Cuban Mahogany and Lutz Spruce 00

[Darryl Young]

Haven't made an update in a while.

Sure wish I had the time to play with tuning 20 or so subpar tops. Maybe then I would have a better idea how/where to shave to get things like I want them. One thing for sure........once you have set the thickness of your soundboard and have the braces glued in place to form whatever pattern you choose.....you've set a lot in stone that can't be changed a lot.

One very important vibration mode that produces the most sound is the monopole mode. It couples with the main air mode (T(1,1)1 mode), and it is the main resonantt mode of the soundboard (T(1,1)2 mode), and it couples with the back at it's main frequency (T(1,1)3 mode). So the top vibrates in the monopole mode at 3 different frequencies and these 3 vibratin modes are responsible for a lot of the guitars sound. So since it is so important and since I don't have a lot of experience........I'm sort of focusing on the monopole mode on my early builds.

With that in mind, each time I carved braces I would re-check how it affected the monopole mode of the soundboard (also calle the ring-and-a-half mode as ideally, there would be a complete ring in the lower bout and a line in the upper bout running along the UTB). So I would look at the current shape of this ring mode and try to figure out where to trim to close the ring in the lower bout as it seems a closed ring would mean the top is in balance with itself in stiffness and mass distribution. The ring doesn't have to close......but it's something to shoot for. I didn't get it closed on my last build and it sounds great. Also, the frequency where the ring develops tells you something about the overall stiffness of the top. Another thing, ideally I wouldn't have a sharp bend anywhere in the ring pattern in the lower bout. Again, I didn't do very well at getting there........but that is where I wanted to be.

So with that said, here is a pic of the progression of the ring mode on this sounboard as I shaved the braces. I wish I had stopped at the pattern I got at 238 Hz........but I thought I could tweak it just a little more and it would get better........but I made it worse, then slightly better where I stopped at 229Hz. Actually, all I did was re-triangulate the lower braces mostly and that must have changed the weight to stiffness enough that the ring changed. Another thing I'll mention that is a repeat from what I heard on Al Carruth's tuning video.......when you are close to where you need to be, go slow as small changes make a lot of difference. This seemed to be the case for me. Right now the soundboard had very, very good sustain and a strong note when tapped. I never got the bottom of the ring to close and when I finally got the left side of th ering closed, the right side opened up!

Here is the pic of the progression (you can decide if it's improving or getting worse <smile>)

TuningProgression.JPG

Probably the most area inside the ring is when the mode was at 238 Hz.........one reason I think I went slightly too far.

Hey Darryl,

Does the Carruth video explain where the brace shaving needs to be done in order to achieve optimum results?

tommy

[Ken Hundley]

Add a very small tuning bar?

[Darryl Young]

Tommy, yes and no <smile>

He does give generalizations........and he tells you what he is going to try and then you watch him do it and then you see the results of it. The X brace probably isn't Al's speciality so he doesn't have it down to a perfect science. He sort of wings it not unlike what I did.

[Darryl Young]

Ken, I may be missing what you are saying, are you suggesting I should add a tuning bar? Is that the same thing as a tone bar?

[Ken Hundley]

Pretty much.....maybe figure out a different location for a very small brace.....carve it to where you like it, or carve it off together if its no help.

[Darryl Young]

You may be right Ken, not sure.

Keep in mind that it is a 12 fret neck on a 14 fret body so the bridge is already lower than it normally is. And since it's a 00 body, there isn't a lot of room below the bridge. I think it would add a lot of stiffness to the perimeter to squeeze another tone bar in there (just an opinion).

While tuning the plate, I bent around the perimeter of the lower bout and it was nice and loose except for 3 places where it was extremelly stiff. The stiff spots were at the ends of the lower X brace arms and at the end of the single tone bar. Since the end of the tone bar was near the end of one of the X brace arms, it was really stiff in that area. I ended up salloping the X braces and the lower portion of the tone bars back a lot while tuning the top (good or bad). It seemed to help the shape of the pattern and really helped the tap tone become clear and sustained. I almost removed the lower peaks of the X brace arms and did remove the lower peak on the tone bar. Of course, I didn't get it perfect either so I'm not claiming what I did was ideal. The stiffness around the perimeter feels much more balanced when flexed by hand though it still is slightly stiffer in the areas previously mentioned.

[Darryl Young]

Wow, it's been awhile!!!

Finally glued the soundboard on the rim assembly tonight. I used fish glue which is what I used on my previous build. First time I didn't run the braces through the sides so I just trimmed the laminated lining and left the mahogany sides intact. On the first try I accidentally trimmed through the lining into the side leaving it thin. When I tried chiseling out the remainder, I busted the side out slightly. I pushed it back in place, ran CA glue in the cracks, and clamped it shut. Would have worked very nice......except in my haste I failed to place wax paper between the clamp and side........and I glued the clamp to the side <smile>. This left a little bit of a scar when I removed it. Oh well.

Here is a shot of the top being glued to the rim. Note the green stripe I used in the outer rosette ring.

IMG_0815.JPG

[Darryl Young]

Removed the go bars last night. The box looks good and the tap sounds great. Only issue is I should have used something between the go bars and the top as the pressure of the go-bars left small depressions in the top. Thankfully they are small and I should be able to steam them out easily (or they may disappear on their own) and most of it will be removed when I add the purfling. Anyhow, I thought I would mention it so the mistake isn't repeated.

[Darryl Young]

Have the box closed now and used my homemade binding cutter to trim the top/back to the rim. Actually, I setup the cutter to remove roughly 80% of the binding channel at the same time. This way the edge of top/back won't interfere while I scrape the sides flat.

I'll try to post pics.