Acoustic Guitar – Preparing the Soundbox

The first step in constructing the guitar is to put together the sound box or body. This comprises the sides, top or sound-board, and back. The body is arguably the most important part of the instrument as it is the component which amplifies the vibration of the stings. It must also withstand the huge forces exerted by the tension of the strings which constantly try to collapse it. It’s quite amazing to think that a properly constructed box of wood only a few millimeters thick braced by light spruce can withstand the the strings constant force of between 150-200 pounds (68 – 90 Kg). That’s the same weight as a 14 year old boy.

The two sides which have been thinned and pre-bent at the factory are still pretty rough at this stage and will be sanded smooth later on. The sides and back are of East Indian Rosewood and in the picture i’m test fitting the sides, neck, and tail blocks. There is a cardboard mold inside the assembly to hold things in the right shape. Once glued together the sides will make a ring which the top and back will later be glued to.

Because the sides have to be glued to the top and back a flexible mahogany strip (Kerfing) is glued around the upper and lower rim of the sides which increases the gluing area between them. The clips and clothes-pegs offer enough force to hold the kerfing while it dries.

Here i’m in the process of clamping two of the 4 spruce braces to the back. The gently curved shape of the brace imparts a smooth curve in the back which gives it huge strength and helps in sound projection. The way the back vibrates is very important to the final sound of the instrument and the size and position of the braces affects this – though with no experience of building a guitar before I have to rely on the wisdom of the plan rather than using my judgment on this one.

This photos clearly shows the curve of the brace. The wooden support underneath it has been carefully filed and sanded so it supports the back during the gluing process.

The photo below shows all the braces and a segmented spruce center strip glued in position. The center strip adds strength to the join between the two panels which make up the back. I decided i wanted to have a nice 45 degree chamfer on the edges of the center strip as it would be visible in through the sound-hole of the finished instrument. This gave me chance to botch together my first jig.

The green part of the jig is sandpaper onto witch a 45 degree ramp decends. By resting the spruce center-strip segments on the ramp and moving them backwards and forwards against the sandpaper I can take off a little material at exactly 45 degrees. If you have ever read “Zen and the Art of Motorcycle Maintenance” you’ll know what I mean when I say this is a very high quality moment. Really the shaping of this little component has very little to do with the function of the finished instrument but I think making this simple jig was the first thing i did on this instrument that was not outlined in the instructions. It might sound a bit odd but it was at this moment that the building of this instrument became totally mine. I had set the level of quality that I wanted to achieve in the coming months.

The strips turned out like this – all nice and uniformly beveled along the top edges.

The top or soundboard of the guitar is made of tightly grained spruce. It is two pieces joined down the center – this has been done already in the kit as it’s quite difficult for the beginner to master at this early stage. The join must be spot on so as not to fatally waken the instrument. You can just about see in the picture I have drawn the positions of the braces onto the inside of the soundboard. These braces also made of spruce will strengthen the soundboard and depending on their shape and mass will contribute much to the final tone of the instrument.

In this picture I have been using a cabinet-scraper to smooth the sound hole rings I had previously glued into recesses in the sound-board. The cabinet-scraper is an amazingly simple tool, just a small sheet of steel with a very tiny turned burr on one edge which acts like a very tiny blade. It takes of small amounts of wood and leaves a very smooth finish.

The x-brace is at the heart of the sound-board’s structural and tonal integrity. here you can see me making sure it’s going to fit ok. The kit came with ready formed x-braces but I split them when i tried to notch them together. The fit should be very tight but I made it too tight casing a split. I decided to order more spruce and make my own braces from scratch.

After making sure the underside of the sound-board is smooth I can glue the reformed x-braces into position. Here I’m using cardboard off-cuts so i don’t dent the delicate braces. This is a dry-run which means there is no glue applied to the components. It’s useful to do this to make sure everything fits together well and that you have enough clamps to do the job. With the set-up in your head it’s much faster to do the actual glue-run which is just as well as the glue begins to set quite quickly. Once the braces are clamped i leave the whole thing to dry over night and wake up each day to the excitement of unclamping. It’s surprising how easy it is to get out of bed when you can’t wait to see how your latest effort has turned out.

After a few days of carful brace gluing the top is almost done. The braces can now be shaped using a chisel. The smaller ‘finger’ braces and the two larger ‘tone’ braces on the right are carved away almost to nothing at the ends. It takes a steady hand and a very sharp chisel to do this without slipping and damaging the soundboard. Another consideration at this stage is the protection of the other side of the sound-board. Spruce is extremely soft and a stray wood shaving pressed into the surface can easily dent it.

To finish the sound-board I take it outside where the light is better and sand everything smooth. I spend a great deal of time fine tuning the shapes of the braces until it looks right to me. The top is quite rigid now though I still treat it like a only child after all this is the element that will be most influential to the final sound of the instrument.

Now that the sides, back and top are ready it is time to glue them all together.

>> Completing the Sound-box

1 Comment to Acoustic Guitar – Preparing the Soundbox

Mark Gough
January 7, 2009

Considering the space you have to work it looks great so far its inspiring thanks I’m thinking of making one myself

leafcutterjohn.com