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This is my article on the guitar as a structure.  This was originally published by the Guild of American Luthiers  in American Lutherie #100, in 2009.  (Their graphics are better than mine!)


All content on this page:  © Copyright 2009 James W. Blilie, 2009



The Guitar As A Structure

(And Some Practical Information On Bracing)

Why does the neck of the guitar break at the nut when dropped off the stage?  Why does the bridge sometimes pull off a guitar?  Why does the neck of a guitar sometimes need to be reset?  Why does a really thick guitar top give less bass response than a top of more typical thickness (a thinner top)?  These are questions that are – primarily – structural. This article is a discussion of the structure of the guitar.  It will cover areas where I think I can clarify the case and add information.  I have seen many ideas on the structure of guitars in print that are simply wrong. My main subject is bracing.  Next will be a discussion of strength and failure (and a few other diversions.)  I will try to give both the technical explanation and some practical advice that a non-technically inclined builder can apply.

I am a structural engineer and have been working in the civil engineering, aviation, and medical device industries for 25 years.  My engineering work has been, in essence, ensuring that structures are strong (or stiff) enough.  I am also a guitar maker since 1998 (I’m building my 59th guitar) and a fingerstyle guitar player.  I have found that my engineering experience has helped my guitar building in many ways, such as reading plans, making jigs and tools, and understanding the stiffness and strength of the guitar.  This is my $0.02 as an experienced structural engineer.


Figure 1 - The Guitar Idealized As a Simple Structure


What is a structure and why is the guitar a structure?  Basically every thing in the world that stands up on its own is a structure.  A house is a structure.  So is the Golden Gate Bridge , a playground swing, a bicycle, a bicycle wheel, an airplane, a pop can, a computer, a CD jewel case, a CD.  A guitar is a structure.  It is structure that is designed to be held by a person, resist the tension of the strings, allow the strings to be plucked, allow the vibrating length of the strings to easily be changed, amplify the string vibrations, and project sound.  It is also a structure for which stiffness is an essential and defining quality.  Stiffness and strength are directly related.  As I will show, strength is much less significant than stiffness for the guitar.  A guitar, as an overall structure, can be simplified as shown in Fig. 1:  As a piece of wood restraining a stretched out spring.


Measuring Structural Factors.  

To explain how the stiffness of the braces (and the rest of the guitar) works, I need to introduce some engineering measurements that are used to define the stiffness of structures.  These measurements are section properties and material properties.  Section properties are inherent to the shape of the structure.  Material properties are inherent to the material it is made of.  They act together to define strength and stiffness.  Because engineers understand section properties and material properties (and properly apply them), buildings don’t fall down and airplane wings don’t fall off.

Most people have a tactile, “horse-sense” understanding of section properties:  If a wood plank is taller or wider, it will be stiffer (all other things being equal – this caveat applies throughout; I’ll not repeat it.  In real life, things are never equal.)  A thick Sitka spruce trunk is stiffer and stronger than a thin one.  A ¼-inch thick guitar top is stiffer and stronger than a 3/32-inch thick top.  These are all differences in section properties.

Section properties are simply the exact way that engineers use to measure a “stiffer than” or “stronger than” structural shape.  Section properties are determined exclusively by the shape and size of the structure.  Section properties are not affected in any way by the material – this is the key point.  A two-inch by four-inch plank will have the same section properties whether it is made of steel or plastic or butter.  Section properties are essentially perfectly understood by engineers and have been tested and confirmed millions of times.  They are simple to compute and formulas may be found in the suggestions for further reading (SFR) at the end of this article. Please note that a number of topics that are introduced in this article are discussed in much more detail in some of these suggested readings.

Separate from section properties the other important factor is material properties.  A two-inch diameter steel pipe is much stiffer than a two-inch diameter plastic pipe (both pipes have identical inside diameter (ID) and outside diameter (OD)).  This is the material effect.  Because the ID and OD are identical these two pipes have exactly the same section properties; they are different in stiffness (and strength) because they are made of different materials that have different inherent material properties. These inherent material properties are also precisely measured by engineers.  The primary material property of interest here is the modulus of elasticity, abbreviated as E, and generally referred to as “Young’s Modulus.”  The details on Young’s Modulus and how it is determined and used can be found in the SFR.

The material properties of wood are quite variable and hard to predict.  The difference in stiffness and density from one piece of (one species of) wood to another can be dramatic.  Therefore, no formula for sizing wood parts is practical where stiffness is critical.  Extensive testing of each individual piece of wood would be required. For wood (and most other materials), the strength and stiffness vary in a linear way with density (specific gravity).  See the table of density versus strength for various wood species below, and the plot in fig. 2.  The plot is a straight line.  The averaged data are linear.  In more homogeneous materials such as metal, the data conform even better to a straight line.  Table 1 lists strength and stiffness material properties for common lutherie woods .  These values were taken from an online reference from the U.S. Forestry Service, USFS Wood Research Lab: Note that the Young’s modulus listed is for the longitudinal direction (with the grain) since this is the only direction of interest here.  This is because all the main loads under consideration are along the length of the grain (with the minor exception of cross-grain deflections of the top; but these are driven more by the longitudinal deflections than by stiffness across the grain.)  Another reference work, Wood Handbook:  Wood As an Engineered Material: contains an extensive table of values (pp. 4-4 through 4-24) and also has nice discussions of wood strength behavior.

Table 1 – Wood:  Material Properties

Wood Species

Young’s Modulus (EL, Mpsi)

Specific Gravity (Avg)

Ben ding Strength (tensile, psi)

Transverse Crushing Strength (psi)

Western Red Cedar





Sitka Spruce





Engelmann Spruce





Honduran Mahogany




No Data

Bigleaf Maple









No Data





No Data

Indian Rosewood




No Data

Brazilian Rosewood




No Data





No Data




Figure 2 – Wood:  Density versus Inherent Stiffness for Various Species

The fact that the relationship between stiffness (Young’s Modulus) and density is inherently linear shows that just changing wood species doesn’t affect the stiffness to weight ratio very much.

The section properties of interest for us are the bending modulus (abbreviated as I; also sometimes called the “bending moment of inertia” or “second moment of inertia”, see SFR) and the area (abbreviated as A, sometimes called “cross sectional area”).  The area is simple for rectangular cross sections:  A two by four plank has area A = 2 inches X 4 inches = 8 inches2. Cross sectional area affects the strength and stiffness of a structure when it is pulled (or pushed) along its length.  Imagine a fishing line holding up a weight.  Think of 3 pound test versus 100 pound test monofilament fishing line.  The 100 pound test line has greater cross sectional area than the 3 pound test line – approximately 33 times the area; but the material is the same.  The greater strength (and stiffness) of the 100-pound line is due to it having larger cross sectional area, its section property is larger.

The bending modulus is a little more complex and has units of inches to the fourth power (in4).  This is described in some detail in some of the SFR.  Think of bending a 2 by 8 plank flat-ways (floor board); then think of bending it tall-ways (floor joists).  It takes much more force to bend it tall-ways, even though the area (and weight) is the same in both cases (8 X 2 = 2 X 8 = 16 in2).  This difference in stiffness is the effect of the bending modulus.  Ben ding modulus (stiffness) is especially dependent on the height of the section.  In fact, the bending modulus is proportional to the third power of the height of the section ( h3).  The simplest formula for bending modulus is for a rectangular section and is:  1/12 w h3.  Note that the stiffness is proportional to the width (w) and to the 3rd power of height (h3)  The 1/12 is a constant that defines the shape as rectangular.

An example:  If you take that 2 by 8 plank on edge and replace it with a 4 by 8, on edge, the area is twice as large.  And, the bending modulus is also twice as large – because the height did not change.  If you take that 2 by 8 plank and replace is with a 2 by 16, then, again, the area doubles.  But, this time, because the height doubled, the bending modulus goes up by a factor of 8 (23 = 8).  Note that the weight of both the 4 by 8 and the 2 by 16 is the same (A = 32); but the 2 by 16 is 4 times as stiff as the 4 by 8 when both are oriented in the tall-ways direction.  Weight follows area (A) directly.


The original (2 X 8) beam:

I1 = 1/12 (2)(8)3 = 85.3 in4

A1 = (2)(8) = 16 in2


The 4 X 8 beam:

I2 = 1/12 (4)(8)3 = 171 in4

A2 = (4)(8) = 32 in2


The 2 X 16 beam:

I3 = 1/12 (2)(16)3 = 683 in4

A3 = (2)(16) = 32 in2


This shows how important height is for bending stiffness.  A guitar top ¼ inch thick is 19 times as stiff in bending as a top that is 3/32 inches thick, while it is only about 3 times as heavy (assuming the same wood – a big assumption in real life.)  Guitar top vibrations are essentially governed by bending stiffness and weight of the top and braces. A little brace height or top thickness goes a long way.


Graphic Data On Brace Stiffness.

  In Figs. 3 and 4 you see section properties plots for four brace shapes:  Rectangular, triangular, trapezoidal (top width = ½ bottom width), and “paraboloid” which is the curved shape that most builders use, more or less.  What you see is that the area (and weight) of the brace changes “directly” with the width of the brace (twice the width gives twice the area and twice the weight).  The bending stiffness of the brace changes by the third power of the height (they fall on slightly different curves because the different shapes mean different formulas, although still proportional to h3.) The thing to remember is that the effect of brace height on stiffness is very strong and much bigger than the effect of height on weight (area).  If you want a light, stiff brace, make it tall and narrow.  If you want it less stiff and maybe heavier, then make it wider and shorter.


Figure 3 – Plot of Brace Height Versus Bending Modulus

Figure 4 – Plot of Brace Height versus Cross-sectional Area  

What Do Changes In Section Properties Mean? 

How does this math and technical junk apply to guitars?  As shown above, the thickness (section height) of the top is the dominant factor for stiffness.  The same thing is true of the braces and the neck and the fingerboard.  If you shave the height of a brace down from ½ inch to ¼ inch, then you have reduced the bending stiffness of that brace by a factor of 8 – it’s now 1/8th as stiff as it was before and the weight is ½ what it was before.  If you left the brace height alone and made the brace ½ as wide as it was before, then the stiffness is ½ what it was before and the weight is also ½ what is was.

Figure 5 – Relationship of Brace Height to Brace Stiffness and Weight

Ben ding stiffness along with weight (mass) controls how the top of the guitar reacts to the vibrations of the strings.  Whether from the twisting of the bridge on a flattop guitar or the up-down pumping of the top of an archtop guitar, the response of the top is dependent on the bending stiffness of the braces and top.  This is why so much effort is put into top thickness and the height, taper, and scalloping of braces.  When you adjust brace height or width you are adjusting the bending stiffness (and mass).

Figure 5 shows the change in bending stiffness for change in brace height for braces of a constant area, and therefore constant mass.


Stiffness and Vibration Response. 

The big question is:  What does changing the stiffness of the top, sides, back, neck, and braces do to the sound of the guitar? A good friend of mine is an engineer and an expert on vibrations.  She gave me a simple way to think about vibration and stiffness.  If you take nothing else from this article, I hope you find this useful:

If you want something to vibrate at a higher frequency, then you should make it stiffer or lighter, or both.

If you want to make something vibrate at a lower frequency, then you should make it less stiff or heavier, or both.

Please note:  As you make something stiffer, you tend to make it heavier (adding material to add stiffness) – this works against high frequencies (lighter would be better).  And, as you make something less stiff, you tend to make it lighter (removing material to reduce stiffness) – this works against low frequencies (heavier would be better, all things being equal). Therefore, the stiffness and weight relationships to height and width in Fig. 5 are important.  In general, if you want to make a plate (guitar top) vibrate at higher frequency, make it thicker and use tall, thin braces, which are lighter for the same bending stiffness.  If you want to make a plate vibrate at a lower frequency, make it thinner and use wider, flatter braces, which are heavier for the same bending stiffness.


How Stiff/Strong Should I Make the Brace

In his article entitled “On the Selection and Treatment of Bracewoods”,  American Lutherie #60, Ervin Somogyi wrote, “… I’d been careful only to use vertical-grained material [for guitar braces].  That was, after all, what everyone had told me to do, since the very beginning.  But I’d not done anything to determine whether any selections of vertically-grained woods really were the stiffest.  I did know, from previous experience of overbuilding, that you could make things too stiff.  And, likewise, too loose.  So, where to draw the line?”  Exactly; there’s the eternal question:  How stiff? After hearing the information presented here, luthiers always ask, “so what brace size/shape/height/width/material should I use?”  I always answer with a question, “What do you want the brace to do?” The braces at the neck end of the top are there for strength.  Upper bout transverse bars, usually the heaviest braces in the guitar, keep the upper bout area from collapsing under the string forces.  The braces around the sound hole reinforce this big weakness in the top, which is right in the line of the force of the strings.  (You would have a hard time picking a worse place to put that hole, from a strength standpoint.) The other braces, in my opinion, are there to control the vibrations of the top and, to a much lesser extent, to react the twist of the bridge (for strength.)  You could make a guitar with no top braces.  It would have to have a very thick top to not collapse.  It probably wouldn’t sound very good.  Clearly, some sharing of the load by the braces is needed in order to free other parts of the top to move more.  The top (and back) is a stiffened-shell type structure, like most structures that we use (airplane fuselage, airplane wing, house walls, floors and roofs, car bodies, boat hulls, etc.), for a very good reason.  A stiffened shell is the most weight-efficient way to make these structures.

I have no formula for a good sounding top/guitar.  I use my ears and tap parts all through the stages of construction so I can hear how they are reacting to vibration.  I think my guitars succeed quite well:  I am pleased with the volume, tone, and overall effect.  But I have no formulas.  I flex and tap, flex and tap – and listen.


What Shape Brace to Use? 

The choice of brace shape (rectangular, trapezoidal, triangular, “paraboloid”, whatever) is purely a matter of personal choice and working methods.  None of them would be noticeably superior for sound quality.  Observe all the different methods that have been used to brace very successful guitars:  X-brace, fan-brace, ladder-brace, lattice bracing, double-tops with foam or honeycomb cores, Kasha-style bracing, etc.  There are many ways to skin this cat; there is no silver bullet. A “paraboloid” (more or less a parabola or half-oval in cross section) brace shape looks nice, is easy to form, does not induce stress concentrations, and is resistant to dent damage (no sharp corners); but these imply nothing about its acoustic performance.  I use paraboloid shaped braces most of the time.  But for some designs, I use rectangular or triangular brace shapes.  You see every possible brace shape in successful existing guitars.  A rectangular brace or brace of other shape, as long as it has the same stiffness and weight and has the same “footprint” (attachment glue area) on the top should give an identical effect.


Figure 6 – Plot of Various Brace Shapes:  Stiffness Versus Height


Special Case: I-Beams. 

One special case of stiffness is the I-beam, which you have seen in steel construction.  The I-beam places most of the working material far from the center of the cross-section.  The I-beam section generally gives the best strength and stiffness for a given weight of material, given practical considerations like fastening it into the structure.  That’s why it is so common in steel construction:  The big cost driver in steel construction is weight of steel.

Most of the time, lighter is better, all things being equal.  Some people use I-beam shaped braces in guitar construction.  If you want really light and really stiff, this is a good way to go.  You can achieve the same effect as an I-beam by various techniques.  You can build (by gluing strips) or carve a true I-beam shape.  You can also laminate a brace.  If you laminate a much stiffer material (carbon fiber, hardwood, fiberglass, etc.) to the top and bottom of basic softer brace material, you have essentially the same thing as an I-beam.  (The top wood itself can act as an I-beam flange; but adding a stiffer laminate works better.)  It looks like a rectangular section, but because the top and bottom laminates are so much stiffer, you can think of them as being wider (not taller) than the rest of the brace.  It’s important to laminate the stiff material to both the top and the bottom of the brace.  Otherwise, you will add little to the stiffness and will just shift the stiffness center of gravity of the section – for which there is no “luthierly” reason.


Strength and Structural Failure. 

Strength follows stiffness very closely.  People generally think of strength and stiffness as the same thing.  The important aspect of strength is:  How strong is the structure compared to the load it must carry? The usual ways that guitars fail are well known.  The headstock breaks off – the crack usually starts right under the nut, the bridge pulls off, the top cracks parallel with the grain, the neck block slips raising the action.  The guitar has plenty of strength to carry the loads of the strings and general handling – the loads for which it is designed.  I have run the analysis on the normal loads on a guitar and the applied stresses are very low, far below the capacity of wood and glue. The margin of safety is large.  Guitar failures occur when loads or conditions are applied that the guitar was not designed for. In my experience, guitar structural failures are caused by two things:  Accidents and weather.  Any of the thin plates on a guitar are obviously susceptible to impact damage.  If they are pressed in a direction they weren’t designed for, they will break.

The headstock breaks off because the guitar gets knocked off its stand and gets whacked in a direction it was never intended to take a load (accident).  It breaks at the nut because that is usually the smallest cross-section of the neck – it has the least strength at that location.  There’s also often a glued joint at the nut area.  Joints are often structural failure points.  They are natural failure points because of changes in section, changes in materials, and stress concentrations.

The top cracks because it was taken into too low a humidity environment (weather).  The wood tries to shrink with the dropping humidity but is prevented from doing so.  It cracks to relieve the stresses induced by the shrinkage.  Cross-grain (transverse) strength is much lower than strength in the grain direction (longitudinal.)

The bridge pulls off because the guitar was left in the car (“just a few minutes,” “I came right back,” “it wasn’t that hot out” – this is weather.)  The glue was heated to the point where its strength fell below the strength required to react the string tension.  Essentially all materials get weaker as you heat them.  I never leave a guitar in a car – it will either be stolen or damaged by heat/cold.  A good rule of thumb:  If the temperature and humidity are comfortable for you (want to sit in that trunk for 3 hours?) then they are OK for your guitar.  If not, don’t do that to your guitar.

Slow permanent deformation of structure under load is called creep.  A neck slip might be caused by creep; but this is very unlikely in my opinion because glues used in guitars are not very susceptible to creep and the typical applied stresses are very low.  Instead, this is almost certainly heat and humidity cycling working on the glue, or a one-time high temperature event (“guitar in the car” again).

Choose good quality wood.  Use fresh, good quality glue.  Make your joints clean and tight and well clamped.  Keep your plate thicknesses, neck section, and brace sections within the normal proven ranges.  You will never have strength problems.


Loads and Stresses on the Guitar.

We are going to look at the internal loads and stresses on some significant locations on the guitar.  Before tackling these, we need to identify the internal loads applied to the guitar.  The applied load on the guitar is the tension of the strings.  This load is distributed (carried) through the guitar as internal loads:  Axial loads, shear loads, and bending loads (also called moments), which are defined briefly below and are discussed in more detail in the SFR. 

Note that all objects deflect (compress, stretch, or bow/bend) when loaded.  The way they carry the load is by deflecting:  like a spring.  These deflections are mostly so small (Young’s modulus is large compared to the load) that you don’t notice them.  Deflection of the guitar (the top, mostly) is what produces the sound.  No deflection, no sound.  But excessive deflection will change the load paths, often resulting in collapse.

Internal loads are the loads inside the structure, not applied from outside. The internal loads react the loads coming from outside the structure.  As mentioned, there are three types of internal load. Axial load is simple:  It is pull or push (tension or compression) acting along the length of an object.  This is tension in a rope, compression in a column.  Shear is also simple, at least in its simplest form. It’s the load acting perpendicular to the length of an object.  Shear is a little harder to explain because most people don’t have an intuitive feel for it. Shear loading is what makes your fence gate sag out of true and become a rhombus shape (not 90-degree corners), as in Fig. 7.

Figure 7 – Shear Loading and Deflection


  Bending moment is more complex.  Ben ding moment is computed as the distance from the load to the point where the moment is computed multiplied by the load (Fig. 8).  Units are distance (inch) times load (pound):  Inch-pounds:  in-lb or in-#. Think of holding a 20-pound weight suspended from the end of a 4 foot long beam.  Then think of holding up that weight at the end of an 8 foot long beam.  It will be twice as hard.  The bending moment is twice as large.

Figure 8 – Cantilever Beam:  Illustrates Ben ding Moment

Shear load in Figure 8:  PSHEAR = P
Axial load in Figure 8:  PAXIAL = 0 (zero, there’s no load acting along the length of the beam)
Ben ding moment in Figure 8, at the support of the beam (left end):  M = P X L
At other locations along the beam, the moment is different, but still P X L.  The distance, L, changes at other locations, so the product P X L changes proportionally.

 All the calculations for the guitar that follow assume that the strings are light gauge D’Addario phosphor bronze strings with total tension of 163 pounds for standard tuning (tension data for other strings can be found in an article on the D’Addario website:  The additional tension applied to the strings by plucking them and fretting them is small compared to the static tension and can safely be ignored in these calculations.  All of the stress calculations use conservative (small) areas and bending moments of inertia.  So the stress numbers are a bit higher than would be expected in real guitars.

Stresses are computed using the internal loads (PAXIAL, PSHEAR, M) and the section properties A and I.  Axial stress is computed simply as the load (P, pounds) divided by the area (A, in2):  P/A = stress, units pounds per square inch (psi).  Shear stress is also computed  as P/A, units psi (symbol t, refer to Figure 7).  Ben ding is more complex.  Ben ding stress is computed:  moment (M, units inch-pounds) times c (distance to the location where the stress is computed in the beam from the neutral axis of the beam, units inch) divided by the bending moment of inertia (I, units in4):  Mc/I, units psi.

Figures 9 through 14 give detailed internal loads and stresses for important locations on the guitar.  Note that all stresses are rounded to 2 significant figures.


 Figure 9 – Loads On the Headstock


Peak compression stress (at top edge of fretboard) = ~640 psi  (bending + compression)

Peak tension stress (at bottom of neck) = ~490 psi  (bending + compression)

Assumes entire neck section is effective and no support from the truss rod.

Figure 10 – Loads On the Middle Part of the Neck (side view)


Stresses similar to those quoted for Figure 9.


Figure 11 – Loads On the Neck At the Heel


FAXIAL = Compressive stress in the guitar top at neck joint:  ~820 psi

Assumes 2-inch width of the top is effective:  A = 2.0*0.10 = 0.20 in2   P/A = 163/0.20 = 820 psi

FSHEAR = Shear stress in the glue at the heel:  ~30 psi (assumes 1 in2 effective)

Figure 12 -- Loads On the Body At the Bridge (side view)

Top stress similar to Figure 11.  Back and sides stresses:  Very low.

FSHEAR = Shear stress in the sides = ~ 163#*sin(2degrees)/((0.07 inch)(3.5 inch))/2(two sides) = 12 psi

Figure 13 – Loads on the Guitar Top at the Sound Hole (top view)


Open hole stress concentration factor = ~3.0

FAXIAL = Compressive stress in the guitar top near the soundhole = ~3.0 * 820 = 1600 psi


Figure 14 – Loads on the Body at the Bridge (top, top brace, and bridge only shown)


Shear stress in the bridge glue:  ~33psi

Peak tension stress in the bridge glue:  ~98psi


Assumes 1 inch long by 5 inch wide bridge contact;

string height above top at saddle = 0.5 inches

FSHEAR = 163#/5.0 in2 = 33 psi

FBEND = Mc/I = 82 in-#(0.5 in)/(5/12 in4) = 98 psi


Note that I do not calculate the bending stresses in the top and top braces.  These stresses are highly dependent on the design of the guitar and will therefore vary a great deal.  (They are also too complicated for the scope of this article.)  However, I have run some numbers and the stresses are low compared to the strength of the wood and glue.


Grain Orientation.

I have often seen the admonition that the grain direction in the braces must be vertical – 90 degrees to the surface of the top or back plate.  There is no structural justification for this (except that gluing a plate to the free edges of the brace grain could conceivably inhibit splitting in the brace.)  Ervin Somogyi’s test data mentioned earlier support this conclusion.  Note that neither of the U.S. Forestry Service documents referenced earlier lists separate strength values for tangential and radial grain directions:  Just the transverse direction (perpendicular to the grain.)  Very few braces ever get very far from square proportions (height and width equal):  The height is rarely much more than twice the width.  Therefore, there is no structural reason to choose one orientation or another.

Unless the grain spacing becomes close to the smallest dimension – what I’ll call the critical dimension, of the part (brace, top plate, back plate, whatever) the part can’t tell the difference between one orientation or the other. I think that the vertical direction is favored for braces because vertical grain is easier to plane, trim, scallop, and taper and perhaps for aesthetic reasons.  Grain direction could become significant for very small braces.  Most braces are taller than they are wide.  Therefore a horizontal grain direction, rather than vertical grain direction, would be more homogeneous.

In contrast with braces, vertical grain becomes very important for tops, sides, and backs.  This is because the grain spacing becomes close to the critical dimension (the thickness) of the plate.  If the thin plates of the instrument had flat-sawn grain, the wood would no longer behave like a homogeneous material.  The difference (in density and strength) between the late wood and early wood is often very large and would become apparent in uneven stiffness in different areas of the plate.


Surface Quality. 

I have regularly seen claims that the smoothness of the internal parts of the guitar (how smoothly sanded the braces are, etc.) causes measurable/perceptible differences in sound quality.  Don’t believe it.  Fine sanding of braces is done for reasons of visual or tactile aesthetics or for traditional concepts of workmanship.  There’s nothing wrong with that.  Just don’t expect any acoustic changes.  Whether you sand to 150-grit, 320-grit, or 600-grit will not affect the stiffness of the braces and therefore has no way to affect the generation of sound.  The sound frequencies of interest to the human brain have wavelengths on the order of 1 to 50 inches (frequencies of a couple hundred Hz up to around 10,000Hz).  There is no physical mechanism by which sanding scratches of much less than one one-hundredth of an inch width/depth could have any perceptible effect on the sound waves.  The sound wave cannot “feel” (be affected by) features this small.  Conceivably, if the surface were left so rough that the bracing was absolutely fuzzy with wood fibers, it might damp the vibration.  But short of major fuzzy (think fake fur glued on), there will be no effect on the sound.


Tapering, transitions, and joints. 

Be very careful with all joints.  They are a natural failure point.  Bad joint design, bad gluing, old glue, bad surface condition, big changes in section, and poor clamping can all cause a joint to fail. Very sudden changes in section lead to failure.  This is because of an effect called stress concentration (see SFR).  Just one example:  A 2 X 6 with a 2-inch hole in its center will fail in tension (pulling straight along its length) before two 2 X 2 boards of the same material, pulled lengthwise together.  The area is the same; but the hole causes the stress to be higher at the edge of the hole (imagine water flowing around a rock in a stream) and therefore the piece with the hole fails at a lower load. Sudden, big changes in section should be avoided.  Smoothly taper joints if possible.  Use long scarf joints.  Use a scarf joint instead of a butt joint.  Taper the ends of braces.  Round off the corners of components.  All these tend to reduce stress concentrations and make a more reliable structure. If you end a brace in the middle of a plate and don’t taper the end, a lot of that brace mass is carrying no load (useless mass) and the end creates a stress concentration (sudden change in section) that will likely be the location of the first crack in the plate.


Personal opinion. 

I have found this to be true for all structures and for guitars as well:  Lighter really is better, all things being equal.  My experience is all with acoustic guitars; mass may have some intrinsic benefit in electric guitars.  It seems to me that lighter acoustic guitars generally sound better than heavier ones.  I have found that the most important factor in achieving a loud, good-sounding guitar is using top wood that is very stiff, particularly in the cross-grain direction (others will disagree.)  A couple of my older guitars had significantly heavier tops due to low stiffness in the top wood.  The resulting guitars did not sound as good as my guitars with stiffer tops.



Material properties and section properties are the two components of stiffness and strength.

Changing materials will change stiffness.  Generally, stiffer materials are also more dense (heavier.)

Tall, thin braces are stiffer relative to their weight than wide, flat braces.

If you want something to vibrate at a higher frequency, make it lighter or stiffer.

If you want something to vibrate at a lower frequency, make it heavier or softer.

Grain orientation isn’t structurally significant until grain spacing approaches the part’s smallest dimension.

Surface smoothness has no effect on stiffness or strength (within reasonable limits.)

Guitars generally have plenty of strength to carry their loads.  Just don’t expect them to carry extraordinary loads:  From accidents and weather (excessive heat, cold, moisture, or dryness.)  I think I have indicated how important stiffness is.  Proper tailoring of stiffness produces a satisfying sound.



Suggestions for Further Reading

Structures:  Or Why Things Don’t Fall Down, J. E. Gordon (there’s a recent printing from 2003 by Da Capo)

Engineering the Guitar: Theory and Practice, Richard Mark French, Springer

“Forces On Archtop Guitars”, Franz Elferink, American Lutherie Number 74, pp. 30, 31

 The web encyclopedia Wikipedia has a number of good, stable articles on some of the topics discussed in this article:

 For more on Young’s Modulus:

For more on bending moment of inertia (I):

For more on bending: Ben ding

For more on stress concentration:

For more on shear stress:

For more on tensile stress:

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© 2015, James W. Blilie, Barbarossa Guitars                        5997 Turtle Lake Road, Shoreview, MN 55126                        This page was last updated:  7-Feb-2015