1. Back & sides
“The back and sides really do influence tone,” says Eggle. “You want the sides to be very stiff and solid because you don’t want energy from the strings to dissipate into them. You want that energy to be directed into the top and then for the top to be banging up and down and getting air moving. The back of the guitar will then reflect sound toward the listener.
“Our guitars are built with a 15ft radius on the back and the tops have a 30ft radius. So the top’s got an arch to it but it’s very nearly flat – while the back arch is more pronounced. I’m actually making my backs thicker and thicker these days. If you stiffen up the back you seem to get more projection and more volume out of the front end of the instrument, which seems to be what people want.”
The back of the guitar also needs to be braced for strength. Unlike the top, it tends be in a ladder pattern in most steel-string flat-top acoustics; it’s less complex than the top bracing because tonal effect is not as critical and there’s no soundhole to consider.
The strip down the middle covers up the join in a two- piece back. Some guitar designs – such as Taylor’s 100 series – have no back bracing at all as the laminate back is arched (much more radically than Eggle mentions above) to provide the necessary strength.