Acoustic Guitar Build – Day Four

Top Soundboard

Mark the centreline with a small notch at each end of the board and then run through the thicknesser to remove the excess rosette until it just scrapes the top of the board. Clean up the back as well.

Tapping the board now should have a kind of flabby sound – a  “wump” as I call it. By thinning it down we have removed the high fundamental “note” which it had when it was stiffer. When we brace it, we will re-introduce that high fundamental and then carve the braces to remove it again.

Note: If we have a high fundamental note then when we play whatever note that happens to be – e.g. a D – then we will get major resonation on that frequency meaning that a D (or any derivative harmonic) will essentially be louder than other notes. We *want* that “wump” sound – it means that we will get an even frequency response from all notes.

Now sand the back of the soundboard – we want a smooth surface for maximum glue contact for the bracing. Go from 80 -> 120 -> 240 grit and finish with yellow micromesh.

Draw the shape on using the template, lining it up with the rosette marks, and make marks on the centreline – 20mm from the bottom and 42mm from the top.

Mark the bridge line using a scale template – calculated assuming a 14th fret join and a 25.4″ (Martin standard) scale.

Mark 1/8″ above the bridge – this designates where the bridge plate starts.

Start to glue all flat braces into place. A tip is to hold them down for 20 seconds so that they will not move as much under the clamp when it is tightened.

Carve the braces – from the centre, remove wood , ramping them down to the board. Leave 1/8″ on the front two, the others need to be carved down to zero height. Once finished, carve in from the sides to form a pyramid shape.

Clamp in the X-braces temporarily. Cut a bridge plate which fits tightly against the two braces and is exactly on the bridge plate line marked previously.

Draw on the bracing lines – you need to check a pattern for this, they are freely available. Using spruce for the braces, cut one for the top and two for the X braces. The top one can be chunky as the nut end of the body is stiff anyway but the others must be carefully measured.


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