Guitar Top/Back Displacement

[scamp]

Before building my next guitar ( it would be my 3rd ) I decided to do some research on improving acoustic performance beyond tap testing etc. which is highly subjective, especially when you haven't built a lot of guitars. Read Trevor Gore's design book and decided that the ability to measure the displacement of the top wood under load using a calibrated technique was important to optimize thickening as well as properly bracing the top so I built a jig for this purpose and proceeded to measure the displacement of the tops, as well as the backs, of guitars I have to see differences etc..

This what I came up with....

Guitar Top Displacement Jig.JPEG

Made displacement measurements of three guitars under a 5 lb load. Made the measurements at the bridge, below the bridge by a few inches ( in more or less the center of the lower bout ) and in the center of the lower bout on the back. Here is what I got ..

Cheap Yamaha FG400A
Bridge Displacement .025 mm
Front Lower Bout .030 mm
Back Lower Bout .041 mm

Martin D28
Bridge Displacement .025 mm
Front Lower Bout .029 mm
Back Lower Bout .037 mm

Kit OM ( Sitka top EIR back and sides )
Bridge Displacement .036 mm
Front Lower Bout .040 mm
Back Lower Bout .064 mm

Scratch Built OOO 12 Fret ( European Spruce top Cocobolo back and sides )
Bridge Displacement .027 mm
Front Lower Bout .032 mm
Back Lower Bout .025 mm

The KIT OM was built very light and the measurements reflect it. It is very responsive and loud for an OM.
The OOO 12 fret back displacement reflects the fact it is cocobolo and even though the back is only 0.1 inch thick it is very stiff. The guitar is more like the Martin D28 but without as much of the jangly sound you get with EIR.

Anyway... Thought I would pass this along in case anyone was interested. For my next build I will try and use this jig to help properly thin the top ( measuring displacement of different top woods I have used ) as well as shape the braces and thin the top edges. Just trying to add a bit more science into this.

Regards

[Srick]

Great post Scamp, although it’s hard to interpret your results. I am going down a similar rabbit hole and the one piece of advice I keep running across is the answer to the question, “How do you know how much to thin?”

The answer being, “You’ll totally know it because your guitar will lose all its tone.”

YIKES!

FWIW, a friend who is a highly lauded luthier, told me that his first twenty builds are sitting in his parent’s basement, all slowly collapsing. We aren’t the first to ponder these questions.

[scamp]

From the data I collected I can definitely see a difference in the deflection vs the top thickness, width of bracing as well as the type of wood. I think the next step is to measure a few different spruce tops ( deflection vs thickness) and see what kind of variability I get as well as tap tone performance for similar deflection numbers. Should be interesting.