Acoustic Guitar Action: Geometry Class
I'm going to talk a little about how to lower the action on an acoustic guitar or bass. That involves modifying the saddle — actually cutting or sanding to make it shorter. Before I get to the no-going-back nuts and bolts, therefore, I'm going to give you a tip for how to assess how much adjustment your saddle will need.
I should mention that setting action is just one part of a good setup and you'll get the best bang for your setup-buck if you perform all of the steps. On an acoustic instrument, I definitely recommend getting your neck relief (and nut height if possible) in good shape before you begin adjusting action. I talk about all off this stuff (and the right order for the steps) in my Sketchy Setups guides — Sketchy Setups #7 is dedicated just to acoustic guitars. If you're interested in getting the best from your setup, that's a great place to start.
But, on with the geometry lesson.
"Why geometry?" you ask. Because we can use some pretty simple geometry to take the guesswork out of adjusting an acoustic guitar saddle. Anything that can make that less daunting is helpful.
If you picture your guitar from the side, you can imagine that the string-line, the saddle, and a line along the top of the frets form three sides of a right-angle triangle. Of course there are tiny deviations in this model but they're all negligible and we don't need to worry about them.
To quote, Pythagoras, "Right-angle triangles ROCK!" Well, he may not have said that but, for the purposes of making acoustic guitar setup more straightforward, they really do rock.
The middle point between nut and saddle is the 12th fret. If we measure our string action at the 12th fret, we can know ('cos geometry) that it's half the height of the upright part of the triangle, i.e. the saddle.
Put it another way, the saddle height is two times the 12th fret action.
See where I'm going with this?
Let's say you measure your action at the 12th fret and decide that it's too high. You'd rather it was 2/64" (about .8mm) lower. To get that 12th fret height lower by 2/64", we need to have the saddle lowered by twice that amount.
Because the saddle height is twice the 12th fret height, removing 4/64" (1.6mm) from it will cause the 12th fret action to lower by 2/64" (.8mm).
Right-angle triangles ROCK!
We can measure and mark that figure on the saddle and modify it with a lot more confidence.
In the real world, you'll want to measure the action on the top and bottom string to determine what changes you want. This will often be a different figure for each. That's ok. Note the desired change for the top string and for the bottom string, double each, and mark the appropriate measurement on the corresponding end of the saddle.
This will all become more clear when I get to the saddle adjustments. We'll look at that next. For now, you've got geometry homework to digest. 😉
This article written by Gerry Hayes and first published at hazeguitars.com