Guitar Design

The more I learn about guitar making, the more I am aware of how complex it all is. The ideas below are the ones that have helped me to best understand how musical instruments function and to get the sounds that I want from them. Other luthiers have different approaches and ideas that with time, dedication and hard work,
also produce excellent instruments.

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 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. 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 transfer 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. But 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. 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 cancellation

On the other hand, guitars with a fixed bridge and no tailpiece have to convert the twisting motion of the bridge as it is pulled and released into up and down movement in the sound board. 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 certain frequencies will disappear. This effect is often called 'phase cancellation'. 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. One key to producing more sound is to deal with these phase problems.

Tension zones

One way of reducing phase cancellation is to have different tensions in the sound board zones around the bridge. An excellent example is the fan bracing used on most nylon strung guitars. The braces above the bridge are closer (and therefore tighter) than the braces below. Below are some samples from this interesting article. The design on the left has gone a step further than the standard fan bracing (right) by being asymmetrical sideways as well as just top to bottom. This means that it will also reduce phase problems from the bridge rocking side to side. Note that most of the fan braces pass directly under the bridge, making the braces an energy link between bridge and sound board.

Lever action

Another important action created by a fixed bridge is to lever the top using the upper edge as a hinge and pumping the whole soundboard like a bellows. X bracing allows almost the whole top of the guitar to be used in this way. Martin X bracing was first used in the mid 1800s on nylon strung guitars, but it wasn't until the higher tension steel string guitars came along that it started to really work. The large X strengthens the weak spot below the soundhole, but more importantly it creates a bass speaker function over the most of the top, while still allowing other zones to function as smaller speakers.

The ringing sound that is characteristic of the heavy X brace system is some thing that is sought after by rhythm players and some flatpickers, but without heavy strings and a thick top the braces don't work together that well. As you can see only the main X has any real joint between braces. (and that joint is weak in one direction) Allowing any brace to move as a separate entity on the sound board can create phase problems and dead zones. In this design the braces aren't really passing under the bridge which means that (unlike the classical system above) it relys on the sound board and the bridge plate (flat bit in the middle) to transfer vibration from bridge to braces. This is not efficient energy transfer.

Dixie Michell has an interesting take on steel string evolution and Scott van Linge has some valid info about the shape of Martin braces.

Here are some designs that integrate phase control with lever action
and good energy transfer

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. The radiating braces pass under the bridge which activates them. The thin curved braces tighten the top making it asymmetrical and adding to the harmonic content.

Graham McDonald's bouzouki (right) also uses lever action by making the upper part of the X brace heavier and attaching it to the sides near the sound hole. Leaving space between the end of the small braces and the sides will lower the pitch of the top and give the design some strong lower mids. 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 help to form tone ring, sides and back into a solid unit.

Another element of this design is that my saddle (the part of the bridge that the strings actually sit on) is directly on top of some main braces. (my saddle is level with the front of the bridge plate above) The direct tranfer of energy from saddle to bridge to braces to top is very important to increasing sustain and volume.

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. The four brace ends near the sondhole go close to the tone ring to form a hinge.

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 device made of metal to increase power and projection. It is 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 right up to the sides. I have heard that Smallman laminates two backs together in a curve to remove all vibration. I find this a bit too extreme (too much weight) 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.

Classical Lattice Bracing

While many makers now use lattice design in their bracing, it is using carbon fiber above and below the braces that takes it to another level by multiplying the overall strength. 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.

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. That is why symmetry is not a negative factor in these designs. Jim Redgate gives a good explanation. 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.

Lattice bracing on steel strings

Because I have always used electric and nylon string guitars, I can't play well on heavy steel strings. This is why I felt the need to create a steel string acoustic that felt like an electric but had enough volume to play lead over the top of rhythm guitars. So my focus has been to take the most efficient bracing design known (the carbon reinforced lattice brace) and adapt it to steel strings.

As you can see from my own bracing design, (shown up the page) this has involved using some X brace concepts. For instance the central X of the lattice has more carbon in it as it passes under the saddle and is the main energy transfer to the other braces. Having distinctly different tensions above and below the bridge by varying the distances between braces is another important variation. All of these ideas have to be tested constantly to see if they work, which is why every hand made instrument should be a subtle variation of the one before. This "evolutionary" process is why hand made instruments tend to get better and better while factory instruments at best remain constant, but more often tend to go downhill as production speed and profit margins become the focus.

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. I would also love to hear from other makers who can add to my own knowledge.