T(1,1)2 and Back Question

[klooker]

I took some measurements after closing the box.

Here are the top taps:

I then tapped the back while holding the top against my stomach and leaving the sound hole uncovered to try to measure the back:

Did my back taps actually measure the back at 196Hz? If so, my T(1,1)2 is actually 217Hz (which seems high) and my back is 200Hz?

This is a traditional Martin Dreadnought design with the wide, low back braces.

Thanks
Kevin Looker

[johnparchem]

217 Hz for the T(1,1)2 is what I would have said from the first graph. With the bridge and strings it should come down maybe to 207 Hz or so. Is the top the Martin x-brace design? If so you can reach in and tune the top after the guitar is finished. Low and wide is a lot less stiff than the taller braces, I can see the backs resonance being around 200. How thick was your back?

[klooker]

Yes, it's a Martin X-brace pattern.

Now that I'm checking my notes, it looks like I neglected to do final thicknessing of the back. The back was supposed to be 2.2 mm but I have 2.8 mm written down which was the original thickness when I glued the panels together. This is embarrassing, but I'll check this evening.

Thanks for the feedback.

[klooker]

Unfortunately the back is about 2.8 mm thick.

I could attempt to thin it while on the body but I think besides reducing weight, I'll be lowering stiffness too so I'm going to pull it, thin it, & re-brace with taller braces. I made a bad assumption that Martin's old style "low & wide" lower back braces would be a good idea to reduce the stiffness of the back to make it more live but still look traditional if someone looked inside. In this case it appeared to make the back too compliant.

I'll also look at the X-Braces some more while it's open to ultimately drop T(1,1)2.

Thanks

[johnparchem]

If you thin the back more you will lower its resonance. I would work on the top a bit to loosen it. It will change with a bridge and strung up. You can tune the inside braces after the guitar is strung up and see its finial resonance. Also you can hear it at that time and decide if you want to change anything.

[klooker]

Thanks John.

Do you know what effect a back with a lower resonance than the top will do to the sound?

[klooker]

One last thing I'd like to add to explain why I thought the Martin low & wide lower back braces would get the back to a desirable frequency.

This is the plot from one of my earlier builds. It's an X-braced, Martin OM shape with low & wide lower back braces per the Grellier plan.

I guess I got lucky on that one. It was one of my first builds & I followed the Grellier plan like a cookbook.

Subjectively speaking, that guitar has a rich and interesting sound. The only negative is that it runs out of headroom when pushed hard probably because there was a lot of scalloping done to the bracing.

Kevin Looker

[johnparchem]

The low and wide is not a bad way to go, especially because you like your last guitar. Was the plot of the last guitar with a bridge and strung up. Its numbers are within my range of where I would like a steel string. If you just closed up with no bindings, I would consider thinning around the lower bout perimeter of the top. Then I would probably finish it and see where I am at. As you go in the process the top frequence will go down. With a bridge and strung up you will have a better idea how to tune.

[Trevor Gore]

Kevin,

Check out pages 2-22 to 2-25 in Design (if you haven't already).

[klooker]

Kevin,

Check out pages 2-22 to 2-25 in Design (if you haven't already).

Thanks!
No, I hadn't reviewed that section. I wish I would have weighed the back before I braced it & closed the box. Reducing to target thickness would remove about 20% of the thickness. That would of course reduce mass but also lower stiffness or is it safe to assume that the braces contribute to the majority of the stiffness?

Also I assume that by lowering the frequency of the top, I could achieve the same flip flop of the back & top frequencies as you demonstrated by adding mass to the back?

[Trevor Gore]

Reducing to target thickness would remove about 20% of the thickness. That would of course reduce mass but also lower stiffness or is it safe to assume that the braces contribute to the majority of the stiffness?

I would expect it to lower the frequency, but how much depends heavily on your bracing scheme, too.

Also I assume that by lowering the frequency of the top, I could achieve the same flip flop of the back & top frequencies as you demonstrated by adding mass to the back?

Yes, but you have to look to where things are likely to end up if you want to keep the preferred separation between the T(1,1)2 and the T(1,1)3 frequencies.