Guitar maker's notebook

The unseen luthier's art -- Brace yourself!

It's ironic that one of the most important elements of an acoustic guitar is a part that's rarely seen by the player -- the bracing.  Here's an example:

 

In an earlier post, I wrote about the plates and how the material is carefully selected and prepared to be light, stiff and responsive.  It seems a bit counterintuitive that we would then glue on other bits of wood to the inside surface.

Fact is, the plates, and particularly the top plate, are under enormous stress.  The steel strings are under high tension and attached to the bridge, located in the middle of the top plate.  Were it not for some carefully located structural elements, the top plate would fail.  One of the primary goals of bracing is to supply structural strength to support the bridge area, but in such a way that the bridge can transmit the string vibrations into the plate.  It's a balancing act.

There are a large number of bracing patterns used by guitar makers, but for steel string guitars many of them incormporate what's often called the "X brace".  As you might expect, the brace elements are crossed in a lap joint and resemble an "X".  The lower arms of the X cross under the bridge to help give structural support while retaining the ability for the bridge to rotate front to back, and thus "drive" the top.  Also under the bridge, you'll find a patch of thin hardwood, which provides structural support to bridge area, while also providing a solid surface where the balls on the ends of the strings can have direct contact with the top, without causing damage.

Above the soundhole, one will usually find a beefy transverse brace which stiffens up the area where the finger board is usually attached, preventing distortion of the upper bout when under tension.  Around the soundhole itself, you'll also find small braces or an extra layer of top material in the form of "donut" which provides to stiffness to the perimeter of the hole and some protection against cracking of the end-grain.

Other smaller, light braces are also usually located to provide additional stiffness to targeted areas of otherwise be unsupported.  These provide some protection against cracking along the grain of the plate, and their stiffness and location can be adjusted to affect the tone and balance of the sound.

Generally speaking, we want the braces to be stiff, but without adding excessive mass to the vibrating top plate.  When preparing the braces, the maker will usually orient the grain 90° to the surface of the plate, and will make the brace taller than it is wide, all to maximize stiffness of the brace, while minimizing mass.  The braces can be further "tuned" by removing material from areas and decreasing the height of the brace where strength is less important.  This is known as "scalloping".

All of the brace elements work together with the materials and design of guitar to give the instrument its individual sound.  What might seem a random pattern of sticks glued to the underside of the top has an enormous impact on the sound.  You can't see the guitar maker's skill in this department, but you sure can hear it in the finished product.

In the photo sequence below, I'll take you through the bracing of an OM-style instrument. 

 

I've glued a circular patch over the area under the soundhole, just a hair larger than the rosette, and I'm thicknessing the patch with a drum sander.

 

Here the sound hole "donut" and the soundhole itself have been cut out.  The other bracing elements are dressed to rough dimension, and then glued on to the underside of the top plate.  Here you see a clever contraption known as a "go-bar" deck, where dowels are used to clamp the elements as they're glued.

 

Once all the elements are glued in place, comes one of my favourite steps in building a guitar -- shaping and scalloping the braces.  My weapon of choice, a 1-inch chisel.  By carefully shaping and whittling, while periodically tapping the top to monitor changes in the top's responsiveness, the braces take shape slowly.

 

And this is the final result.  Almost a shame to hide it inside the guitar where no one will ever see it! 
 
 
 

"My guitar has an adjustable neck, but I can't get the action low!"

I wonder how many guitar technicians have heard that?  Worse still, how many clients come in AFTER they've broken or stripped the "adjuster"?  Truth is, very few acoustic guitars have adjustable necks, but almost ALL modern-day steel-string guitars have an adjustable "truss rod".  Believe me, NOT the same thing at all.

Guitar set-up was on my mind this week, as I did the final set-up on my "Gemini" guitars -- matching 6- and 12-string dreadnoughts -- see the Gemini photo album.  Personally, I never want to see one of my guitars come back with a damaged truss rod, so I thought I would blog a bit on the subject. 

"So, just how does someone get a "good action" on a guitar?"  To reiterate, you DON'T get it by cranking on that truss rod adjuster!!  Of course, I'm assuming that the guitar doesn't have any fret issues or other problems that could cause noises.  But even brand-new, high-end factory guitars in a music store will require some additional set up to play properly.

What is commonly referred to as "action" decribes the height of the strings above the fingerboard.  If they're too high, it takes more effort to press the string down to "fret the note" and the guitar is hard to play.  If they're too low, the strings will vibrate against frets and cause all manner of unpleasant noises.  The height of the strings is determined by the height of the nut slots at one end, and the bridge saddle at the other end -- turning the truss rod adjuster doesn't affect either of these!

"So what DOES the truss rod do, and why is it adjustable?"  The truss rod is embedded into length of the neck shaft and is used to fine-tune the straightness of the neck when it is under string tension.  The picture show a neck blank where I've routed a slot to receive the truss rod assembly that is sitting loose on top.  While the string tension will cause the neck to want to bend slightly, the truss can be adjusted to apply counteracting pressure.

 

"So, you adjust it so that the neck is perfectly stright, right?"  Well, in fact, no. A small amount of "relief" is required so that the "action" can be set low, without the strings vibrating against frets.  We generally check the truss rod setting with the strings tuned, and a capo placed on the first fret.  We then fret a string on the 12th or 14th fret, and observe the "relief" or gap between the string and the 6th or 7th fret.  If there's no gap, either the neck is too straight, or maybe even bent backwards!  We're looking for a 0.010 to 0.020" (0.25 to 0.5 mm) gap, depending on playing style -- about the thickness of a standard business card.  To decrease the gap, we turn the truss rod adjuster a little bit clockwise, to increase the counteracting pressure.  We ALWAYS measure the change, and if it doesn't change, we don't just keep cranking -- a stripped adjuster or broken truss rod is usually a very expensive repair!

 

"My neck relief is within spec, but the guitar is still hard to play!" With the neck relief adjusted, the technician will then measure the gap between the strings and the 1st fret, and the 12th fret.  These indicate how much the string slots in the nut need to be deepened (or filled!), and how much the bridge saddle needs to be raised or lowered.

The gap at the first fret should be around 0.010 to 0.020" (0.25 to 0.5 mm) -- again about the thickness of a business card, and the height above the 12th fret 0.060 to 0.090" (1.5 to 2.3 mm), depending again on playing style.  If the gaps are way off, the adjustments must be made to the nut and saddle in steps, as lowering the saddle, will also tighten the gap at the first fret.

 

"OK, my guitar is now really easy to play, but never seems to be in tune -- what's wrong?"  You may have noticed that saddle on an acoustic guitar is on a "slant".  The length of the high E string is shorter than the low E string.  We call this "compensation".  When we fret a note, the string is slightly stretched.  Compensation helps to adjust for the effect this stretch has on the pitch of the fretted note.  Guitar makers use various methods to figure out the average compensation across all strings, and build this into the slant of the saddle.  But, that alone is not perfect.

If you play the open harmonic at the 12th fret, and then play the fretted note, they should sound exactly the same -- I actually measure the pitch with an app on my iPhone.  If they don't, we say that the "intonation" must be adjusted -- effectively lengthening or shortening the string at the saddle to fine-tune the compensation.  I won't describe the method in detail, as it's not something a novice should attempt -- but if you really must do-it-yourself, drop me a line and I'll point you to a tutorial on the net.

After checking the intonation string-by-string, I ended up with my 12-string saddle with a bunch of guide marks, telling me where to file the "string-break" peak for each string:

  

I then removed the saddle, filed the break points, polished the saddle, and then reassembled the guitar.  You'll notice that the break points for the high octave strings on the 3rd and 4th pairs are at the leading edge of the saddle, and the lower pitch strings "break" over peaks toward the back edge.

  

Now this is 12-string that is easy to play -- AND plays in tune!