My top construction gives a characteristic sound that is distinctive and easily identifiable, and consistent even between different styles of guitars and different woods.
Obviously I want the top to have a good bass response without sacrificing the trebles. There have been several approaches to solve this problem from other builders, some of which involve splitting the bridge so that the treble and base strings oscillate the bridge separately. (Kasha et al)
My design keeps the bass and treble strings hooked to the entire bridge because I want the entire top to respond to every string. The bridge is wider than most to distribute the energy directly across the top grain and to engage side braces coupling the width of the vibrations more completely. Tight stiff tops favor treble response. Loose tops favor bass response but sacrifice definition and sustain. A top that is stiff in the center, under and around the bridge and tapers off to very thin edges can be stiff enough for the local, short coupled treble vibrations beneath the bridge and still allow the entire top to flex under the influence of bass strings. This design copies good acoustic speaker design with flexible margins to enhance the bass, and a relatively stiff center section for the tweeter action.
It is also important to keep the bridge from twisting over from the torque produced by the pull of the strings. Stiffness under the bridge spreads the torque out over the top. At the edges of the top the force from the twisting bridge is no longer vertical but rather puts the edge in tension. The top wood can resist a pulling force much better than it can resist forces at right angles to the grain. So, to prevent the bridge from moving, it is necessary for the top to be stiff under the bridge, but not necessary for it to be stiff at the edges of the top.
The diagram shows the bracing pattern, which is fairly traditional looking. It employs an x-brace and with tone bars and a bridge plate. The shaping of the x-braces and the use of an equalization brace behind the bridge plate execute the plan.
Notice that all braces that tie into the X-brace have their highest parts located nearest the center of the guitar. That is where stiffness is needed. The braces are also tall and thin. Strength is much increased with increased height, and only slightly increased by making a brace wider. Therefore, tall thin braces have less mass and more stiffness. There is a little scalloping on the x-brace but not much. Most of the “scalloping” is below the bridge near the lower edge. Scalloping is more of a corrective measure than a design element. Ideally, there should be no scalloping at all, just a gently tapering brace perfectly proportioned to support the load with minimal mass. text.
I like to use padauk for the bridge plate for several reasons. It is quite musical. Tap a plank and you will see that it rings like Brazilian. At the same time, it is lighter than most really hard woods, so it fits the good stiffness with minimal mass criteria rather well.
The top is arched like the side of a barrel. It is mostly cylindrical. My form is cylindrical, and the sides of the body are sanded down to a cylindrical arch to match the top.
In the view to the right you can see the equalization brace below the bridge plate. It is quite thin and rather high in the center but tapers quickly to terminate in each brace. It was recommended to me by an excellent Luthier (my brother Stephen Kinnaird in Nacogdoches Texas) and having tried it, I like it. The articulation of each note seems enhanced and it is consistent with my dual design philosophy of stiffness with out mass and graduating plate reinforcement.
I believe that the neck plays an acoustic as well as mechanical role (a false dichotomy anyway). Necks need to be stiff. They need not to vibrate. They need to transmit all of the mechanical energy of the string to the top and not squander any of it in small oscillations of the peghead and the neck itself. The goal is to make a neck that has a very high modulus of elasticity. An adjustable truss bar does not appreciably affect the modulus of elasticity. It moves the plane of vibrations back or forth to compensate for string tension but it does not itself resist those vibrations. The adjustment itself, that a truss rod affords, is not a bad thing, it just isn't a sufficient thing. What is needed is a neck that is both very stiff and very light. (Lightness seems to be a mantra that players recite when trying out a guitar.) Carbon Fiber comes to the rescue. The stuff is expensive but it is light and very stiff.
My necks come in two basic lengths: twelve fret and fourteen fret.
Both necks now have truss rods to facilitate control of neck relief.
Carbon Fiber reinforcement is buried on either side of the truss rod channel. These bars extend through the joint between the peghead and the neck, traditionally the weakest point of the neck most prone to breakage.
The 14 fret neck does have a truss rod in addition to the carbon fiber reinforcement. This is a concession to the dreadnaught crowd that feels more comfortable knowing that they have some measure of control over neck relief. The neck is over-engineered. This gives the builder a feeling of comfort.
The truss rod can be seen next to the neck. It is a two-way adjustable rod from LMI. The channel into which the rod fits is flanked on either side with a bar of 1/8 x 3/8 carbon fiber. The bars extend out into the mortise to engage the eventual spline that fits into the mortise. There is a lot of reinforcing strength in the spline that is fastened with epoxy into the mortise. The spline then fits into the heel block about 3/4 of an inch. This internal strength allows for a very narrow external neck heel which, on a cutaway is nice because it just isn't in the way when playing up.
I enjoy the feel of a sharp chisel gliding through hard wood. Carving the neck is one of the pleasurable parts of making a guitar that I would never turn over to a computer driven router or even a pantograph. Each neck I make is a little different. It is a derivative of the interaction between the wood, the tool, my mistakes and corrections, and even a little momentary inspiration as I work with the medium. What you get is a hand made neck. Much the same can be said about the rest of the guitar. I bend the sides by hand over a hot pipe, etc. etc.
Beginning the process of revealing the hidden peghead within a block of Mahogany.xt.
Peghead is mostly liberated at this stage. The volute is perhaps too big. I like it but may reduce it later. This is a neck for a 12 fret 000.
I make mostly bolt on necks. The reason for this has to do with my guarantee. Neck angle is guaranteed for life (mine). I want to be able to get the neck off easily if the occasion arises. Tops are notoriously prone to vertical fluctuations and the neck needs to be able to keep up with that. (C.F.Martin's first boss, a German, had designed some necks whose angle could be adjusted with a key inserted into the heel of the instrument.) I have never had a bolt on neck fail, and I believe that this mode of attachment is gaining wide acceptance in the guitar making fraternity.
The Spanish put the heel block in upside down. There is no need for a large internal heel extension glued to the back to counteract neck torque. Witness all the high tension steel string guitars that get by just fine without one. But, what really needs support is the fingerboard extension. I personally do not like to see that part of the fingerboard behave like it was independent from the rest of the fingerboard. On guitars that depend on the top to support the FBE, that seems to be the inevitable outcome. If the heel block has a well supported cantilevered extension that goes out to the first major top cross brace (above the soundhole) then the FBE can be kept in line. The block would look like a Spanish heel put in upside down.
Koa guitar in the mold showing the laminated heel block. There is the mortise on the outside surface which accepts the spline from the neck. Bolt holes are barely visible on the inside surface. Obviously I glue on the back before the top. This block does not extend out as far as some of my later models which may go about 3/4 of an inch deeper toward the soundhole. I have noticed absolutely no diminishing of the sound by this kind of block. Guitars made this way have been my best sounding instruments (a fact that has more to do with successful top bracing than the heel block design). What I have noticed is rock steady FBE's.
Here is the latest rendition of the laminated heel block. It has a great reach under the top and it is thin and light and strong. This block will be machined and upcoming photographs will show how the fingerboard and heel attach.
This is the laminated heel block. I make my plywood out of thin sheets of Mahogany. Great ply.
This block will be about the thickness of the lining and unlike the larger solid block, it is in no danger of splitting.