Acoustic Guitar Construction

Evening gentlemen, my 10th build is a Martin OM size guitar which should have a lower bout depth of 41/8”. Sadly, after radius sanding the rims i’m going to be shy by around 1/4 to 5/16” - depending on my top and back thicknesses. Any idea what affect a 1/4” shallower body will have on the volume and bass etc.?
Also, I was thinking of freeing up the top, to compensate, by not tucking in the lower legs of the X braces. I’ve never had the nerve to do this before, incase the top belly’s up - what the thinking on this? Thanks in advance.

What I'm about to say is based on equations, not experience, so take it with a grain of salt. But...

A smaller volume of the body cavity will raise the lowest natural resonance of the guitar, which is the vibration of a plug of air in the sound hole (the Helmholtz frequency--like blowing on a glass bottle). This is usually in the 90 to 110 Hz range. The lowest note on a guitar is about 82 Hz, so the Helmholtz frequency is important to bass response.

You can lower the Helmholtz frequency back to where you want it by using a smaller sound hole. Exactly how much is hard to say.

If R is the radius of the sound hole, and V is the volume of the body cavity, then the Helmholtz frequency is proportional to R/V (actually SQRT(R/V)). So in theory, if you reduce the volume by a certain percent, then you should reduce the radius by the same percent. Thereby keeping the same ratio (and frequency) as before.

Good luck.

Greg

I don't think that amount of depth change would have much effect, but Greg's more scientific approach would be something to consider. As to the lower legs of the X-brace, I do not tuck mine and have not had a problem with any top bellying. Check the recent comments in the Soundboards section.

Greg - thank you for the reply and info. Its a tad over my head but I think I understand the reasoning, just not sure how much I’d need to reduce the sound hole diameter by - would 1/8” make a difference or are talking more?

Carl - I did find the discussion in the soundboard section, if others have done it without any issues later on, then I’m happy to try it. Does it make big difference to the tone, volume or sustain compared to tucking the braces in?

NeilG495 wrote: ↑Wed Feb 18, 2026 4:19 pm Greg - thank you for the reply and info. Its a tad over my head but I think I understand the reasoning, just not sure how much I’d need to reduce the sound hole diameter by - would 1/8” make a difference or are talking more?

Well, I'll take a crack at it. You said the depth was supposed to be 4.125, and it ended up 0.25 shorter. So that's 6.1% smaller.

Now, suppose the plan for the sound hole is 3.75 (typical for an OM). That's a radius of 1.875. Reduce that by 6.1%, we get a radius 1.76, which is a diameter of 3.52. So it's been reduced by 0.23. So, reduce the diameter about 1/4 inch.

Greg

Greg - Thank you for the calculations, I’m up for trying something new with this build so a 1/4” smaller soundhole and untucked X braces it’ll be. I’ll come back with the results once it’s finished, it might be a while as it’s still only a hobby for me - but I do love it.

Perhaps you can recover some height by installing the kerfing proud of the rim. The sides are going to be routed away for the binding and the sides won't be left anyway above the level of the binding.