“In very general terms the top is the ‘speaker cone’ of the guitar,” offers Eggle. “It’s the thing that responds to the vibration of the strings and beats in and out, which starts the air moving.
“When we’re selecting a top, one of the first things that we do is check the stiffness. I’m after tops that are stiff across the grain. With a weaker piece of wood, you’d have to leave it thicker to give it sufficient strength. But that extra thickness would also make it less responsive, meaning it would take longer to react to vibration from the strings.
“By contrast, a stiffer, stronger piece of wood can be made into a thinner top so the response is more immediate and dynamic. What you’re looking for is a ’board that’s perfectly quarter- sawn; the growth rings will lie at right angles to the top of the guitar.
"A slab-sawn ’board is often a lot more pretty to look at, with more figure exhibited – but it’s also less stable and won’t have as much strength. Some pieces of wood are just naturally better, acoustically, than others. Generally, if you get a bright, glassy, resonant sound just by tapping it, that will translate into the instrument.”
Underneath the top will be glued a (usually) maple bridgeplate that is there to strengthen and protect the softwood top where it’s drilled to accept the string- anchoring pins. Guitar makers have used different materials over the years; size and material drastically affect the tone of the guitar (Martin’s 1970s rosewood bridgeplates are often cited as the ‘bad’ example.
The company returned to using maple and continues to use it to this day). This bridge itself is glued to the top, ideally to a section of bare, unfinished soundboard. Its size and material affects tone significantly, in the same way it does for fingerboards.
3. Forward shifting
You’ll see this term used to describe where the whole X-brace is shifted forwards so the centre of the X is roughly one inch from the edge of the soundhole (instead of two, which is broadly speaking ‘standard X-bracing’).
In theory, forward shifting enables the top to vibrate more because the bridge isn’t sitting directly above the very strongest part of the X: so it’s louder and bassier. That’s how Martin used to do it on the fabled pre-War models. It’s very reductionist to talk tonal attributes as a result of measurements in isolation, but this at least explains the term.
There are many approaches to top bracing. The most popular for flat top steel-string guitars is the classic X-brace introduced by Martin in the mid-1800s (classical guitars are usually fan-braced). “The top’s nowhere near strong enough on its own,” explains Eggle, “so it needs bracing. A set of 0.012 strings on a standard scale acoustic generates around 180lbs of pull. That’s the weight of an average bloke – imagine that being supported by a bit of spruce 2.5mm thick for 40 years! That’s a big ask, really.
“I keep coming back to the traditional X-bracing – it works for us. Each brace is tapered: that’s a strong shape. We could leave them square, but that would retain a lot more mass [and deaden the sound] without adding strength; each brace gets its strength from its height, not from its width.”
5. Tone bars
In addition to the main X-brace, most steel-string flat-tops will have two tone bars behind the bridge, to support the back of the top. The position of these and the extent to which they are shaped or scalloped has a big influence on the vibration of the top, and therefore the sound of the guitar, just as it does with the other remaining braces.
An extension of the tapering that Eggle describes above, many quality steel string flat- tops have carefully scalloped bracing. That’s where meat is removed along the length of the braces – it’s enough to ensure the top is free to vibrate fully, not so much to compromise structural integrity.