Thoughts on Structure and Tone in Acoustic Guitar Design

This is the talk I was going to give at the Santa Barbara Acoustic Instrument Celebration, but when I got there, there was nothing in the program!  Oh, well…

For the visuals, you can cut to the chase:  http://www.rickturnerguitars.com/pdf/05_FJ10

 

and then read, or:

Acoustic Guitars…

Structure, Tone and how to Reconcile the Two…

September, 2016

By Rick Turner

Historical Background and My Own Journey

          I started my lutherie career as an apprentice repairman in the caveman era…1963…in a shop in Boston called the Stringed Instrument Workshop.  The work done was crude by modern standards; there was no ASIA, no Guild of American Luthiers, no Internet, and there were but two books on guitar making, both small, both from England, both written to show how to build a classical guitar with a wee nod to steel strings.   Yet we had guitars coming in then that showed some of the now-well known structural issues that plague guitar players and provide job security for luthiers.  Three of those problems were (and remain) the need for neck resets as the geometry of the instrument changes with time and stress; the classic top crack (or two) next to the fingerboard with the top shearing into the soundhole…an issue that I call a tectonic plate shift disaster; and the hump or ski jump where the fingerboard transitions from being on the neck to being glued to the top.  These are not new problems; they all have to do with how the neck intersects with the body of the guitars; yet very few luthiers have taken the time to deal with the issues other than overbuilding the upper bout or making necks easier to reset.

As I started to build acoustic guitars in the mid-1990s, I decided that if I were to do this, I wanted to “bring something to the table” other than just making nicely decorated traditionally built instruments in the Martin/Gibson lineage.  I’d owned (and still own) a number of Howe Orme instruments made in Boston in the late 1890s; I’d first seen them in my 1963 apprenticeship, and they made an everlasting impression for two reasons, both of which involve very clever engineering.  The great thing is that the Howe Ormes are not only engineered brilliantly, but they are built very well…up to the standards of Martin in the 1890s, and they sound both unique and great.  When I showed one of mine, a 12 fret 000 sized one in spruce and Brazilian rosewood to Martin Simpson, his reaction was, “Is this the best vintage guitar ever?”

 

Adjustable Tilting Neck

The two key features of the Howe Orme guitars are the adjustable tilting neck with the cantilevered fingerboard not touching the top and the “longitudinal belly ridge” where by the center third or so of the top is cylindrically arched giving it fantastic longitudinal stiffness allowing the top to be quite thin, yet stiff enough for steel strings which were indeed available in those days…a full 20 years before Martin made a production steel string guitar.   Note also that tilting necks were a feature on some guitars from the 1820s coming out of the Johann Stauffer shop in Vienna where C. F. Martin learned guitar making, so in my way of thinking, a tilting neck with cantilevered fingerboard is a traditional way to build!

Of these two Howe Orme design features, I chose to start with the tilting neck system. For one thing, it takes care of the whole issue of a neck reset; it takes longer to find the right Allen wrenches to do the adjustment than it does to do the job itself. You can literally reset the neck angle, and therefore raise or lower the action in about ten seconds. Two added advantages is that by not gluing the fingerboard to the top, you have the option of getting more vibrating area from the soundboard, and the fingerboard is no longer subject to dipping down or kicking up past the neck joint, particularly if you use carbon fiber under the fingerboard to stabilize the playing line of the fret tops.

I have modified the Howe Orme system which used a metal fitting at the heel cap as a hinge, and then two bolts threaded into the heel face under the fingerboard (which has a stiffener extension of the neck) which bear against the body close to the top.  The problem with the Howe Orme design is that as you adjust the action, you also change the overall playing string length; the two are locked together.  My approach is to replace the hinge with a bolt so the hinge point itself can be moved in and out.  This allows for a bit of overall adjustment of intonation as well as full neck angle and yaw adjustment.

 Structural Fingerboard

All of this brings up the whole issue of guitar playability from first fret to last.  When designing a guitar or bass, you have to start with a straight line…a side elevation representation of a typical string.  Add your playing length defining nodal points, the string nut and the bridge saddle, and do whatever you like with the string afterlength, nut to tuning peg, saddle to bridge or tailpiece.  Now the next thing is the very gently curved even line of the fret tops from first to last, curved for “relief”.  It is the job of the fingerboard below the fret tops and the neck below that to perfectly maintain the perfection of that relief line.  It should not deviate where the neck joins the body or where the fingerboard is (perhaps) attached to the top of the guitar, and no reasonable and predictable amount of guitar geometry shift, whether that’s from long term stress or from humidity changes, should change the perfection of the relief line of the tops of the frets.  This perfection of line over time is something that “traditionally built” acoustic guitars just do not maintain.  The fret at the neck to body joint define a hinge point, and sooner or later, the fingerboard extension will “drop off” over the body or ski-jump up.  We’ve gotten so used to this that it’s believed that we need to build in “drop off” over the guitar top.  In fact it’s just a workaround for a bad design; the neck and the guitar top support the fingerboard, and therefore the relief line so differently that we’ve just learned to jury rig the fix, often during a refret or fret level, crown, and polish.  And in neck resets, there is often the need to taper shim the fingerboard extension to get it anywhere near the correct relief line.

My solution to this has been to make what I call a “structural fingerboard”…I dado two 1/8” x 1/8” slots into the underside of the fingerboard and glue in two ½” tall x 1/8” wide carbon fiber bars from the nut end of the fingerboard to the very end.  A fingerboard made this way is stiffer than most completed necks, and since the stiffness is designed in to best support a constant relief line, playability is assured.

As for the carbon fiber over the body, I deal with it in two ways:

  • Relieve the underside of the CF for clearance with a cantilevered fingerboard. It’s amazing just how stiff the fingerboard extension remains even with the CF down to less than ¼” tall.  There is also no lack of volume of the notes played on the cantilever.
  • For a non-tilting neck, I cut two slots right through the top and as needed, into the neck block and any upper bout transverse braces so there is no change in structure for the fingerboard support through the neck joint area.

The Flying Buttresses

          Over the years I’ve repaired a number of guitars and a mandolin where one or two top cracks appeared next to the fingerboard and the top under the ‘board was shifting into the soundhole like a tectonic plate shift after an earthquake.  Clearly it took a glue joint failure in the upper transverse braces to do this with the guitars; most likely the instrument got hot enough for the brace glue to at least soften, and with a glue like Titebond, 130 F. is enough to do that.  It does not seem that a French slipper foot neck block nor massive bracing can prevent this collapse of the top under the fingerboard; the shear force of 160 or so pounds over years and with heat will do this.  I recently repaired a nice B. C. Rich dreadnought with this issue, and upon researching the guitar, I found that this is one of the better known failure modes for these guitars.  The worst I’ve fixed was a ‘70s Martin D-45 with buckled rosette, and the oldest were a Gibson A style mandolin, and one of my Howe Orme guitars.

 

So how does one design around this problem without just massively bracing the upper bout of the top?  For me, the answer was at the Cathedral of St Denis in Paris, the first of the Gothic cathedrals to feature flying buttresses to support the walls without requiring truss chords inside the building.   I’d been aware of the concept of flying buttresses, but it wasn’t until I saw the building and then stepped inside that I really got it how effective the engineering is.   The cathedral builders wanted no truss chords inside the building to interfere with the soaring lines of the ceiling which is 28 meters…92 feet tall.  The problem is that a roof with no truss chords (the horizontal beams tying one end of the truss to the other in tension) puts outward pressure on the walls, pushing them to collapse.  The answer was flying buttresses…supports outside the building directing the outward horizontal pressure down to the ground.  The Basilica of St. Denis was finished in 1281 AD, and after more than seven centuries, the walls still stand.  http://uk.tourisme93.com/basilica/high-gothic-architecture.html

 

Since the collapsing force on the neck block area of a guitar is inward, not outward, my solution was to put two or four carbon fiber tube flying buttresses inside the guitar from the upper part of the neck block down to the sides just below the guitar waist where the nearly in-line and vertical plane of the sides is extremely strong in compression and where the pressure will be distributed to the sides and not the top.  This allows me to put in very light bracing in the upper bout since the top is no longer tasked with supporting the approximately 160 pounds of compression from the tension of the strings.  Since I’m cantilevering the fingerboard above the top, that area is now able to contribute to the tone and volume of the guitar.  In tapping the upper bout vs. the lower bout of a completed guitar, I find a rather obvious “tweeter/woofer” type of response, and in listening to what these guitars actually do, I hear really wonderful harmonic sustain and an evenness of response that I really like.  The guitars are not necessarily louder than a more traditional design, though they do punch out well, but it’s that harmonic content that I really like.   If one preferred a more traditional Martin or Gibson sound, it would certainly be easy to block the upper bout with more braces specifically to control tone, but those braces would not be needed for neck or fingerboard support.

 

The real test of my approach to these structure issues was the guitar I built for Henry Kaiser to take to Antarctica in 2001 on his National Science Foundation Artist in Residence Grant.  Henry wanted me to design “a real guitar made out of wood”; using carbon fiber to reinforce the wood was fine, but Henry wanted this to be a part of the whole experiment.  Success or failure were both open options, and either would yield a learning experience for both of us.  This was to be a somewhat experimental guitar; I knew it wasn’t going to be a disaster, but I was aware that the climate might wreak havoc on it.   The guitar design itself was part of the grant proposal kind of summed up as “let’s see what happens.”  The climate even in the “summer” is brutal.  Outdoors can get down to minus 50 F. with humidity down below 5%.  Then the instrument…and people…go inside to normal room temperature, the snow they drag in melts, and humidity may go up to the low 20s for a while.   In other words, it’s guitar hell.

Henry spent close to two and a half months down there, and with the adjustable tilting neck, he was able to change the action for standard or slide playing with ease.  When he got back, the structure of the guitar was perfect.  The neck needed absolutely no truss rod adjustment, the action was just fine, and amazingly, though there were a few nicks, dings, and scratches, the basic integrity of all the wood was just fine…as it has remained for about sixteen years now.

To see the inside story on how I approach acoustic guitar design, check out this article that was published in Fretboard Journal a few years back:

http://www.rickturnerguitars.com/pdf/05_FJ10_Turner_all.pdf

 

 

 

 

 

 

 

2 thoughts on “Thoughts on Structure and Tone in Acoustic Guitar Design

  1. Caught your post on FaceBook. I have been playing guitar since 1971, own 12 guitars. I would greatly like to share my.ideas with other luthiers and designers. I see way too much adhesion to traditionality when it comes to guitar building, and i am one to go beyond preconceived expectations. In 1971-3 i knew a guitar designer named George Overholtzer, from Chico Ca. He built a Martin type guitar, submitted it to the NAM show in i think 1972, and won best of show. His process for pretreating Sitka spruce was the majic that won him that prize. His under study David Heitzman told me what this wood pretreating process was about. This treatment runs paralell to why Stratovarious violins became so famous, despite tbe fact that they were otherwise like many others built of that era.
    My quest is to discover the obvious of what works, as opposed to the established way of doing things. The development of the guitar has always been one of repressed efforts due to it’s acceptance by the unwashed masses as viewed by ivory towered elitists bent on maintaining the “Lute” as the instrument of choice by those elitists. It was not until 1956 when Segovia was finally able to prove to those elitest snobs that guitar is more thana percussion instrument! Currently have built a Franken resonator guitar using a Fender Squire “Mini Strat” neck. Initially the sound buzzed, and ratteled, yet now am getting this to have a unique sound very much with over tones of accoustic metal. I would love to share with you some of my ideas, and would love to participate in developing more instruments towards the alternatives.

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    1. Frankly, my eyes glaze over when I read about undocumented “magic” secret treatments which supposedly result in superior instruments. And Stradivari instruments, by the latest credible information, were as they were because of Strad’s genius at carving each piece of wood slightly differently to maximize performance. It wasn’t varnish or the canals of Venice or midnight mojo; it was just great understanding of each individual top and back set. And, unfortunately, Segovia grabbed a death grip on the development of the classical guitar; it could only be ONE thing. Now that he’s gone, the classical guitar has blossomed in form, construction, and function with the likes of Smallman, Redgate, Ruck, and the development of the double topped instruments, many of which would have been rejected out of hand by Segovia. Yes, he was a genius. He was also an autocrat.

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