WO2010148431A1

WO2010148431A1 - Soundboard for stringed musical instruments

Info

Publication number: WO2010148431A1
Application number: PCT/AU2010/000771
Authority: WO
Inventors: Bradley R. Clark; Alex R. Edwards; Daniel L. Jack
Original assignee: Australian Native Musical Instruments Pty Ltd
Priority date: 2009-06-24
Filing date: 2010-06-22
Publication date: 2010-12-29

Abstract

A soundboard for a stringed musical instrument, such as an acoustic guitar. The soundboard has a generally planar form with a longitudinal axis defined between a heel end, at which a neck of the instrument would be attached, and a body block end opposite the heel end. The soundboard has a primary structural zone extending generally along the longitudinal axis of the soundboard and having a width (measured across the soundboard laterally to the longitudinal axis) which is substantially greater than its thickness (measured into the plane of the soundboard). Secondary zones, laterally adjacent the primary structural zone, have a material thickness which is less than that of the primary structural zone. A method of designing a soundboard for a stringed musical instrument is also disclosed.

Description

SOUNDBOARD FOR STRINGED MUSICAL INSTRUMENTS

FIELD OF THE INVENTION

The present invention relates to musical instruments and in particular to a soundboard construction for a stringed musical instrument such as an acoustic guitar. It will therefore be convenient to describe the soundboard in relation to that example application. It should be understood however that the soundboard is equally suited to other stringed instruments. BACKGROUND OF THE INVENTION Acoustic guitar fundamentals

There are two general types of contemporary 'acoustic guitar', those made with nylon (or gut) strings and those made with steel strings. The overall geometry and functionality of each remains similar, and so the general 'physics' or structural dynamics of each share common elements: - a forcing function, or input, from striking of the strings

^■ coupling of the strings to the soundboard (or soundbox) by means of a bridge and saddle assembly at one end, and a nut at the other end, causing vibration and resonance of the guitar

^■ fluid-structure coupling of the guitar and the surrounding air creating audible output

The perceived quality of the sound produced by a guitar is dependent on the modal characteristics of the system, which are largely influenced by:

^■ the materials used in construction (predominantly timber)

^■ the overall geometry and connectivity of the elements ■ the dimensions, distribution of mass, and orientation of the guitar's panels and their stiffening/strengthening elements

A typical example of a prior art acoustic guitar is shown in Figure 1. This figure shows the main features of such a guitar, and Table 1 (appearing at the end of this description) sets out the nomenclature used throughout this specification to refer to those features. Service loads on the guitar A guitar is subjected to a series of loads during its service life;

^■ internal residual stresses resulting from the construction processes ^■ sustained load from applied string tension

^■ transient load from vibration of the strings when played

^■ external loads and reactions exerted by the player in service

^■ exposure to changes in ambient conditions ■ accidental external loads from transit and handling

Each load set results in a different set of stresses and displacements. The structure of the guitar must be suitably robust to withstand 'reasonable exposure' to the above load sets without compromising the payability, tuning stability or integrity of the instrument. The string load set is of primary importance to the design of the guitar, and the magnitude of these forces is readily calculated by a function of the unit weight of the string, the scale length of the guitar and the frequency each string is tuned to.

The string load set is applied at an eccentricity to the soundboard via the bridge and saddle assembly. So, the strings exert both an in-plane force on the soundboard, as well as an out-of-plane rotation. The string load set is sustained over the life of the instrument. Materials

Timber is both orthotropic, meaning it has different properties in different orthogonal directions, and hygroscopic, meaning it will absorb or lose moisture in relation to the temperature and humidity of the surrounding air.

The strength, elastic modulus, shear modulus and Poisson's ratio are different in the tangential, radial and longitudinal direction.

The shrinkage/swelling due to variation in timber moisture content is also substantially different in the three orthogonal directions. Review of traditional soundboard design The X-brace pattern

Guitar designs and structures have varied little since the mid 1800s when The Martin Guitar Company produced a 'steel string' acoustic guitar, as opposed to the existing gut strung 'classical guitars'.

Martin's goal was apparently to use steel strings to create a louder instrument more akin to the relatively loud steel strung banjo. Banjos were a favoured instrument at the time for reason of their loudness or acoustic output. Martin braced the guitar soundboard in a pattern that has become known as the 'X-brace' pattern. This pattern has become the industry standard, and there are few substantial variations.

Traditionally, X-brace soundboards have uniform thickness of between 2.8 to 3.2 mm, and are usually braced by 5 main braces and 4 smaller struts or braces. The braces typically have dimensions between 7-1 Omm in thickness and

10-18 mm in height. The braces are generally made of Spruce or a similar species of wood and are higher than they are wide, the lesser dimension being adhered to the underside of the instrument's soundboard. A typical example of a prior art X-brace soundboard is shown in Figure 2.

Traditional departures in detailed design and construction between nylon and steel string guitars result from the difference in applied forces when a guitar of given scale length is fitted with steel strings or nylon strings. The load applied to a guitar with 'regular gauge' steel strings is generally about 75kg, about double the load compared with fitment of 'normal tension' nylon strings.

The most significant detailed design differences between steel string guitars and nylon string guitars have traditionally been the soundboard bracing pattern, neck reinforcement and heel design.

Recent research and development undertaken by the present inventors has focussed on optimising the structure and performance of steel string guitars.

However, the structural principles, optimisation routines and resulting soundboard structures described below are also valid for application to nylon string guitars, and other stringed musical instruments.

X-brace performance observations Time-dependent, permanent deformation can be observed in many traditional X-brace soundboards, particularly behind the bridge where 'bellying' is often observed to varying degrees along with rotation of the bridge assembly.

Due to the orientation of the bracing elements with respect to the soundboard with the X-brace pattern, there is also a tendency for curvature to be induced when the guitar is subject to changing ambient conditions as the soundboard's dimensional change is restrained by the braces.

In some cases, the deformation and curvature mentioned above can adversely affect the payability of the guitar. In extreme cases, some luthiers will attempt to 'reset' the neck angle to restore the payability of the instrument. This is a major operation, and is generally not economical or practical.

Prior art literature review

Many studies have been documented by other researchers, with the aim of better understanding the structural dynamics and fluid-structure coupling of guitars.

More recently, numerical methods such as finite element methods have been utilised to study the modal characteristics of guitars. Most previous studies focus on the performance of the soundboard, due to the importance of this part of the guitar on the overall acoustic 'sound quality'.

Previous studies of the guitar soundboard have generally assumed some initial soundboard geometry and bracing arrangement (usually a variation of the X-brace pattern) and then calculated the response of the system using numerical methods. This process is essentially design by informed trial and error, a process whereby design modifications are heavily dependent on the designer's intuition and interpretation.

The vast majority of documented studies in this field are concerned with variations of traditional brace patterns for either steel or nylon string guitars.

Sensitivity analyses have been undertaken to explore the relative influence of the various features of the traditional X-brace soundboard construction, for example.

The foregoing discussion of past research, including any documents, products, acts or knowledge, is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other country on or before the priority date of the claims herein. SUMMARY OF THE INVENTION

Using numerical analysis and design methods, the inventors have developed a radically new soundboard design for stringed musical instruments such as acoustic guitars. Broadly, the soundboard of the present invention has a generally planar form with a longitudinal axis defined between a heel end, at which a neck of the instrument would be attached, and a body block end opposite the heel end. The soundboard is characterised by: a primary structural zone extending generally along the longitudinal axis of the soundboard and having a width, measured across the soundboard laterally to the longitudinal axis, which is substantially greater than its thickness, measured into the plane of the soundboard; and secondary zones, laterally adjacent the primary structural zone, having a material thickness which is less than that of the primary structural zone.

Further, preferred, features of the soundboard may be as defined in the dependent claims 2 to 20 attached hereto and hereby made part of this disclosure of the invention by cross reference. The invention also relates to a method of designing a soundboard for a stringed musical instrument as described in the New Approach below.

Comprises/comprising and grammatical variations thereof when used in this specification are to be taken to specify the presence of stated features, integers, steps or components or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 shows the main features of a typical acoustic guitar, and includes the nomenclature used throughout this specification to refer to those features;

Figure 2 shows an internal plan view of a typical prior art X-brace soundboard;

Figure 3 shows an internal plan view of a soundboard made in accordance with a preferred embodiment of the present invention; Figure 4 shows a perspective view of the soundboard shown in Figure 3;

Figure 5 shows an alternative perspective view of the soundboard shown in Figure 3; and

Figure 6 shows a side elevation of the soundboard shown in Figure 3. A NEW APPROACH TO SOUNDBOARD DESIGN Optimisation and analysis process

Recent research undertaken by the inventors differs from previous studies of the guitar soundboard in that rather than postulating an arrangement, then undertaking the analysis to test and verify the results, a set of governing criteria are applied to determine an arrangement by automated computer methods. The static and dynamic performance of the resulting arrangement can then be calculated using standard numerical methods.

The inventors have developed a novel soundboard design using numerical analysis methods manipulated by optimisation routines. In this context, 'optimisation' refers to an automated decision making process whereby material that is not forming an effective part of the structure in a certain arrangement and loading condition is 'removed' from the analysis. This process is repeated, until termination criteria are met. The optimisation process involves two pieces of software; a Finite Element

Analysis (FEA) package, known as Strand7®, is controlled by a purpose written optimisation program, referred to herein as iCarve, via an Application Programming Interface (API).

Finite Element Analysis Software Strand7® (see http://www.strancl7.com) is a commercially available FEA package that is widely used in structural and mechanical engineering design offices. Strand7 has an Application Programming Interface (API) which allows software written by others to interact with, and manipulate the 3-dimensional finite element model. Strand7 incorporates a variety of solvers that can be used to calculate stresses, displacements and the dynamic response of structures. Evolutionary Structural Optimisation Program iCarve is an optimisation program that was purpose written by the inventors to interface with Strand7. The general procedure used by the program is referred to in the prior art as Evolutionary Structural Optimization (ESO).

Existing versions of ESO programs allow removal of entire elements from a finite element mesh. iCarve differs from earlier versions of ESO programs in that it allows progressive thinning of elements within the finite element mesh, within specified thickness limits. A structural analysis model is built using Strand7. This model represents the geometry, loads and material properties of a guitar. This can be used to calculate the forces and displacement of the structure. The geometry of the structure is represented as small elements. The elements are an approximation to the behaviour of the full structure and, generally, the more elements the better the approximation. Initially the thickness of the elements in the soundboard are set to the maximum value chosen. iCarve interacts with the Strand7 model and extracts analysis results. The process works iteratively, manipulating the model and then recalculating the analysis results.

On the basis of the contribution of each finite element, iCarve has a decision making algorithm that determines areas where there is excess or underutilised material, and then removes this material from the analysis model. This process continues until the criteria for stopping are met. The result of this process is a distribution of material that better matches the loads that are applied to the model.

The resulting distribution of material is a function of the type of results that are extracted from the FEA program and used by the optimisation routine. iCarve has been developed to allow the soundboard to be optimised with regard to;

^■ membrane stress (in-plane)

^■ bending stress (out-of-plane)

^■ combined stress (Von Mises)

^■ deflection (strain energy function) Objectives of the research

The objectives of the inventors' research, included:

^■ Identification of the natural string force load path' considering the orthotropic nature of the soundboard construction

^■ Determination of a material distribution commensurate with applied forces following the naturally stiffest load path given prescribed soundboard geometry and boundary conditions

^■ Limitation of rotation of the bridge assembly observed in many X-brace soundboards

^■ Limitation of deformation behind the bridge observed in many X-brace soundboards

^■ Minimisation of induced soundboard curvature due to change in ambient conditions ^■ Incorporate a hierarchy of primary and secondary structural zones, to enable the secondary zones to be 'tuned' without compromising the primary load path

^■ Minimisation of the weight of material included without compromising the strength of the instrument

^■ Maximise acoustic volume of the instrument Description of the model

A three dimensional finite element mesh was constructed to represent the guitar's front, side and back panels and features. The model is mainly comprised of plate elements.

For the purposes of study of the soundboard, external node restraints were applied to the relatively rigid heel elements.

In preliminary analyses, string forces were modelled as externally applied loads to the bridge rather than (internal) prestress loads. Change in temperature load cases were modelled as thermal node loads.

Similarly, change in moisture content load cases were also modelled as thermal loads, by relating the unit shrink/swell values to the thermal coefficient of expansion of the material.

Initial conditions In its initial state, the geometry of the finite element model is per the idealised finished specification of the guitar, with the exception of the soundboard. The soundboard initially has the overall geometry imposed by the curved sides of the guitar, but is set to have an initial uniform thickness throughout.

Interim state As the optimisation routine progresses, the plate elements comprising the soundboard (identified by unique property group numbers) are progressively thinned according to the degree of utilisation of the element for the particular stress state being extracted from the FEA program and used by the algorithm.

The plates forming the soundboard have a maximum thickness equal to the initial thickness. That is, material can not be 'added' to the initial finite element mesh, thickness can only be incrementally reduced. A minimum thickness is defined by the user. Without a minimum thickness being specified, material could be removed entirely. For most runs, 2mm was used as a practical minimum thickness for reason of general robustness.

The progressive thinning increment is defined by the user. For example, an element within an analysis run with an 8mm initial thickness and a 2mm minimum thickness could have its thickness reduced up to 12 times if the progressive thinning increment is set to 0.5mm.

The decision to remove an element is based on its "stress" as a percentage of the maximum stress in the soundboard. Initially any element with stress less than 0.5% of the maximum stress is thinned. Stress may be a stress or it may be a derived result representing deformation contribution at the bridge.

The model is then rerun and if any elements have a stress less than this value they are again thinned. Eventually no element will be thinned and so the percentage is increased to say 0.75% and the process repeated. Typically the solution might require up to 200 iterations and several hours of computer time. The evolution of the soundboard geometry at the completion of an iteration of the algorithm can be viewed in Strand7. Termination criterion The termination criterion that was selected was a 'minimum average thickness' of the soundboard. That is, the optimisation routine terminates when the volume of the material remaining in the soundboard after a particular iteration has an average thickness equal to the volume remaining divided by the plan area of the soundboard, which always remains constant.

In many general structural engineering problems, it is usually good practice to govern an analysis by prescribed material stress limits and/or deflection limits. Although it is certainly desirable to know that operational stresses are within the elastic limits of the soundboard material to minimise permanent (plastic) deformation, it is more pertinent in the case of a guitar soundboard to work with parameters that are more intuitively understood, and easily related to the volume and therefore the overall mass of the soundboard. In other words, it is probably more informative for a guitar builder to know that a solution has an average thickness of 4.0mm, rather than knowing the maximum extreme fibre stress at a point within the mesh. The benefit of termination of consecutive analysis runs at a given average thickness is that results can be compared across constant soundboard volumes, with varying distribution of mass over the area of each soundboard. The question can be asked: how best to distribute an average of 4mm of soundboard material if the aim is to utilise combined stresses, or bending stresses, or minimise deflection under a given load case?

Actual elemental stresses can be contoured and viewed upon termination of the optimisation routine by running the linear static solver in Strand7.

Modal analyses and thermal loads can also be applied within the FEA program once iCarve has altered the mesh according to the parameters set by the user.

Discussion of preliminary results and prototype guitars

The arrangements resulting from the application of the above process are radically different to traditional bracing systems. A fundamental principle was questioned by the inventors' approach and research.

The traditional X bracing pattern appeared to be stiffening the structure. Since it was postulated that sound is produced by a moving panel, that is by achieving amplitude of the membrane or soundboard, the relatively stiff nature of the traditional soundboard was questioned.

In order to achieve sound or acoustic output, it was postulated that the soundboard needs to achieve amplitude where it would appear that relatively stiff braces by nature restrict amplitude.

The fundamental frequency of a system is proportionate to the square root of stiffness divided by mass. So, increasing the stiffness of a system of given mass increases the fundamental frequency of the system. Conversely, adding mass to a system of given stiffness reduces the fundamental frequency of the system. So, it is well understood that it is not only the structural stiffness of the soundboard that is important in consideration of dynamic response, but the distribution and participation of mass within the structure for a given mode of vibration is equally important.

A stiff soundboard of low modal mass will be easily excited, but will be unable to reproduce or amplify low frequencies efficiently. A less stiff structure with a higher modal mass will have a lower fundamental, and will more readily reproduce or amplify lower frequencies (provided there is not so much modal mass that there is insufficient input to excite the mode).

For reference, a standard six-string acoustic guitar at concert pitch will produce fretted notes ranging from 82Hz(bottom E) to 1046Hz (High C).

At the outset of the research, it was thought that by optimising the distribution of material for 'structural strength' performance and utilisation reasons, it was possible that the resulting distribution would also display good acoustic or tonal properties. Various analysis runs were undertaken, using a variety of initial conditions, termination thicknesses and material properties with varying degrees of orthotropy. The governing stress or deflection function was also varied.

Additionally, physical tests were undertaken on samples of the materials to determine more accurately the properties of the timbers used in the construction of a guitar.

It was found that once the ratio of longitudinal to radial elastic modulus is greater than about 10-12, the resulting material distribution tended towards a central thickened zone of primary structure of initial (maximum) thickness, surrounded by adjacent zones reduced to the minimum thickness. By stark contrast to the traditional methods the thickened zone of primary structure is much wider than traditional braces; in some embodiments this primary structural zone is 185 mm in width, and yet only a maximum of 8mm in thickness, being substantially lower or less thick than traditional braces.

The extent of the primary structural zone was found to vary according to the type of stress being used in the optimisation calculations. However, the optimisation routine did (predictably) utilise the full initial thickness with little graduation in areas of primary compression flowing around the soundhole and up towards the heel end of the soundboard.

Where bending stresses were utilised, the primary structural zone extended further down behind the bridge towards the end block. Where bending was included as combined stresses, the primary structural zone tapered in plan and graduated in thickness towards the end block. Where extreme fibre stresses were utilised to include for bending stresses, an array of ribbing resulted in the lower bout towards the end block.

There was also a tendency for the compressive force to strut directly into the waist, where the soundboard acts as a deep beam (in-plane shear) spanning its shortest dimension which occurs at the waist. The orientation of the struts is tangential to the shear wall action, aligned with the curved sides of the upper bout just above the waist.

Importantly, the various optimisation runs undertaken all resulted in the clear distinction of a central zone of primary structure necessary to carry the string force set, and adjacent zones of secondary structure that complete the soundboard diaphragm but are not utilised in carrying the primary string force load set.

For practical reasons, the inventors believe that the regions forming the secondary zones, particularly in the larger areas in the lower bout will require reinforcement strips to prevent splitting in service. These reinforcement strips are able to be used as a means of tuning the thin secondary zones of the soundboard to sympathetically vibrate with the played strings, and achieve amplitude and desired tonal characteristics.

Preliminary results show that the fundamental frequency of this type of arrangement is significantly lower than published data for traditional X-brace designs, which are generally found to have a fundamental in the vicinity of 160- 165Hz.

It was postulated that this approach may result in an acoustically louder guitar. The acoustic guitar is a box or cavity which amplifies the resonance or vibration of played or plucked strings.

Prototypes produced demonstrate a substantial advantage in acoustic output or volume when compared to traditional X-brace designs (and when compared with applicant's current "inverted A" pattern as sold under the brand Cole Clark).

The research approach directly takes into account the orthotropic nature of timber. Timber is an order of magnitude stiffer in the longitudinal direction compared with the radial direction. In contrast to the traditional X-brace design, a soundboard produced in accordance with the present invention includes thin areas free to flex in the radial direction in the secondary zones around the soundboard's boundary. This in turn allows the primary structural zone to oscillate with greater amplitude and results in a soundboard capable of greater acoustic volume.

By comparison, the centrally located primary structural zone, with grain orientated along the longitudinal axis of the guitar, leaves greater areas of soundboard unsupported by stiff structural members. Larger areas being allowed to vibrate demonstrates an advantage over the numerous smaller areas bounded by the more numerous braces characterised by the 'tradition X brace' system or similar multi brace or lattice brace approaches. The fundamentals of the bounded or smaller areas are typically in the low midrange of the instrument's sound spectrum: 200 to 600 Hz: These frequencies are often noted to be less desirable.

The lower frequencies from 60 to 150 Hz are considered desirable by many guitarists.

Stability

Timber, being hygroscopic, will absorb or lose moisture when subjected to changes in ambient conditions. The shrinkage/swelling due to variation in timber moisture content is substantially different in the three orthogonal directions. For many species, the ratio of the tangential to radial to longitudinal shrinkage is about 100:50:1.

Although most guitar makers will recommend that exposure to severe changes in ambient conditions should be avoided or at least minimised, it is inevitable that a guitar will certainly be subjected to moderate changes in ambient conditions, being relative humidity and temperature, during its service life.

Since traditional soundboards have bracing in an X or crossed pattern, curvature is induced in the soundboard due to the introduction of differential shrink/swell properties by virtue of the orientation of the braces glued (at an eccentricity) to the underside of the soundboard. A traditional brace will not undergo significant change in dimension in its length when it loses or gains moisture. However, the soundboard will change it dimension significantly in the lateral (radial) direction. The result is a tendency for an X-braced soundboard to become concave (sink inward) when the soundboard loses moisture relative to the moisture content at manufacture/assembly. Conversely, the soundboard will become convex (pop upward) when the timber gains moisture and the soundboard expands against a x-brace which is relatively stable along its dimension. The primary structural zone identified in the inventors' research has its grain orientated along the longitudinal axis of the guitar, aligning with the grain orientation of the soundboard material. Hence, the potential for significant curvature to be induced by differential shrinkage or swelling of the substructure of the thickened primary structural zone relative to the overlying soundboard is minimised. The result is a more stable soundboard through very substantial variations in relative humidity.

Timber creep

The relatively high narrow braces, especially the area surrounding the two 'tone braces' behind the bridge in traditional X-brace designs have a propensity to flex or go beyond the point of 'plastic deformation' over time, for reason of the sustained force, known as creep.

In contrast, the primary structural zone identified in the inventors' research better accommodates the axial, bending and shear forces around the bridge which tend to cause deformation over time in more traditional designs. Experimentation by exposure to the triggers of creep (sustained force, elevated temperature and humidity) demonstrates that the 'panel' is substantially less prone to deformation. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

To assist the further understanding of the invention, reference is now made to the accompanying drawings which illustrate a specific preferred embodiment. It is to be appreciated that this embodiment is given by way of illustration only and the invention is not to be limited by this illustration.

Referring now to Figures 3 to 6 of the drawings, there is shown a soundboard 10 for an acoustic guitar made in accordance with a preferred embodiment of the invention.

The soundboard 10 is generally of planar form and has a longitudinal axis 12 defined between a heel end 14, at which the neck of the guitar would be attached, and a body block end 16, opposite the heel end 14. In plan view, viewed from the inside of the guitar, (see Figure 3) the soundboard 10 includes an upper bout 18 and a lower bout 20 separated by a waist region 22.

The soundboard 10 of the invention is characterised by a primary structural zone 24 extending generally along the central longitudinal axis 12 of the soundboard 10 and having a width W, measured across the width of the soundboard laterally to the longitudinal axis 12, which is substantially greater than its thickness T_p measured into the plane of the soundboard (see Figures 4 to 6). The soundboard 10 also includes secondary zones 26 and 28, laterally adjacent the primary structural zone 24 having a material thickness T_s which is less than the thickness T_p of the primary structural zone 24.

The maximum thickness T_p of the primary structural zone 24 is preferably within the range of about 8 to 15mm, and more preferably about 10mm. The maximum width W of the primary structural zone 24 is preferably within the range of about 150 to 200mm. The minimum width could be in the range of 15 to 60mm. The thickness T_s of the secondary zones 26 and 28 is preferably within the range of about 1 .5 to 4 mm, and more preferably about 2.2mm. It should be understood however, that these dimension are given merely by way of example and would vary with the species of timber used, as well as other design choices made in the optimisation process. Consistent with conventional soundboard design, the soundboard 10 of the embodiment of the invention shown in the drawings also includes a sound hole 30 and bridge support region 32, both located on the longitudinal axis 12. However, in this instance, both the sound hole 30 and bridge support region 32 are also located on the primary structural zone 24 of the soundboard 10. The width W of the primary structural zone 24 is greater than the width of the sound hole, such that side portions 34 and 36 of the primary structural zone 24 surround the sound hole 30. Similarly, in the embodiment shown, the width W of the primary structural zone 24 is greater than the width of the bridge support region 32. In a finished guitar, a conventional bridge and saddle may be mounted to the upper, or outer, surface of this region of the soundboard.

From Figure 3 is can be seen that the width W of the primary structural zone is greatest at approximately a mid point between the sound hole 30 and the bridge support region 32. In this embodiment, this mid point is also approximately the mid point between the heel end 14 and the body block end 16 of the soundboard 10. However, the inventor's optimisation and analysis process has demonstrated that other alternative plan view shapes for the primary structural zone 24 are possible. It can also be seen from Figure 3 that the width W of the primary structural zone 24 approaches a minimum at or near the body block end 16 of the soundboard 10. The width of the primary structural zone at the heel end 14 of the soundboard may be similar to that at the body block end 16 or, overall, slightly wider. In the embodiment shown in the drawings it is actually split into two portions at the heel end 14 of the soundboard 10 (to allow access to the truss rod nut).

In an alternative embodiment (not shown in the drawings) the sound hole 30 may be in a different location, not on the longitudinal axis of the guitar. For example, it could be located within one of the secondary zones 26 or 28, or it could be in another part of the instrument altogether (such as a side wall).

Referring now to Figure 6, there is shown a side view of the soundboard 10, with its outer surface 38 facing upwards in this figure and its inner surface 40 facing downwards. It can be seen that the thickness T_p of the primary structural zone 24 is substantially uniform over at least half of its length and is graduated to a minimum towards the body block end 16. Indeed, in this embodiment, the thickness T_p of the primary structural zone 24 is substantially uniform from the heel end 14, around the sound hole 30 and through the bridge support region 32, and then graduates to a minimum near the body block end 16. However, once again, alternative thickness profiles have been found by the inventors' analyses to be suitable.

In the latter regard, the width and thickness of the primary structural zone 24 will, to a large extent, be determined by the properties of the material from which the soundboard is made. The elongated shape of the primary structural zone 24 is determined by the orthotropic nature of timber. The properties of the timber employed will thus determine the most suitable shape, including its width and thickness and the graduations of those dimensions, as required to withstand the axial, shear and bending stresses imposed on the soundboard by the strings of the instrument. In the embodiment shown in Figure 6, it can be seen that the thickness T_p of the primary structural zone 24 is graduated towards the body block end 16 of the soundboard such that its minimum thickness is substantially the same as that of the secondary regions 26 and 28. Thus the body block end of the primary structural zone 24 gradually merges into the material of the secondary zones 26 and 28. However, this is not essential and the thickness of the primary structural zone 24 may simply graduate to a minimum which is closer to the thickness of the secondary zones, more along the lines shown in Figures 3 to 5. In these figures, the thickness is similar but is not exactly the same. Suitable timbers for use in manufacturing a soundboard in accordance with the present invention include but are not limited to the various species of Spruce, Cedar, Bunya, Koa and similar tone woods that are suitable for stringed instrument construction. The soundboard may be fabricated by taking a uniformly thick sheet of timber and routing out excess material so as to leave a resulting shape as shown in Figure 3 to 6, for example.

Alternatively, the soundboard may be made by laminating together two or more sheets of timber so as to build up the required thickness for the primary structural zone. In this way, the additional layer (or layers) may extend through the region forming the primary structural zone but do not need to extend into the secondary zones to any significant extent. This increases utilisation of the timber and reduces waste.

An additional benefit of laminating two or more sheets of timber is that the finished soundboard is less likely to split because the structure of the grain in each sheet of timber will be slightly different, even though the general direction of the grain is consistent. Any weaknesses on one sheet would be partially compensated by the other sheet.

To eliminate or at least minimise the possibility of the thin secondary zones splitting, reinforcement strips 42 may also be provided. Whether these are actually needed in any particular instrument would depend on the nature of the timber used. It can bee seen from the embodiment shown in the figures that the reinforcement strips 42 are wider, than they are thick, however this need not necessarily be the case. This minimises their impact on the tonal character of the soundboard. Of course, they could also be used to tune the soundboard, together with variation of the thickness of the secondary zones of the soundboard itself.

In an alternative embodiment, not shown, the primary structural zone may include one or more cavities to reduce weight in that portion of the primary structural zone whilst maintaining its stiffness. This arrangement could be compared to that of bridge box girder sections which are generally designed to maximise stiffness while minimising weight. In this way, the weight, and hence acoustic properties of the soundboard, may be tuned in accordance with the mass of the timber from which it is constructed, or the style of playing of the musician.

It will be appreciated that the stiff, beam-like structure of the primary structural zone 24 achieves great rigidity around the bridge support region 32 of the instrument. However, the graduated thickness of the primary structural zone 24 also allows it to function like a leaf spring which is fixed at the heel end 14 of the soundboard and is relatively free at the body block end 16. This leaf spring effect of the primary structural zone 24, together with the relatively thin secondary zones 26, 28 of the soundboard surrounding the primary structural zone 24, enable the whole soundboard 10 to act as a diaphragm which resonates in response to vibrations induced into the primary structural zone 24 at the bridge region 32. In a finished guitar, the bridge/saddle assembly mounted to the leaf spring effectively "pumps" the whole of the diaphragm which forms the soundboard of the instrument. In prototype guitars this effect has been found to produce a substantially louder guitar with superior low frequency performance and greater tonal character. Although a preferred embodiment of the invention has been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention. Such variations are considered to fall within the scope of the appended claims. Table 1 - Components of an Acoustic Guitar

(Refer to Figure 1) a. headstock b. nut

C. strings d. soundhole e. saddle f. bridge g- body block end h. soundboard i. lower bout j- waist k. upper bout

I. heel m. neck n. tuning machine

Claims

CLAIMS:

1. A soundboard for a stringed musical instrument, the soundboard having a generally planar form with a longitudinal axis defined between a heel end, at which a neck of the instrument would be attached, and a body block end opposite the heel end, the soundboard being characterised by: a primary structural zone extending generally along the longitudinal axis of the soundboard and having a width, measured across the soundboard laterally to the longitudinal axis, which is substantially greater than its thickness, measured into the plane of the soundboard; and secondary zones, laterally adjacent the primary structural zone, having a material thickness which is less than that of the primary structural zone.

2. The soundboard of claim 1 wherein the primary structural zone is widest at a position between the heel and body block ends of the soundboard.

3. The soundboard of claim 2 wherein the primary structural zone is widest at about midpoint between the heel and body block ends of the soundboard.

4. The soundboard of any one of the preceding claims wherein the width of primary structural zone is a minimum at a position adjacent the body block end of the soundboard.

5. The soundboard of any one of the preceding claims wherein the thickness of the primary structural zone is graduated to a minimum adjacent to the body block end of the soundboard.

6. The soundboard of any one of the preceding claims, further comprising a sound hole located on the longitudinal axis of the soundboard within the primary structural zone, the width of the primary structural zone being greater than the width of the sound hole such that a portion of the primary structural zone surrounds the sound hole.

7. The soundboard of any one of the preceding claims, further comprising a bridge support region located on the longitudinal axis of the soundboard within the primary structural zone, the width of the primary structural zone being greater than the width of the bridge support region such that the primary structural zone surrounds the bridge support region.

8. The soundboard of claim 7 when appended to claim 6 wherein the primary structural zone is widest at about a midpoint between the sound hole and bridge support region.

9. The soundboard of claim 7 or claim 8 wherein the primary structural zone is of substantially uniform thickness over at least half of its length but is graduated to a minimum at a location between the bridge support region and the body block end of the soundboard.

10. The soundboard of claim 9 wherein the thickness of the primary structural zone is graduated from a maximum at the bridge support region to a minimum which is substantially the same thickness as that of the secondary regions.

1 1 . The soundboard of any one of the preceding claims wherein the soundboard is made of timber having grain which extends in a direction substantially parallel to the longitudinal axis, the primary structural zone and secondary zones being formed by routing away portions of the timber.

12. The soundboard of claim 1 1 wherein the soundboard is made of two or more sheets of timber which have been laminated together to create a desired maximum thickness for the primary structural zone, the grain of each sheet extending in a direction substantially parallel to the longitudinal axis.

13. The soundboard of any one of the preceding claims wherein the primary structural zone includes one or hollow cavities to reduce weight of that portion of the primary structural zone whilst maintaining its stiffness.

14. The soundboard of any one of the preceding claims wherein the primary structural zone functions as a leaf spring which is fixed at the heel end of the soundboard and relatively free at the body block end.

15. The soundboard of any one of the preceding claims wherein the secondary zones span between the primary structural zone and peripheral edges of the soundboard.

16. The soundboard of claim 15, further including reinforcement strips spanning the secondary zones.

17. The soundboard of any one of the preceding claims wherein the primary structural zone has a maximum thickness in the range of 8-15mm and the secondary zones have an average thickness in the range of 1.5-4.0mm.

18. The soundboard of claim 17 wherein the primary structural zone has a maximum thickness of about 10mm and the secondary zones have an average thickness of about 2.2mm.

19. The soundboard of any one of the preceding claims wherein the primary structural zone has a maximum width in the range of 150-200mm and a minimum width in the range of 15-60mm.

20. A guitar comprising a soundboard as defined in any one of the preceding claims.