In response to musicians and makers who have asked about my
bracing etc. I will try to outline some ideas behind
my instrument designs

The art of making musical instruments involves infinite subtleties.
The information below is just basic concepts and physics.

Engineering, electronics and most modern design disciplines are driven by the need to be constantly better than what went before. Acoustic musical instruments on the other hand don't seem to have much evolutionary force driving their development. Most musicians want to play copies of the instruments that their heroes in the past played and few of them have an opportunity to find out what a really good instrument sounds like. Among the high end factory made instruments are some rather nice ones, but they don't compare to a guitar hand made by a good luthier. (This may sound biased, but it's an opinion I first heard from the creator of Maton guitars)

Greg Smallman

Once when I was arriving to do the sound mixing at a concert. I heard a loud classical guitar as I approached the hall and I thought they must have set up the sound system without me. When I entered the hall I saw two guitarists practicing a duet with no mikes. One of the guitars (a Greg Smallman type with graphite lattice bracing) was filling the hall with sound. The other (a high end factory guitar) was barely audible. This experience among others, has convinced me that Greg's approach to guitar making is probably the most important development in acoustic instrument design for a long time.

Having said that, I will point out that I have never tried to copy Greg Smallman's guitars. Rather I have found that every Greg Smallman and Peter Biffin concept that I have encountered has been relevant to my own exploration of acoustics and has become a dominant influence in my guitar designs. The ideas set out below are taken from many sources including my own experience.

Sound board and braces

I define the sound board not as the top of the guitar, but as a specific area of top around the bridge that has the function of converting the bridge energy into sound. In most classical guitars this area finishes at the brace below the soundhole, whereas in most steel string guitars it continues up to the big brace where the fingerboard ends. The size of the sound board and how it functions is determined by the braces. Three functions of the braces are to strengthen the guitar, to isolate the moving sound board from the rest of top, and to tranfer vibration from the bridge to the whole soundboard.

It starts with strings

Many people visualise the guitar strings pushing up and down on the top of the guitar. This is not the case. The main energy of the strings is produced by them changing length as they vibrate. That means that the string energy is moving at right angles to the direction you want the soundboard to move. A guitar with a tailpiece like a mandolin easily converts the energy by pushing the bridge straight in as the string shortens. The more the strings are angled over the bridge the harder it pushes. But because the downwards pressure is continuous, it damps the sustain and low frequencies as the angle gets steeper. This gives tailpiece guitars a distinctive percussive sound much loved by jazz players..

Phase

On the other hand guitars with a fixed bridge and no tailpiece have to convert the twisting motion of the bridge into energy in the soundboard. The most obvious problem is that as the front of the bridge moves down, the back of the bridge moves up. If different parts of the sound board move in opposite directions then no sound is produced. This effect is often called 'phase cancellation' or 'moving out of phase'. Quite a large percentage of the energy produced by the strings is lost by different parts of the instrument moving out of phase with other parts. The key to making a powerful instrument is to deal with these phase problems.

Control with braces

For fixed bridge guitars, the idea is to convert the rocking energy of the bridge into pumping the whole sound board up and down. To do this, a good bracing design will have strong support around the bridge. (Many common designs fail in this regard, as is evident by the bridge being twisted when you look at it from the side.) I always have the strongest part of the bracing just in front of the bridge. As well as anchoring the bridge it provides a tense area for the high frequencies. The sound board where it meets the sides should be loose, allowing the whole sound board to move up and down with the front of the sound board acting like a hinge. If the front moved up and down as well, it would move out of phase with the area behind the bridge. The sound board should actually lift slightly as the strings are tightened.

Tone ring

In my current designs, none of the braces actually reach the sides of the guitar. This allows some powerful bass to happen but makes the edges of the soundboard a bit fragile. The way I deal with this is to use a special heavy lining joining the top to the sides. This lining is a solid wooden construct that supports the sound board in the same way a banjo rim holds a banjo head. The weight of this construct adds to the projection of the whole instrument. Bluegrass banjos use a similar metal device to increase power and projection called a tone ring.

Speaker box

I make every other part of the body as solid as possible to reduce any phase cancellation. I use the thickest wood I can bend (about 3mm) and I make the back braces very solid. I have heard that Smallman laminates two backs together in a curve to remove all vibration. I find this a bit too extreme for my guitars but I do it on my mandolins. Designing the body to not vibrate is a principal of speaker box construction. Graham Caldersmith gives a good explanation of this.

Soundhole and body tuning

Another form of phase cancellation is that air passing through the sound hole moves from the outside of the sound board to the inside of the sound board. This air pressure moving back and forth cancels certain lower-mid frequencies. It really comes down to tuning the whole thing so that you don't lose an important frequency. Also the resonant frequency of the body itself (between 90 and 150 hz) creates a dominant overtone that becomes part of the guitars sound. (this is called the Helmholtz resonance) This tuning of soundholes and body chamber is something I won't go into here, but I would recommend looking at speaker box design as well as different guitar designs.

Bracing designs for a fixed bridge

Steve Klein's bracing (below left) uses 'flying braces' going from the bridge plate to the sides without touching the face. These make a hinge to convert the movement of the strings changing length into pumping the sound board up and down.

Graham McDonald's bouzouki does something similar by making the upper part of the X brace heavier and attaching it to the sides near the sound hole. This is an interesting example of mixing a central X brace with supporting lattice braces. Graham has produced some excellent books on bouzouki and mandolin making.

Below is a very simple but effective design I used on a small steel string guitar. I have isolated the sound board from the rest of the top with a tone ring made of heavy wood. (the sawcuts are filled to make it solid) The heavy side braces also help by linking it to the back.

Another element of this design is that my saddle (the part of the bridge that the strings actually sit on) sits directly on top of some main braces. (my saddle is level with the front of the bridge plate below) For some reason with standard Xbracing (over 90% of all steel strings) the braces only meet the front tips of the bridge. This results in a sagging bridge and upper-mid tone loss.

Having the sound hole in the sound board can be negative factor, like a hole in a speaker cone. In this design, I am making it part of a hinge that the soundboard swings on. That's why the braces near the sound hole are stronger and go closer to the tone ring.

Jim Redgate (left) and Gil Carnal (right) use the fine lattice that is typical of Smallman's nylon string guitars. You can see how Jim isolates the sound hole whereas Gil incorporates it into the bracing as in steel string designs. Note the resulting difference in sound board size. It is important to remember that more sound board doesn't mean more sound or more bass. Note the tone ring and stabilizing braces in Jim's guitar.

Physicist Michael Kasha and luthier Richard Schneider evolved a system of multiple tuned braces. The design has many good ideas, including having all the braces touching the bridge. Moving the soundhole right out of the way frees you from having to deal with the structural problems it causes, as well as removing most of the phase cancellation caused by the air moving back and forth through the hole. As you can see, the braces here look like a tuned set of resonators. A given string pitch will find the ones that move in sympathy with it.

Lattice braces, in total contrast to this, are so integrated that the braces aren't separate elements. It is better to think of them not as braces, but as a modification of the sound board effecting it's weight and tension.

I fact, before I got into lattice bracing I was experimanting with a braceless design that used a second layer of Cedar laminated to the sound board. My talamar has one of these tops. The sound is beautiful (you can hear it on my audio page) but it is not as powerful as a graphite braced instrument.

X bracing

My understanding of guitar history is that the X brace was first developed by Martin to deal with the extra weight of steel strings that tended to sag the soundboard in front of the bridge. The resulting extra bottom end made it a very popular way to brace a steel string guitar. As the guitars got bigger and the bridge was moved even closer to sound hole, (a bad move I think) the X brace design became pretty much the only effective way to brace a steel string. Currently almost all steel string guitars are braced like this. Scott van Linge has some interesting info about the shape of braces.

There are two basic problems with the X brace. One is that joining the central X by gluing the endgrain of one brace to the side of another brace is weak and vibrations are lost there. Another problem is that an X brace big enought to stabilize the top of a steel string guitar is quite heavy, making the instrument inefficient and needing a lot of power to drive it. Now consider that by using a very small amount of carbon fiber above and below the braces in an X join, both of these problems no longer exist.

Carbon fiber

When you laminate a straight layer of carbon fiber above and below a wooden brace with the two layers meeting at each end, the wood no longer has any real function except to hold the two layers of fiber apart. So the wood can be joined to any number of similar structures without any loss of strength. The form which Greg Smallman used is a lattice, which is an array of X braces.

Lattice

There are special properties in a lattice design. If you make up a reinforced lattice, (either free floating or on a sound board) you will find that it can only twist in a couple of ways. Every other form of flexing is prohibited by the lattice structure. The most common types of out of phase sound board movement (top to bottom and side to side) simply cannot occur. Adding this 'phase control' to the incredible strength to weight ratio of a carbon reinforced soundboard, you can begin to see the powerful potential of this design.

High note

At a folk festival some years back I met a young player with a Gibson f series mandolin. I loved the bottom end on the Gibson, but he was impressed by the mandolin I had, which had full power and sustain right up to the highest note. (something that was notably lacking on the Gibson) It occured to me that in all the years working with cabon fiber, I had never had to think twice about the top end of the instrument. Hard, light carbon constructs are a perfect resonator for high frequencies. By coupling that with a loose edge for bass response, (something I learned from the Gibson mandolin) you can have the full range.

I will continue to add information to this page and will try to answer questions