view of an application of the harmonic board according to the invention to a grand piano; Fig. 2 shows a part cut-away plane view of the harmonic board shown in Fig. 1;

Fig. 3 shows a detail, exploded and partly in section, of the board shown in Fig. 2 ; Fig. 4 shows a section from A to A of Fig. 2; Fig. 5 shows a partly exploded view of Fig. 4. DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an exploded view of a grand piano 11 on which a harmonic board 10 according to the invention is mounted.

The harmonic board 10 is shaped like a harp, it is equipped on one side with staples 12 to support the strings and is associated in a traditional manner with the soundbox of the piano 11.

The harmonic board 10 is made from a base panel 14 made of a material, for example expanded resin, of synthetic material, light wood such as plywood or balsa, or some other material with similar characteristics, with a specific weight of less than 0.15 grams per cm³.

The panel 14 includes, on at least part of its surface, a plurality of through apertures 13 (Fig. 2) which open onto the respective faces of the panel 14. In this case, the through apertures 13 are circular and made over the whole panel 14 except for a perimeter band used to attach the harmonic board 10 to the soundbox of the piano 11.

The through apertures 13 are arranged aligned so as to achieve parallel rows according to the desired pattern.

Transverse stiffening elements 16, in this case shaped like tubes or little pins and made of stiff resin, are inserted into the through apertures 13. In this case, each through aperture 13 is connected with the adjacent through aperture 13 by means of grooves 15 made on the surface of both faces of the panel 14.

In this case, the grooves 15 are orthogonal to each other (Fig. 3); according to a variant which is not shown here, there are grooves 15 positioned obliquely to each other which connect the apertures 13 diagonally.

The grooves 15 and the transverse stiffening elements 16 act as a housing seating for a plurality of reinforcement threads or filaments 17, which pass alternately from one side of the panel 14 to the other.

In this case, the reinforcement filaments 17 intersect each other in correspondence with the grooves 15 and achieve on both faces of the panel 14 a reticular structure of the desired pattern, in this case a square mesh, which has the function of stiffening the panel 14 in both a lengthwise and transverse direction.

To be more exact, the function of the reticular structure is to connect the two opposite surfaces of the panel 14 and make them solid, creating a monolithic structure wherein all the material collaborates in resisting the bending stresses, such as those imparted by the strings of the piano when they are made to vibrate .

According to the invention, above the reticular structure consisting of the reinforcement filaments 17 respective covering layers 18 are applied, which are made of light, highly resistant fiber material.

According to a first embodiment, the covering layer 18 is associated with the panel 14 by means of resins which can be polymerised.

According to a variant, the covering layers 18 are made of carbon fiber.

According to another variant, at least one covering layer 18 is achieved by super-imposing, for example by means of polymerisable resins, several layers, each of which has a different orientation of the fibers and/or a different value of resistance. Although the harmonic board 10 according to the invention is extremely low in weight, it is extremely resistant to outside mechanical stresses due to the fact that the light material of the panel 14 is enclosed inside a rigid, monolithic structure, with high resistance. Thanks to the application of the covering layers 18 consisting of resistant and super-resistant fibers on the faces of the board 10 which are most exposed to stress, the most opportune material works in the most efficacious manner . Moreover, the dimensional stability, which is long term and unaffected by changes in outside climatic or environmental conditions, is ensured by the inclusion of multiple connections established by the transverse stiffening elements 16 and the reinforcement filaments 17. Maximum versatility is guaranteed for the board 10 by the following factors: the fact that it is possible to control the arrangement of the fibers in the covering layers 18; the number and resistance of the reinforcement filaments 17; the thickness of the panel 14; the density of the through apertures 13 and therefore the density of the transverse stiffening elements 16. These same factors make it possible to achieve zones with differing resistance on the same board 10, or to achieve boards specifically designed for particular uses and applications. In the particular use as a soundbox for a stringed musical instrument, and specifically for a piano, this versatility makes it possible to control all the desired and sought after sound effects, and also to differentiate characteristics according to the spectrum of frequencies associated with the specific zone or band of the board 10.

Claims

1 - Harmonic board in composite material used as a construction element in the production of resonance chambers for stringed musical instruments, structures for boats, furniture, etc. which are subjected to mechanical bending stresses, the board defining a substantially parallelepiped structure, or comparable thereto, consisting of an upper face on which the mechanical stress acts and which is subjected to compression, a lower face subjected to tension and an intermediate volume, the board being characterised in that it consists of a base panel (14) made of light material defining the shape of the board, the two counter-opposed faces of the panel (14) being connected by a plurality of transverse stiffening elements (16) housed in through apertures (13) uniformly distributed on a substantial part of the surface of the panel (14) , the faces of the panel (14) being lined with a respective covering layer (18) made of high resistance fibers impregnated with resin.

2 - Harmonic board as in Claim 1, which includes lengthwise and transverse stiffening elements consisting of reinforcement filaments (17) housed in grooves (15) made on the counter-opposed faces of the panel (14) , the reinforcement filaments (17) passing alternately from side to side of the harmonic board (10) through the apertures (13).

3 - Harmonic board as in Claim 2, wherein the grooves (15) connect the adjacent through apertures (13) with each other.

4 - Harmonic board as in any claim hereinbefore, wherein the through apertures (13) are arranged along a plurality of parallel rows.

5 - Harmonic board as in any claim hereinbefore, wherein the panel (14) is made of a material with a specific weight of less than 0.15 grams per cm³. 6 - Harmonic board as in any claim hereinbefore, wherein the panel (14) is made of expanded resin and/or synthetic material .

7 - Harmonic board as in any Claim from 1 to 5 inclusive, wherein the panel (14) is made of light wood, for example balsa.

8 - Harmonic board as in any claim hereinbefore, wherein the panel (14) is curved or pre-bendable according to a desired shape . 9 - Harmonic board as in any claim hereinbefore, wherein the panel (14) consists of two sheets defining the outer faces and enclosing an inner space.

10 - Harmonic board as in Claim 9, wherein the inner space is filled with light material, for example expanded material.

11 - Harmonic board as in any claim hereinbefore, wherein the covering layer (18) is made of carbon fiber or high resistance light fibers impregnated with resin.

12 - Harmonic board as in any claim hereinbefore, wherein the covering layer (18) is made as a multi-layer with layers having a different orientation of the fibers and/or different resistance parameters.

13 - Harmonic board as in any claim hereinbefore, wherein the transverse stiffening elements (16) are made of rigid resin impregnating the reinforcement filaments (17).

14 - Harmonic board as in any claim hereinbefore, which includes zones or bands defining different resistance parameters and/or rigidity.

15 - Harmonic board as in any claim hereinbefore, wherein the resistance and/or rigidity of the board can be made different by acting on at least one of the following parameters :

- material and/or thickness of the panel (14) ; - density of the through apertures (13) and consequently of the transverse stiffening elements (16);

- thickness and/or constitution of the covering layer (18) ;

- material and/or density of the reinforcement filaments