Neck Block Shift and Soundboard Shear

Neck Block Shift

This article is intended to draw attention to what I believe to be an intrinsic design flaw found in many acoustic guitars. In my experience, some of the most dramatic negative results of this flaw can been seen in center soundhole, X-braced, 12-string guitars as seen in the following photo of a 1996 Guild D25-12 NT. You can read more about the disassembly and rebuilding of this particular guitar in my article » 1996 Guild D25-12 NT Makeover.

However, as I hope to demonstrate, neck block shift can be an issue regardless of where the soundhole is located, regardless of brand, and regardless of the number of strings involved.

In the worst case examples two (2) catastrophic failures have occurred: The neck block has shifted away from its intended position and, as a direct result of that shifting, the soundboard has literally sheared off into the soundhole.

I have stated many, many times that I believe it is just a matter of time before a given traditionally-built acoustic guitar will require a neck reset. I have also stated that I do not believe this should continue to be the case! I believe it is entirely possible to construct an acoustic guitar that never requires a neck reset. For all my scientific friends out there who lovingly wait to pounce on my spurious claims, I realize this is entirely unprovable, thus it will remain a theory on my part, because "never" is not an experienceable, measurable occurrence. :-)

I have written articles documenting neck resets on Guild 12 strings. One article in particular is dedicated to the topic, in which I go into more detail about the topic of neck resets in general, regardless of brand. Rather than repeat all of it again here, you are welcome to read the article » The Infamous Guild Neck Reset.

In short, there are two primary factors that contribute to the need for a neck reset:

Neck Block Shift
Soundboard Bellying

The front plate (top, soundboard) and the back plate of a typical wooden acoustic guitar are held apart by the thin, shaped sides. The most common approach to joining the two sides together involves the use of a Neck Block and a Tail Block. You can read more about these components in my article » Neck Blocks, Tail Blocks and Linings.

In its simplest form, the body of such a guitar is comprised of 6 basic components:

The Soundboard
The Back
The Bass Side
The Treble Side
The Neck Block
The Tail Block

Lest I be accused of over-simplifying acoustic guitar construction, there are a few more components needed to complete the body: Kerfing (or tentalones, or solid lining) is attached to the inside front and back edges of the sides to give the front and back plates something to be glued to. The sides are often reinforced with tape or bracing to mitigate splitting. The back plate (the "back") is typically braced, or may be domed and left brace-less (by laminating and/or pressing). The front plate (the "top", or "soundboard") is also braced and may contain other structural elements.

NOTE: Guitars built using the so-called "Spanish Heel" method, where the sides are let into slots cut into the heel of the neck, do not have a separate Neck Block. The "head block", as it is called, is simply an extension of the neck. As incredulous as it may sound, such instruments can still suffer from what I refer to as a "Neck Block Shift", which I will endeavor to explain, later in this article.

NOTE: Depending on design, some non-traditionally built guitars have dramatically reduced the risk of the Neck Block Shift issue, with some having potentially overcome it completely. Guitars built with one-piece rounded backs, such as Ovation/Adamas or Canna, do not have separate Sides, Neck or Tail Blocks, per se. Guitars having offset soundholes, such as Batson or McPherson, incorporate a different approach to bracing. And so on.

The Neck Block Shift

The neck block may be the single-most critical component of the acoustic guitar, being the "hub of the wheel", the structural intersection of the back, sides, soundboard and neck. A neck block is not supposed to move in relation to the other components. The primary force acting upon the neck block is the result of the constant pull of the strings at the bridge. The strings are anchored to the tuning machine heads which, in turn, are secured to the headstock which is a section of the neck which is either glued on or bolted to the neck block.

The neck block may suddenly rotate forward in response to a sudden change in load and/or a sudden failure in structural support. In the worst-case scenario, the soundboard will split along one or both sides of the fretboard extension, and literally shear off into the soundhole. Stop to consider that, on all center soundhole, X-braced guitars there is NOTHING in front of the fretboard extension except 12.5 square inches of missing soundboard. It is into the abyss of the soundhole that the neck block, the neck attached to the neck block, and the section of the soundboard glued to the neck block all seemingly seek to plunge.

After a sudden shift forward of the neck block, there may be a spring back to the original position which may, in turn, leave little visual evidence of the trauma. If there is a split in the soundboard (or two splits, one on either side of the fretboard extension), this may or may not be apparent, depending on the severity of the shift.

In addition to a crack in the wood, there may be a visible mis-alignment at the soundhole, as you can clearly see in the photos, below. Sometimes such splits are not noticeable, not right away, and sometimes not ever. And other times there is such significant forward movement that not only is the soundboard jutting out into the soundhole, but the neck has moved forward into the body, curling the binding in with it. You can tighten and loosen the strings and watch the deformation occur in real time. Severe damage, such as you see in the photo of the 6-string Martin D-45, below, is an example of sudden, catastrophic failure.

This unrequested realignment of components most always occurs as the result of glue failure, resulting from some combination of temperature issues and string tension. We have all heard the tale of a guitar left in a vehicle on a hot day. Glue softens and parts slip, slide and shift. Guitars saved from hot cars and allowed to cool may have all their adjoining pieces of wood continue to be perfectly glued together - and yet those parts have shifted away from their original position(s). That is not a good thing.

Heat is a primary facilitator of glue failure, but excessive moisture can also cause failure. In other cases, too little glue may have been applied, and the bond simply fails under load. Such failure may be partial, where two components remain attached to one another, but are now misaligned. A partial failure is due to what is sometimes referred to as a "creeping" nature of some adhesives. A complete failure can also occur, where two components are now completely separated from one another.

Whatever the extent of and reason for the glue failure, the result is a neck geometry change, a forward-shifted neck. The strings pull the neck and neck block in the direction of the bridge. If the glue has been softened, or has let go altogether, then the wood pieces move. Depending on what releases first, and how long the guitar is subjected to the conditions that promote the damage, a shearing of the soundboard may or may not accompany the "Slip."

So, which do you think is more likely to be the cause of this Gibson's condition: a gentle relaxing and re-aligning of glue joints or sudden, catastrophic failure?

These neck block shifts and soundboard shears definitely occur on jumbo and dreadnought sized guitars, both 12 string as well as 6 string instruments. But what about small-bodied guitars?

On the parlor guitar shown in the photo below, a Guild F-20 from the 1980s, everything appears intact: the fretboard is still attached to the neck and the fretboard extension is still glued down to the soundboard, the neck is still attached to the body, even the pickguard is still right where it was originally attached.

But the damage has been done on this small instrument that features a lower bout width of a mere 13-3/4 inches (That's just a fraction of an inch wider than a Martin 0-18). The soundboard has split along the grain, shearing off and plunging forward into the soundhole.

What if there is no splitting of the soundboard? Does that mean that everything is fine? Look at this next example, which does not have any visible splits or cracks along the fretboard extension, at least, not yet. In the first photo, below, look at the stress fractures in the finish emanating from the neck heel/body joint out across each side of the upper bout.

Here is a second image of that same guitar. Note the collapsing of the soundboard (beneath the fretboard extension) into the soundhole as a direct result of the forward shifting of the neck.

As I mentioned earlier, guitars built using the "Spanish Heel" construction method can still suffer from a forward shifting neck, even though there is no formal (separate) Neck Block. To illustrate this, here are photos of a guitar built this way, having no center soundhole.

There is tremendous tension applied to the structure of the steel string guitar. We know that a chain is only as strong as its weakest link. A similar understanding must be held for the structure of the acoustic guitar. If there is a weakness, longitudinally, between the Headstock and the Tail Block, given the opportunity, it will be exploited under string tension. On this particular instrument (shown above), the weakest point was to be found in a section of the soundboard directly in front of the fretboard extension (the same place a soundhole would begin on an X-braced guitar). You can clearly see the crease where the soundboard has compressed and buckled under the tension, permitting the neck to shift forward. The action is high and the saddle is already buried in the slot. Since the guitar has been built using Spanish Heel construction, and the components are assembled with epoxy, a neck reset is not viable. This unfortunate situation renders a 2 year-old guitar at "end-of-life."

Shift Happens

I have been writing and droning on about this issue for years in the hopes that others will gain an understanding of what I believe to be a common design failure, a mistake that can be corrected, preventing this catastrophic mishap.

There is far too much movement potential in the design of many (most?) acoustic guitar upper bouts. Guilds, Martins, Gibsons and loads of other makes and models have all shared this design flaw at one time or another and are prime targets for this catastrophe. A sudden change, such as a guitar falling over or out of a stand, or being pulled out of a tight-fitting case by grabbing the neck and yanking it free, or even by being dropped while still in its case may be enough force to shift the neck block forward and split or shear the soundboard. Then again, as we can observe in the photos of the 12 string (shown at the outset of this article), a guitar may sit in its case, strung to pitch and be undisturbed for years, and a shift can occur. It is important to realize that we are talking about extremely small measurements of movement, but dismiss this movement at your (or your guitar's) peril.

Imagine your reaction if the wheels on your car were to fall off after {n} number of miles. I am happy for you if neither you nor anyone else is injured or worse, and thrilled if you have a current Roadside Assistance program and can get back on the road with little interruption to your plans. But, just in case you were never informed, wheels are NOT supposed to fall off cars! Acoustic guitar neck blocks are NOT supposed to shift, and their soundboards are NOT supposed to crease, collapse, split or shear!

Countless instruments continue to suffer the fate of a shifting neck block, with and without the oft-accompanying soundboard shear. Why? How did it all get started? Hasn't anyone tried to stop it? Is there anything we can do, or has all hope been lost?

Mitigation

Once upon a time, guitars suffered an even more dramatic fate than neck block shift and soundboard shear. It was known as a buckling and collapse! When the first steel string instruments began circulating, replacing their cat gut / nylon string siblings, builders and players alike were able to share in a new experience where guitars collapsed, imploding under the profound string tension. They folded in half upon themselves like a wooden beam snapping under a heavy load.

Long, long ago in a shop far, far away, a lone luthier looked down upon the wreckage of an imploded guitar and thought, "Maybe if I glue this monster brace across the upper bout section of the soundboard, that'll fix everything." And the transverse brace (upper face brace) was invented, just like that. At least, that is how I imagine it was invented.

Word spread and soon luthiers throughout the land were incorporating the transverse brace into their own guitar builds, thinking to themselves, "What a great idea." And it was a great idea, because fewer guitars were completely collapsing and folding in half than ever before. Some even thought that if one transverse brace is good, two has to be better (see below)! And the people were happy.

The idea behind the use of the transverse brace that runs from side to side across the upper bout, just above the soundhole, is that if sufficient support is provided, the fretboard extension (and the soundboard beneath it) won't plough downward under string tension.

But the transverse brace didn't stop neck blocks from shifting. And it didn't stop soundboards from shearing.

Less long ago than the first long, long ago, and in a shop a little bit closer than the shop that was far, far away, a lone luthier looked down upon the carnage of a soundboard sheared off deep into the soundhole and thought, "Maybe if I glue this little strip of wood across the upper bout section of the soundboard, in the only space left between the transverse brace and the neck block, that'll fix everything." And the upper transverse graft, or "popsicle brace" was invented, just like that. At least, that is how I imagine it was invented.

Fun fact: This extra strip of wood is reportedly found on 12-fret Martins dating back to the 1850s and yet it wasn't added to 14-fret Martins (guitars having greater string tension) until around 1939.

And others went and did likewise, thinking to themselves, "What a great idea." And it was a great idea, because it was fun to say "upper transverse graft," and maybe even more fun later to say, "popsicle brace." More importantly, it didn't stop neck blocks from shifting. And it didn't stop soundboards from shearing. But hey, just be happy your guitar didn't fold in half!

And so guitar makers everywhere added the transverse brace and popsicle brace to their designs and everyone everywhere could now purchase guitars with these new innovative features. And neck blocks still shifted. And soundboards still sheared. But guitars (mostly) stayed in one piece.

And the people were happy.

And a guitar maker here, and a guitar maker there apparently grew concerned about the fact that soundboards continued to shear, because strange additional braces began to appear in the upper bout area. The area that once was occupied by the transverse brace alone, was now occupied by the transverse brace, the popsicle brace and additional braces.

The Wing Grafts

Let's Talk Guild forum member GardMan has pointed out that sometime around the Summer of 1974, Guild makers added two "wing" grafts, popsicle-style, to the otherwise unoccupied area of the underside of the soundboard in the upper bout. Depending on make and model, these patches range in size, from a healthy 3/4" width down to a paltry 3/8" wide.

They run diagonally from the transverse brace near the neck block out toward the centers of the shoulders of the upper bout, resembling outstretched arms or wings (hence the moniker: "wing" brace). It is interesting to note that when a soundboard shears along the fretboard extension in one of these guitars, it always happens between the edges of the fretboard extension and these diagonal brace(s). Their positioning/placement begs the question: If sufficient force and/or inertia applied to the neck block/upper bout area were to telegraph along these braces and be concentrated at the soundhole, would this not contribute to a shearing of the soundboard along on or both sides of the fretboard, instead of preventing it? Regardless, any positive contribution of the wing grafts is questionable, at best. But one thing is clear: they are utterly useless at stopping a soundboard shear, as the photo below clearly demonstrates.

The Neck Block Extension

During that same Summer of 1974 and the addition of the wing grafts, a design decision was made at Guild to add a small block of wood to the top front of the neck block on 12-string models, forming a ledge or shelf, purposefully extending the neck block. In theory, this would add more real estate to glue the soundboard to, well above and beyond what you would expect to find in the traditional 6 string guitar neck block. The extra buttressing would potentially aid in resisting the pull of the strings on the headstock and the resultant tendency of the fretboard extension to want to depress the soundboard, pushing it in toward the back of the guitar, as the cantilever of the neck tries to pivot at the body joint, essentially folding the body in half. Sounds good, right?

At whatever point the neck block suddenly shifted and/or tilted forward, it simply took the section of the soundboard that was glued to it along for the ride, shearing away from the adjacent wood of the soundboard. This occurred in spite of all that extra buttressing of the additional ledge added to the neck block, or the support of the transverse brace, or the addition of the wing grafts.

You may notice a theme developing.

And we must also recognize the Soundhole Support Plate, or "soundhole patch." Many builders decided to replaced the ubiquitous "3 Sticks" soundhole bracing with a thin veneer than sat between the transverse brace and the X-brace. Such an application is certainly warranted if the reasoning is to support the thin soundboard in the area where the giant hole has been cut. This makes even more sense if you understand how much soundboard is removed when large rosettes of shell and plastic or wood purfling are installed.

But I don't believe everyone shared the same reasoning behind the initial introduction of the plate. It was thought that this would help to counter the shift and shear issue. Once again, we have an implementation of a design feature based on a false premise (more on this in a moment), as demonstrated over and over again. Below is a photo of one of these soundboard plates or patches still appearing to be perfectly intact. The only problem here is that the soundboard has sheared clean into the soundhole, ignoring the wing grafts, the upper transverse graft ("popsicle" brace), the upper transverse brace and, as though it wasn't even present, the soundhole patch.

The A-Frame

A more recent design that seeks to improve on the popsicle brace/wing grafts is seen in the A-Frame brace, where two sticks are glued to the soundboard running between the transverse brace and the neck block. They splay out and across the parallel grain of the soundboard, crossing the point where the fretboard extension lays (on the other side of the soundboard). The idea behind this design appears to be that the force applied by a shifting neck block could be distributed to some point outside(s) of the soundhole. Enough force supplied by a forward-shifting neck block could produce a distortion in the soundboard at the intersection of the base of the legs of the "A" and the X-Brace but, overall, it is not a bad idea. But is it enough?

Let's look just a bit deeper into the force(s) at work behind the shearing of the soundboard.

Traditionally, acoustic guitar fretboards have been made from very dense hardwoods, such as Ebony and Rosewood. The grain direction of the fretboard runs lengthwise from the headstock, down the neck and out onto the soundboard. It will expand and contract from side-to-side when exposed to moisture variations. Traditionally, the fretboard extension is glued down to the soft Spruce or Cedar soundboard beneath it. The grain direction of the soundboard also runs lengthwise, and it will also expand and contract from side-to-side when exposed to moisture variations. The soundboard is glued down to the neck block.

The neck block's grain direction is perpendicular (rotated 90°) to the soundboard and fretboard. Traditionally, the neck block is positioned in such a way as its greatest potential for expansion and contraction will occur in the direction of the movement of the sides. This can get a little confusing as, technically, any significant expanding and contracting of quartersawn sides is occurring radially, in relation to the annular rings. The neck block glued to those sides is expanding and contracting tangentially. As a result, there is potential for greater movement in the neck block, up and down, than in the sides glued to it. Of course, wood moves in more than just one direction. In this photo I have drawn a double-headed arrow indicating the direction the neck block will expand and contract due to moisture variations (radially, in relation to the annular rings). Let your imagination be your guide, and consider what may occur should the moisture content change. The fretboard and the soundboard are trying to move along one path, albeit at different measurements of movement, and the neck block is trying to move along a path that is perfectly 90° to the grain direction of the soundboard. It doesn't take much movement to result in a problem.

Soapbox: It's Damage, not Mojo!

There is a place in the life of the struggling musician (or any entrepreneur) where budget constraints dictate, well, everything. There are those of us who may desire more than we currently have, but we can't always have all that we desire. C'est la vie! If I could make a suggestion, however: Resist the temptation to justify legitimate structural damage to your guitar, damage such as a soundboard shear, by referring to it as 'Mojo.'

When a stress crack appears in the foundation of a high-rise, it is not referred to as "Mojo." When a stress fracture appears in the engine mount of an aircraft, it is not referred to as "Mojo." When a crack runs across the windshield of your car, you don't call it "Mojo" (well, some of you might, but you shouldn't). The shearing of an acoustic guitar's soundboard is called "Damage," not "Mojo."
To repair damage to a foundation, it usually involves more than a little glue, as does the repair of an engine mount, or a cracked windshield. Not wanting to spend the money to effect such a costly repair is perfectly understandable, and drives so many decisions we are all witness to. Who would ever want to spend money, voluntarily, if they could just ignore the problem? It is not as though lives are depending on the structural integrity of my acoustic guitar!

As an owner, when my guitar is structurally damaged, my choices are fairly straightforward: I can choose to do nothing about it, and live with the damage. I can spend money to repair it (or attempt to repair it). Or I can start fresh with another (undamaged) instrument.

As a builder, perhaps I can invest some time into better understanding the issue.

Identifying the Problem

To begin to understand what is allowing a soundboard to shear on most traditionally built, center soundhole guitars, we need look no further than the soundhole, itself. To put things in perspective: when tightened, guitar strings are quite capable of ripping the bridge off the face of the guitar. A 4 inch diameter hole has been cut into a thin wooden plate in front of that bridge, in a location that, in the case of a 12-string guitar, is supporting well over 200 lbs of pressure/tension (in some cases, 250+ lbs).

In relation to the body of a guitar, the neck is a cantilever. It is affixed to the body at the upper bout. On many (most?) acoustic guitars the neck joint can act as something of a hinge, allowing the neck/neck block/entire upper bout (as a unit) to tilt or pivot forward. This action will result in the concentration of force onto the precise location of the soundboard where 12-1/2 square inches of very needful support material is now non-existent. A 4" diameter hole resides where that wood used to exist. What could possibly go wrong? The fact that soundboards having no soundholes, or significantly offset soundholes, do NOT experience shearing along the fretboard extension is clear evidence of the actual problem.

I believe that most of the steps taken to mitigate the neck block shift have focused on the hinge aspect of the cantilever neck that I mentioned, above. It is assumed that the neck is hinging at the neck/body joint, and therefore the fretboard extension must be supported by the massive transverse brace, else the neck will fold forward. To help visualize this, see my illustration, below.

And yet, neck blocks still shift forward, and soundboards still shear along the fretboard extension.

A body of evidence has piled up around us over the decades, evidence that clearly demonstrates the design of the support of the upper bout of most acoustic guitars is flawed. Believe it or not, the force that is responsible for the neck block shift and soundboard shear is NOT the hinging motion I am showing in the photo, above.

(I think I have used the word "forward" at least ten times in this article up to this point. That might provide you with a clue as to where I am headed.)

Instead of insisting that the neck is pivoting at the edge of the body, understand that the neck is attempting to plunge FORWARD into the body of the guitar, toward the bridge. Very little, if any attention has been paid to this fact, as builders everywhere jumped on the hinging motion bandwagon and never looked back. Me included. The first couple of guitars I ever built had transverse braces and fretboard patches and wing braces, oh my! Back then I lacked the understanding to see how, by just emulating yesterday's designs, I was doing nothing to resist this forward shifting potential.

Stop that Shift!

Stop the shift before it starts. There is a worthy saying: An ounce of prevention is worth a pound of cure. Mitigation efforts have consistently failed due to the assumption that the force to be countered is downward.

NOTE: By the time the downward movement is occurring, or even about to occur, the damage is already done!

The neck has already shifted in toward the body and it is too late to do anything about it. By understanding that the force to be countered is forward, one can realize the folly of adding more cross-bracing in the upper bout and begin to explore the design changes necessary to stop the shift before it starts.

CLUE: If we prevent the neck from moving forward, it CANNOT hinge downward!

If we prevent the neck block from shifting forward, the soundboard cannot shear. If we prevent any forward movement on the part of the neck / neck block / upper bout, we never need to reset the neck (unless there is a problem with the bridge/bracing that causes the soundboard to lift, but that is an entirely separate issue).

To better support a center soundhole design, it is necessary to first address the structural integrity of the guitar. One approach to a solution reinforces the upper bout. Traditionally, by (wrongly) focusing on the hinging potential of the neck, a multitude of unnecessary bracing has been added to the design, which does not prevent the Neck Block from shifting (or soundboards from shearing) as we have seen.

The neck must be prevented from shifting forward. It must be prevented from movement of any kind.

Immobilizing the neck block requires re-thinking its relationship to the entire upper bout, if not the entire body of the guitar. If there is any potential for movement, once conditions are right, movement will occur. If the upper bout can move, it will. I have shown that this can affect any guitar, regardless of soundhole placement. If a neck remains perfectly flat, with no forward bow, and the guitar develops a high action, what caused it? If the bridge on that guitar has not moved, and if the soundboard on that guitar has not lifted ("bellied"), what is responsible for the high action? In other words, when your guitar needs a neck reset, the predominant reason will be that the neck block has shifted forward, away from its precise location when the guitar was first constructed.

In 2014, by focusing on the forward shifting potential of the Neck Block, not its downward, pivoting potential, I developed a successful take on the "upper bout reinforcement" approach. By unifying the soundboard above the waist, the back, the neck block, and the sides into one (nearly) immovable mass, I eliminated the potential for forward movement. It is certainly not the only way to address the issue, and it has trade-offs, but my guitars built with this feature have not experienced neck block shifts. There are no soundboard shears/splits along the fretboard extension. More importantly, there have been no neck resets required.

Prevention

Call me "old-fashioned", but I believe that to effectively prevent a catastrophe I need to understand the cause.

When I build a center-soundhole, X-braced soundboard, I remove what would otherwise be the sole remaining structural support that had a whisper of a chance at resisting the forward movement potential of the neck / neck block into the body. I MUST do something to restore that support, or I will continue to suffer neck block shifting and soundboard shearing. It may take decades for it to appear, but it will appear.

In the case of the sinkhole in the photo, above, support is required BENEATH the road to support the WEIGHT that is pressing DOWNWARD.

On my guitar, the soundhole is not actually a sinkhole (though it can become one, given the proper circumstances), as the issue is not about weight pressing downward (though that is exactly how it has been treated). Rather, there is a constant lateral force being applied that is sufficient to distort/compress a perfectly round hole into an oval, and that force resides at 321 My Neck Block Lane. Using the sinkhole analogy: my car is stopped at the edge of the hole, brakes applied. A big truck has pulled up behind me, bumped into me, shifted into low gear and pressed down on the accelerator pedal.

I am not suggesting this is an issue for Nylon string guitars. The relationship between the forces and components is different. Using my silly truck-car analogy, in an exaggerated comparison, you can reverse the order of the vehicles and visualize the difference.

I altered the build design of my center-soundhole, X-braced acoustic guitars to eliminate forward movement of the neck into the body by modifying three aspects of traditional design:

Adequate reinforcement: I reinforced the entire upper bout of the soundboard above the X-brace.
Interlocking components: For additional insurance, I "locked" the soundboard into the sides.
Adequate adhesive: I used a non-slipping epoxy to fuse these critical components together.

I have found that laminating one or more hardwood Mahogany veneers to the underside of the softwood soundboard is sufficient to support anything I can throw at this guitar, I am careful to ensure that none of my straight-grained layers are parallel with one another.

For the soundboard in the photo, above, I have set the laminated support back from the rim of the soundhole, giving the guitar a more traditional appearance when viewed from the front. Below is another reinforced soundboard in which I have bound the soundhole.

Once my new soundboard is complete and ready to install, I "lock" that reinforced, laminated section into place. Instead of simply notching the sides (rims) to receive the upper arms of the X-brace, I "let in" the entire upper bout to the body by lowering (terracing) the sides from X-brace arm, around the upper bout and across the neck block, to X-Brace arm.

Knowing that heat plays a role and is a factor in many neck block shift scenarios, I attach this new soundboard using an adhesive that is both non-creeping and heat resistant: epoxy.

If you are concerned about the added weight of the lamination, don't be. I have eliminated the transverse brace, fretboard patch, wing grafts, and any other useless addition of wood to the upper bout. With a 2-layer wood lamination (three layers total, including the soundboard), the total weight ends up a bit lighter to what it would have been with a traditional design. The soundboard can be made even lighter by replacing the two layers of Mahogany with a quasi-isotropic carbon fiber plate. Remember, in addition to losing the extraneous upper bout bracing, I also remove material from the sides, kerfing, and neck block (to lock the upper bout in place.

NOTE: Earlier, I mentioned a trade-off of this design. I realize that this approach has immobilized a large section of the soundboard, specifically an area of the upper bout that has never been known to produce the majority of the sound we associate with the acoustic guitar. Any alteration anywhere on any soundboard will affect its sound. The question should be, "If anyone even notices a difference in sound, will they like the sound of my guitar after I have immobilized the neck block?" I have had no issues with this.

Rigidification

If "ri-gid-i-fi-ca-tion" it is not yet a proper lutherie term, I vote that it should be. In my endeavors to eliminate the negative results from neck block movement, I have accepted the need to immobilize the body shell as much as is reasonably possible. This starts with the sides, themselves. For maximum rigidity, these can be laminated using a vacuum press or clamped into a form. Using a pre-bent solid wood outer layer (the same wood you would have used for a solid side, just thinned a bit more), along with two (2) or more very thin inner layers, a very stable shell is formed that maintains its shape outside of a body mold.

Laminating hardwood neck and tail blocks stabilizes them against seasonal effects. Laminating the linings adds even more strength. My linings are made from 4-ply laminated Spanish Cedar.

Tying the neck block to the tail block using carbon fiber rods takes the model a step further. For center soundhole soundboards, it is not advisable to run rods directly from neck to tail, unless you are willing to sacrifice access to the interior via that soundhole. Instead, the rods can be diverted to the waist, such as may be seen in luthier Rick Turner's approach for his Compass Rose guitars.

This is how I brace my own center soundhole soundboard guitars (see below). One pair of rods runs from each side of the neck block to a block attached just below the waist. Two additional rods, one on each side, run from the waist to the tailblock. Six rods in total are used to stabilize the neck block. All the blocks are laminated Spanish Cedar and/or Mahogany.

Eliminating the potential for forward movement of the neck block results in eliminating the need to reset neck geometry caused by neck block shift. Additionally, by eliminating neck block shift, we eliminate the shearing potential of the the soundboard.

Beyond the Center Soundhole

While I still build the occasional center soundhole guitar for the discerning client, I eventually moved beyond center soundhole designs, joining a growing contingent of luthiers who had chosen to explore the possibilities of "Life beyond the X-brace." There are several fantastic instruments that have been built over the last few decades as a result of efforts of these pioneering guitar makers.

Moving the soundhole out of the path of the strings introduces several benefits:

Structural Support: Reintroducing long-grain wood where a massive sinkhole would otherwise be cut creates the opportunity to further explore the forces acting upon the guitar.
Good-bye Soundhole Shear: And good riddance. There is nothing to shear.
Bracing Design: A larger canvas is available upon which to paint a soundscape. Not having to think about the sinkhole opens a whole new world of bracing design.
More Wood to Vibrate: The upper bout, once unredeemable sonic wasteland, is freed up to contribute to the overall responsiveness of the soundboard.

The potential to produce a more responsive guitar having even greater sustain should be reason enough to consider incorporating this design into your own builds. Returning strength to soundboard in front of the fretboard extension allows for exploration of new bracing patterns, as it is no longer necessary to compensate for the weakness introduced by the soundhole. Additionally, by immobilizing the neck block, removing the ability for it to shift forward, neck resets can become a thing of the past.

Question: Can a steel string guitar built without a center soundhole still experience a Neck Block Shift?
Answer: Yes.

Question: But how?
Answer: Put simply, Compression -> Collapse.

Do not underestimate the pull of those strings, being exponentially more of an issue as the the string tension increases (heavier-gauge strings, longer scale lengths, more strings: 7-string, 12-string, etc). Though a shearing of the soundboard is so unlikely as to be (I won't say impossible, but I will say) improbable, the slightest forward movement of the neck block can still cause trouble. If even the smallest FORWARD movement of the neck block is permitted, the typical heeled neck will simply hinge at the edge of the upper bout and rotate DOWNWARD, deforming the soundboard, altering the neck geometry, raising the action, and necessitating a neck reset. Relying solely on conventional (traditional) means to reinforce the soundboard against that lateral movement (such as by adding a transverse brace) will not prevent the issue from occurring. I have witnessed this too many times.

I remain convinced that it is worth the effort to work toward the balance of immobilizing the neck block while supporting the freedom of movement of the soundboard necessary to produce the most pleasing and enduring acoustic guitar ownership experience. There are so many possibilities . . .

Summary

I applaud any and every attempt to immobilize the frame of the acoustic guitar. The structure that supports the soundboard must be rigid. By eliminating the potential for movement of that structure, failure of that supporting structure due to the pull of the strings can be completely prevented.