In this page are written my general ideas and the concepts from which my work as a classical guitar maker has developed. These are views that have come from many sources, including discussions with other makers, reading from books on theoretical acoustics and my experiments as a maker. It is not intended to be definative and this discussion is an invitation to anyone who wants to contact me with their own ideas and engage in an exchange. I am sure that some of the following are just wrong, but I hope there is possibly enough that is of value to other makers to smooth the way for them to make their own great guitars
Re-inventing the wheel
Doing your own experiments is probably the best way for a maker to understand what is happening with the instruments they make. I have my own strategies that have grown from my own experience, that seem to me right; however in no way do I suggest there is a right or wrong way to make a guitar, if it works it works! So here follow some analysis and some descriptions, some part of which are the ideas I work with, and some are ideas that my work in repair of some great musical instruments have inspired.
The Guitar is a complex interrelated system, so to make a coherent picture I shall break down the whole into component parts, discuss resonances, stategies to move these and simple visualisations. Then move on to ideas like coupling, finally bringing together a whole guitar concept, that is actually nearer to how I work with intuition and my wood.
Suggested reading for those interested in acoustics are two books that have informed some basic science, and encouraged some experiments.
John Backus - The Acoustical Foundations of Music
Arthur H. Benade - Fundamentals of Musical Acoustics
Not a substitute for, but a guide to make at least some observations in this wonderful experiment in acoustics and music that is the guitar makers art.
A vibration of the string begins as an impulse from a "plectrum" that travels along the string at the speed of sound, around the string from bridge/saddle to nut and back. The frequency is fixed by the mass, tension and length of the string. Tonality can be described as a perception of the mix of components of the harmonic series, whole number ratios 1:2:3:4:5:6 etc that are created by the way the string is set in motion (and it's reflections from the instrument). Harmonics can be played at the 1/2, 1/3, 1/4, 1/5, etc points (nodes) on the string length. Plate harmonics are more complex, and form the basic design features for guitar and orchestral stringed instruments, upon which the strings act.
Harmony is built similarly, for example the ratio 1:2 is an octave and other pure intervals are 2:3 - 5th, 3:4 - 4th, 4:5 - major 3rd and 5:6 - minor 3rd.
Temperaments, using perfect intervals as above were used in the past, and have a unique charm. However due to the growth of bigger ensembles and orchestras, the tunings needed to be standardised (these pure ratios did not add up to 12 equal intervals per octave) and the system of equal temperament was introduced (12th root of 2).
Perhaps worth mentioning are the 3rd intervals where the comparison of the pure 3rd to the equal tempered 3rd ratios are very different, as much as a quater of a semitone (24 cents); it has been suggested that this is why our mind interprets the minor 3rd as "sad" because it is flat, and the major 3rd as "happy" because it is sharp compared to the pure interval.
Internal resistance to bending in real 3 dimensional strings cause the harmonics to become progressively sharp due to this "friction", for this reason we have to compensate and adjust the string length, a little longer at the bridge. As higher harmonics become more in-harmonic, this may explain why steel strung guitars with higher energy levels in this range have a ringing character, while nylon strung guitars have a more fundamental tone. The presence of some high harmonics in the tone (being sharp) could be interpreted as sweet. Human psychology and interpretation of sound are difficult to measure and often contradictory.
The strings vibration is passed to the saddle, bridge and then to the front, and then further to rest of the guitar; from which it is radiated to the air. The speed of sound is not fixed, but depends on the material through which it travels as sound waves. So we have string, saddle, bridge, front, the rest of the guitar and the air, all with different speeds of sound.
Two ways to look at the bridge.
The string under tension provides a force along the string
1) Break Angle at the saddle, at this point the string will try to straighten, causing it to pull up at the tie block, at the same time pushing down on the saddle block; creating a Twisting Moment, that tends to try to distort the front.
Strategies to resist twist. We can stiffen the bridge itself, e.g. make the saddle and tie blocks longer even to the end of the bridge. This will have an impact on the vibational modes and harmonics of the bridge and affect the whole system, we will look at this later. We can add a harmonic bar under the top at about the bridge position (usually in front, we will see why later). This stengthens the saddle block but leaves the tie block relatively free. This type of strategy is also used in steel string guitars X bracing that spreads the force in front of the bridge further reinforced to stop the top buckling. Another method is to glue a plate "veneer" under the bridge strengthening against twist and allowing the natural harmonic break at the wings of the traditional design.
2) Break Angle. Here we look at the resultant force at the saddle - imagine an arrow on a bow string force. This is measured by bisecting the angle at the saddle, and gives us the direction of this force:
a) High Saddle the direction will be more forward and could be beyond the saddle base, issues such as a bending saddle, or the wood in front of the block splitting or even breaking away. We should also consider if this is efficient, the direction of the vibration falling outside the base of the saddle.
b) Low Saddle with very little break could buzz due to not enough pressure on the saddle, but provided there is enough angle for a clear note the direction of this force will be more downward toward the base of the saddle. Could there be a loss of efficiency due to less force.
I wonder if the combination of these two break angle forces are actually how the vibration transfers, and it is perhaps a mistake to forget either.
Bridges are made from Indian or Rio Rosewood, Ebony or Maple usually; It is worth saying that Brazillian Rosewood is an endangered species and I will urge makers not to consider buying this timber. There are legal and export issues as well as conservation to consider!
At the other end of the string is the nut, enough break angle is also needed to avoid buzz. The tension of the string pulls between the nut and the saddle, and will place a force on the front, at an angle to the face. We can call this the String relative to the Front angle. What effect can this quality have?
a)High Angle. the nut in this model is elevated above the saddle and the string approaches the front at an angle getting closer as it nears the bridge. There is a new resultant pair of forces that can be calculated, with one component along the front from the bridge, and a second at right angles UPWARD from the bridge. We should consider how this force will resolve over time. I have worked on guitars that have this kind of set up, and this force to pull up the front needs a strategy to resist it. I have often seen a harmonic bar placed under the bridge, and it can be used to resist this and the twist of the bridge at the same time. Also seen have been some form of lattice strutting. I have had to lower the action on this type of guitar in the past as the top drifted up. Is this a solution though as the force is restored, as the maker intended.
b)Low Angle, where the string originates from a point lower than the saddle will produce a second component in a DOWNWARD direction. Makers have introduced ideas like a dropped upper bout to lower the neck angle and sometimes a tailpiece. This kind of set up is most common in the carved jazz style guitars and of course mandolins, banjo's and orchestral strings, where the down force is critical. This set up is usually braced with strong X or A frame inside, and the violin family have a combination of sound post and bass bar. I've seen this on flat top guitar, the Django "gypsy guitar" springs to mind.
c)Parallel to the Front, here there is no resultant force either up or down at the bridge, so we could describe this as a State of Balance. So no special strengthening is need to stop the bridge from riding up or down.
Consider the three above conditions, and ask are any more efficient at transmitting vibration, include the use of extra strengthening that could be considered a friction or damping force. In the long term what are the implications for the action. I have to admit that my preferance for the parallel option is part of my general liking for structral balance, lightness and reduction of tensions in the instrument; and the longevity of the playing action as the guitar matures. Can reduced tensions and restraint allow a longer decay, we normally say "sustain".
Wood Science. European Spruce has the longest cell length of any wood, being both relatively hard and strong; Cedar is a much softer wood and also lighter. Whatever your choise you will have to consider the sound (Timbre) your Timber will make and also it's strength to resist colapse under the forces presented by the strings.
Cut of the log: Quarter Sawn timber is sawn radially with the grain 90' to the face, and will show ray figure where cross grain cells laid down by the tree from the centre toward the edge add stringth and stiffness to cut boards. They also affect the way the wood moves during drying or moistening humidity conditions. Split tops result in the wood cells in line of the grain are more likely to be parallel to the face, and the strength and spring of the top will be at it's maximum along the grain, a very important dimension for the sound board. It's worth mentioning that most trees seem to twist as they grow and lay down cells in a slight helix, so you may find there is a gradual angle quarter rotation along the board. Split tops produce the longest grain (carpentry term) of all cuts, short grain being weaker and more likely to fracture.
Modulus of Elasticity. This is the degree of spring or resistance to bending of the material, and affects the speed off sound through the wood. In the three dimensions this effect differs so that the wood along the grain is at its stiffest and strongest, while in the quartered (radial) across the grain it is less so, and still less in the across grain in line with the trees growth rings.
Stability:- Wood is hygroscopic in nature, so relative humidity in the air will either dry or add water to the wood till it reaches stability. Expansion or shrinking is caused and is different again in the above dimensions. Warping can be caused in some cases, but also shrinking and swelling of plates will result in tension changes in the top and back. Most change will be across the grain, and all timbers vary! Further as wood takes up more moisture it becomes weaker, and as it dries it becomes harder and stiffer.
Thickness; the concern for makers are weight, stiffness or spring and strength. These parameters will change the vibrational harmonic modes of the plate, you may have seen holographic photos of these modes as frequency generated patterns of harmonic areas are created on guitar tops. Another way to see these modes is to place some fine dust on the plate, and play a note to vibrate the plate; the dust should eventually settle on the nodes where the vibration is at minimum. This can help to visualise how standing waves behave, we can use this to consider strut placement, and develop strategies for efficiency improvement to the frequncy spectrum. The sound waves are generated by the strings so the angle it makes to the front can also be considered in this context.
Thin tops, perhaps have a faster response or "quicker attack" and may give up energy from the strings to the front quicker, so that energy from this source also decays quicker. The wood will be lighter and have a lower resistance to bending, and could break up into higher frequency harmonic modes. Some strategy could be to maintain this energy in the top plate to improve sustain by isolating it from the sides and back to prevent "leakage" of sound energy. To resist string forces strutting will need to be relatively stronger perhaps by using heavier fans, Harmonic bars and more recently lattice bracing systems. Another important consideration is the responce of very thin fronts to drying air conditions; Shrinkage can be much quicker in thin wood and tensions can build up very quickly, and I have seen splits in tops during quite short cold dry spells. Low relative humidity in heated homes usually brings more split repairs to my workshop, and very often in very thin fronts, guitars of very high quality.
Thick tops on the other hand have a slower response so "softer attack"characteristic taking up string energy slower, so the string is able to continue to pass energy for longer. What would cause this? Thicker tops are heavier, with more spring so the inertia to start movement is higher, but once set in motion its higher bounce character keeps it going. It is possible that more energy is needed to get the vibation going, as there is more internal resistance to overcome, but also the top needs less strengthening bars so perhaps there is less interference with the natural harmonic content. Thicker timbers have a slower response to drying air, so we can expect the shrinkage to be slower, perhaps a less critical change in plate tensions and there is more wood to resist splitting.
Asymmetrical thickness. Another idea has been floated that the two sides of the front would work in opposition to each other in the 2nd plate harmonic, and could cancel each other out, so that a non symetrical treble and bass front would be non cancelling. By making one side thicker, say the treble side; or by graduating the fan struts from one side to the other and or a strategically placed harmonic bar under the bridge. Another system we often see is the harmonic bar adjacent to the sound hole angled to create an assymetric shape. My reservations are that this could affect the way the fundamental plate harmonic would work with reference to balance, and is there really anything to this idea of phase cancelling in the 2nd mode.
Even thickness tops have a more even elasticity across the board (always supposing the same degree of quarter cut), look closely at the ray figure it needs to be consistent, and should bend with an even curve when flexed. So the speed of sound should be more even due to the consistent spring. Thinning the front toward it's edge is a strategy that may allow a less restricted movement in the fundamental mode, perhaps even lowering the fundamental frequency. How does this work? Making the edge thinner will make it less stiff and this flexibility will change the way we look at the way the top is fixed to the sides. We can imagine this change as moving from a Fixed toward a Hinged edge, thereby making the wave length (single curve shape) longer and thus a lower frequency; leading to lower resonances is the whole instrument. Also because the fundamental is easier to move, a shift in energy toward the lower harmonics is likely. Finally feeling the flexibility with your fingers over the whole top, flexing along and across the grain, and at the edge we can look at this strategy as one of the ways of moving resonances in the guitar. This is for me one of the parts that make up the whole guitar resonance, as we shall see later.
Harmonic Bars used in early musical instruments, are fitted across the grain inside the top and back. A good early example are viols, lutes and early guitars. They were used for structural stability and to create some type of arching, and will have had an effect on the plate harmonics. Modern guitars still use them either side of the sound hole and under the fingerboard, and also on the guitar back for the same reasons. They prevent collapse in front of the bridge and between the soundhole and neck, where the main force of the strings are focused. Usually made from straight grain and mostly split wood is prefered for its maximum strength, they are usually straight in shape and stand higher than the gluing edge; however I have seen curved bars under the sound hole that have pivoted and led to a very distorted front plate. Mostly the edge is scalloped to the sides, though an interesting variation from Jose Romanillos had bars with full height at the sides scalloped toward the centre line, acting like an arch transfering the weight to the sides, at the same time allowing more flexibility in the middle of the board to react to humidity and avoid splitting top or back. Steel strung guitars often use heavy X bracing to spread the much heavier load and this is often supported with further heavy bars to prevent distortion, often with a hardwood bridge plate to help prevent twist and provide a fixing for the pin bridge design. We mentioned the angled harmonic bar above in assymetric design. The harmonic bars on the back provide the back arch along with the foot and perimeter, I have tried an X formation in the lower bout and its something worth trying, but I can't report any advantage. There is a theory that the back when tapped should sound some part of the harmonic series. This was demonstrated to me once though not very convincing, however the lower and upper bout do make obviously different tones (who hasn't played "bongos" on the back) and a harmonic relationship is probably a good idea.
Fan Strutting is probably one of the areas that many books have focused on as the centre of art and design for Spanish Guitars, due to the wide variety of combinations of angles, sizes, shapes, numbers and forms. Usually one of the questions a player will ask is about the strutting. Light struts of wood split for the longest grain are placed under the front in the lower bout and we can look at these in a variety of ways. Usually they are placed in the form of a fan, radiating from a point above the sound hole focusing on a point for illustration, at the 12th or 9th fret, or perhaps a moving "eliptical foci" between the two; picked as an example because both of these points are significant to the string in the harmonic series. Other patterns are used such as, radiating from the end block, or extended beside the sound hole under arches in the harmonic bar(s), or an X figure under the bridge sometimes modified to a star or radiating configuration focusing to a point or line under the bridge. All of the above could have a bridge plate or Harmonic bar at the bridge. Lattice bracing is a cross hatch usually set at 45' to the grain to support a very thin front.
What do they do? The forces from the string are trying to twist the bridge and sometimes trying to pull up or down the front, they can be looked at as being placed to resist these forces, added strength. However some makers have attributed more qualities, here are some ideas. They may pass soundwaves around the soundboard? This idea seems to be that vibration travels along the bars and helps the sound spread out. Another idea is the creation of nodes, and could explain the choice of a 3,5,7 or 9 strut designs, as the focus of nodes and anti-nodes in the harmonic content of the vibrating front.
Creating Form. Sometimes called disciplin, here we make the top form an arching of which there are many types. In this description we see the fan struts anf indeed the harmonic bars working with the front to create a form that is strong enough for the strings forces. Perhaps we can look at relative weights of materials, so for thinner tops heavier or stronger strutting is needed, while a heavier thicker top will only need lighter strutting. Perhaps a consideration could be the weight of the plates producing a more powerful sound wave in the air. The form itself provides some structural strength, and these ideas will be significant when we consider the whole working instrument, factors I will address later in this analysis.
Solera Method. I will talk about this more later, but briefly here because it is on the solera that I and many others create front plate arching. Another method could be the use of a former dish to create the arching by glueing bars and struts pressed against this "dish". This last method is used by many makers to create the back arching, more later.
Before we move on with this subject I want to quickly talk about some recent types of front. New materials like very stiff strong and light carbon fibre and nomex are being used to strengthen the plate itself. They are used in combination with wood which imparts an attractive tone, and can be incorporated as a thin layer between very thin light wood tops, or as a waffle like lattice glued to the underside of very a light front. These guitars usually also use a form or dome, and are designed to reduce internal resistances to a minimum, to improve efficiency, while at the same time keeping the structural integrity. The guitar could still be made in the traditional "Spanish" way, but often this type of top demands overall changes to the whole guitar. We shall see more about this later. I have never worked with these ideas, because the beautiful tones and interactive musicality of "Spanish Guitars" are the sound palette I prefer. There are some good qualities in these systems that I like, for example the very thin sandwich using nomex produces a very (locally) loud guitar, but the ones I tried were uneven and musically difficult for me. I have repaired many big name lattice guitars, and they are very easy to play, even and musical too; I have never found them to be significantly louder and have doubts about projection, and tonally they cannot compare with a good Spanish Guitar musical beauty (my own musical taste.)
CNC Radius Dishes. Supplied by American luthier suppliers the manufactured dishes I've seen on sale were 25 foot and 15 foot radius dishes, as used on C.F. Martin steel string guitars. Many makers use these, incorporating them into solera and or mould and produce very good guitars. This arching seems to be very good at cutting the very high harmonics that steel strings are so good at producing, however I'm not convinced that nylon strings that are less productive of high energy in this range need to be so restricted. The Dome top covers the whole surface and there is no flat component without the tension of being bent, and this may be why some of the archings below are more traditional in Classical Guitar. The 25ft dish is usually used on the front, while the 15ft is usually reserved for the back, more about this later. My friends that I've spoken with learning in Lewes Sussex, use this method and they make very nice guitars.
Flat top pushed up in the lower bout. This type of arching I developed as a student at the London College of Furniture, shaping my solera/mould to a dish in the lower bout. The perimeter remained on a flat plane, with the bridge the focus of the doming, while the upper bout remained almost completely flat, and was combined with a neck set to produce a parallel string/front angle. The centre line has a parabolic type curve from the neck to the end block. I found this produced a guitar with good fundamental energy and an even response across the strings when combined with a thinner plate edge. This strategy was developed from the flat front guitars I began making at L.C.F. during my first year there. I have seen a number of this model guitar in recent years and 20 to 30 years on they maintain shape and action stability, with remarkably few splits, tonally they have mellowed but retain their "balance power and projection" mentioned by John Mills in the 1980's.
Flat top pulled down at edge all around the lower bout. This arching is a combination of flat top in the upper bout and the parabolic curve in the lower bout centre line. The perimeter is flat around the upper bout but falls from the waist to the line of the bridge and continues lowered, around to the end block. There is another mix of plate resonances and so another tonality and balance. This was the arching for we created at the Romanillos courses in Siguenza I attended in Segovia Spain. This does produce a very musically beautiful instrument of great sensitivity; such that I built dozens of this type and is a feature of my model 3 guitars. The lower resonances are very strong with a good balance and projection, while the very high harmonic content is there too for the sweet tones of Spanish Guitar Music. If any maker wants to learn about this form please contact the Romanillos family and attend a course, where they may learn from great makers directly.
Flat top pulled down at edge in line with the bridge. Again this arching has a flat upper bout and this time a flat centre line, combined with a single curve in the lower bout. The perimater will be flat in the upper bout falling from the waist to the bridge line, then rising again up to the end block that is level with the upper bout. This is perhaps the simplest arching, as if the lower bout is "flexed". Plate resonances, harmonics, tonality and balance are subtly different from the above. Great guitars were made by some notable historic figures using this arching including Torres, Manuel Ramirez and Hauser to name a few. Again a beautiful musically sensitive instrument that is balanced, powerful and projects a range of tonal paette. I discussed this strategy with Jose in Guijosa Spain and in England and adopted it to my model 4 guitars.
Double curve or carved top. These do not normally appear on "flat top" instruments except after resolution of stresses, but some makers double curve (like a subtle form of violin arch). Mostly this arching occurs on jazz guitars and can be pressed or carved arching, and is usually found on orchestral string where a shorter louder response is required (bowing provides a constant input of energy to strings). This works in combination with f-holes beside the bridge, cutting the stiffness beside the bridge thus reducing sustain. Usually the stings are taken over the bridge to a tailpiece and the break angle at the bridge is critical in producing the correct down pressure to balance the sound post/bass bar resistance to collapse. I have made several violins and set up and repaired hundreds, but is another area not really part of this discussion, still of interest as many design issues are similar. Jazz guitars often use A bracing a kind of double bass bar set up or X bracing similar to the steel strung guitar.
Front/Bridge. There is a compelling argument that the bridge is a part of the guitars front stutting (fitted outside) and therefore part of the arching, so different shape bridges and how they break up harminically could influence the shape of top plate harmonics as above. Having carried out an experiment on a variety of bridge forms and sizes, they did affect the responses quite significantly. This is subjectively my experience, the type with extended tie and saddle block produced less high harmonic content in the tone, perhaps because the extension of these bars across the top removed the harmonic break at the bridge wings of a traditional design. I found I liked a bridge that was about half the width of the lower bout, more about proportions later. Narrow bridges will naturally increase break angle and reduce gluing area. It is important to the guitar arch that the bridge is shaped to fit exactly the front form as distortion and added tensions can be introduced.
So far we have looked at the strings, bridge and top, and some strategies to move resonances and discussed the harmonics. We now can procede to the much more speculative interreations with the rest of the guitar, perhaps the more intuitive art of creating a whole supportive musical instrument. There are many conflicting views of what is going on here and so I ask for patience in the reader, as there really isn't any proof except the empirical experimental results of makers, who may not agree with each other. I will try nonetheless to give some strategies, some from other makers, and those that form the basis of my visualisation in my mind as I work with the wood in my hands making each guitar.
Back, Sides and the Helmholtz Resonance. So far we haven't talked about this much, and it is important to look at a few things happening here. With the front, the back and sides form the "cave" with the sound hole as it's entrance that produces the Helmholtz resonance. Simply, the bigger the cave the longer the standing wave thus lower the resonance; also the bigger the sound hole the further outside the cave will extend again lowering the resonance. There is probably a ratio of standing wavelength to sound hole area to achieve the best result, you can read the science. Most mathmeticians tell me the actual results compared to the calculated resonance finds that the resonance tends to be lower in real guitars, just a bit. We can guess that the cause of this lower frequency is due to the flexibility of the sides of our cave, not taken into the calculation. We could then tentatively suggest here is an argument for thinner back and sides and very flexible front, if we want to lower the body resonance. Guitars take a few knocks over the years and a practical lower limit of a 2mm back, before splits become very likely is a suggestion. The Helmholtz resonance usually comes out at about G on the bottom string, of a classical guitar and a bit lower on Dreadnought types, perhaps a bit higher on small bodied early guitars.
Sides. If the body depth is increased we get a lower resonance, something I found when making Steel strung guitars for customers; it is possible there is a standing wave consideration for the ratio of depth to outline shape and sound hole. The sides have to be bent to outline shape, and a thickness much more than 2mm is difficult to work with, especially at the waist and upper bout. I bend the wood dry, but wet or with a bending machine seems to make little or no difference. You could increase the thickness by using laminates in a mould, to give you more rigid sides or heavier to reflect away the soundwaves from the front (useful if trying to keep energy in the front.) Struts on the ribs may have a similar role, or some makers contend they help spread the sound from the front to the back, my experiments with these showed no significant change. Ribs are usually fixed to the front with small corner glue blocks, and to the back with a kerfed or unkerfed lining. In my guitars the lining is glued on after the sides have been bent and scraped clean inside, before assembly as making the body freehand without a mould, it helps keep the side shape. I work free hand so that I can make guitars where the front is very hard, just a little bigger, for more flexibility.
Back. This plate can be looked at a simple reflective surface for the standing waves inside the body, and some makers argue this idea; another is that sound waves pass through the sides to the back; both these are probably at least in part right. There is another approach that sees the back and front as a matching or complimenting pair of plates, but more eo about this later. With some systems, like the super light lattice top guitars, isolating the back and sides to reflect most vibration back onto the top will use heavy sides and back, even press moulding a jazz style moulding for extra stiffness; prefering fixed walls for the helmholtz resonance.
Thickness. For vibration loss and reflective concepts Heavy or thick backs make sense. If this option is chosen then a relatively heavy guitar can be expected, the back is often Rosewood. If the top is fairly thick spruce the weight of the back could be matched to the front with the bridge; this will lead to a thin back and light guitar.
Back fitting. We can try to get some type of matching from the method of fitting the back to the sides. With the CNC dish type front arch there comes the complimentary back dish of 15ft radius; two dishes csn be used, 1) to glue the back bars to the back, an 2) lined with sandpaper to shape the sides and blocks for fitting. A similar method can be used to fit a back in the other archings, and it is a fairly straight forward easy method, using off the shelf CNC dishes from luthier shops. Of course makers can make thier own dishes to compliment the front arching. There is another old method that compliments the pulled down type archings above, where the back bars are glued to the guitar and the back pulled down over them and glued at the same time. The "Torres back" is difficult joint, and the advantages are possibly a strees free joint when carried out well, and it is a "perfect" joint too with maximum control. However because of the difficulty and the size of the gluing surface with animal glue, it is essential to always do a dry run as there is no time for surprises once glue has been applied, you need to be quick working in a warm draft free workshop. I've had most success with this method that probably suits my way of working, and achieved a more reliable back/front tap tone matching.
Double tops. Briefly, a second front material can be attached to the inside of the back bars, as a kind of sympathetic plate. Because the bars are glued to a plate on both side this adds to the strength and rigidity of the combined structure, allowing very light thin bars; however it is unlikely to be any lighter than a single back because the outer back has to be able to stand up to life as a guitar and resist a few knocks.
It is thought that similar and coinciding resonances tend to support each other, Sympathetic resonances can be a problem, we have all had that annoying buzz from a loose strut or hardware during repair work. Wearing my repairmans hat, solving such issues are a regular experience, and you may ask why try to build in such resonances into a guitar; well the simple answer is it's usually the better instruments that have these problems and unresponsive that very rarely do. So there is something happening, how can we use these qualities.
The first one is quite easy to demonstrate, The front fundamental : body resonances. The helmholtz body resonance is usually around G on the bottom string of the guitar as described above; and the front fundamental is in the same area, but can be lowered to F# or even F by thinning the plate edge. Actually matching can lead to a surge at the resonance peak reflecting back onto the string, so it may be better to adopt the stategy of the violin makers who tune the plates to mismatch by between a tone and a semitone to avoid wolf notes. F to F# fits this ratio. To check sing into the sound hole and listen for the resonance peak and compare to a tuner for the pitch, then play the bottom in the middle of the string 12 frets above the fretted note till you see an unusual vibration of the string, the centre of this "wobble" will give the pitch of the front fundamental. There may be issues for the way the player has to play around these notes, I find I naturally play resonance peaks so as not to rattle the strings, as most good guitars seem to have them somewhere.
Front to Back coupling. This is the controversial theory that the front and back tap tones should match. In some way it depends on where you tap it, but lets say you find the lowest fundamental notes on both plates. Again you may want coincidence or a slight mismatch as above, or choose say the helmholtz to match. In this case we now have a resonance matching relationship between the front, back and body; and this could be a pretty powerful supportive system, to learn to control. Getting this much matching is no easy task and a bit of an art form of it's own, but we are talking about an inbuilt harmony in the structure.
Neck synchronised to match. This is a stretch of the imagination! but lets look at it just the same. In this idea we need minimum tensions in the guitar structure so everywhere works efficiently together. The string is in balance with the top (parallel angle) with the neck attached at the mid point (12th fret) and to work with the coupling noted above imagine this. The string is vibrating, in the up half of the wave it pulls the bridge/front and the neck up; then in the down half of the wave it pulls the bridge/front and neck down. This will happen at the mix of harmonics in the tone at the speed of sound. We have the front vibrating and the neck vibrating. This neck vibration travels along the neck passing the body join (appui or someting like fulcrum in violin) where the phase is reversed as it travels on into the heel and further into the back. The speed of sound along the grain of timber is faster than in air. So if the front and back have very similar resonances and are 180' out of phase the body will act like a vibrational pump with a directional wave through the sound hole. If the phase is a little out we could get the interesting effect of chorus to fill out the tone. As an idea this is not really supported by science because nobody has tested the theory, so it's open for discussion. However it does provide an explanation for the guitar that seems to project a tone over a longer distance to an audience than a seemingly louder guitar at close quarters. Also I might suggest that sound waves radiated to the air by a heavier front may be more powerful, and sent in a directional arc as opposed to all around, use this energy to best advantage for normal concert conditions.
The Solera is the assembly jig used to make a traditional assembly Spanish Guitar, and there are reasons why it is so effective. To create the above conditions, arching and neck angle it provdes a platform to consistently achieve the same form for your guitar. It is fairly versatile in that it can incorporate side moulds and a go-bar deck and if you wish to vary the shape a way to fit side "square" brackets. If you wish to make a variety of archings you can have another solera that all your fittings work with. If Studding and nut/washer sets are used for the go-bar posts the height is adjustable making it possible to use self adjusting spring loaded go-bars instead of bent sticks that sometimes break. You can build your guitar onto or around the front which can be your outline if you choose not to use a mould.
In this short paragraph I'm going to remind you of the Harmonic series 1:2:3:4:5:6:8, I left out 7 because it doesn't fit into our 12 semitones scale. You will remember they create the perfect intervals that are our basic harmonies. Well it may come as no surprise that the proportions of our instrument also have harmonic relationships, we will sometimes have to invert an interval so 2:3 = 5th, invert to 4:3 = 4th, so 4:5 = major 3rd, invert to 8:5 = minor 6th; perfect intervals not equal temperament. So here we have a few: Neck 1:2 String, Body length 5:8 String. The ratio of upper bout :waist :lower bout = 4:3:5, Bridge 1:2 lower bout. Just keep looking and you will find these ratios, I once used this system to draw up a plan of the classical guitar and it looked remarably like the late model Torres plan we had at the London College of Furniture.
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