The 'three inch rule' for scallop peaks - O'Brien

[Brian D]

A few years ago I learned the math used to build tuned wind-chimes. They, like marimbas, etc are vibrating rods. The rule was the nodes occur at 2/9ths of their length. I keep finding peaks on soundboard braces that are very close to 2/9ths of their length. On the 00 I am working on now, the shoulder brace is located at 2/9ths of the length (I was looking for a node to hold in order to tap and listen). The mechanics of a vibrating soundboard is beyond me, but if a brace is similar to a marimba, maybe 2/9ths would be a good general rule.

[Tim Benware]

Good info.
Somewhere on the UMGF, John Arnold listed all the measurements he took of a 1935 D-18, what he considered to be a very great sounding guitar. I will try to find those this evening and post it if I can find it.

I'd like to see that.

[Dave Bagwill]

Here ya go:

One of my favorite guitars of all time is a 1935 D-18 #61033.
All measurements are in inches.
X-brace angle is 97.5 degrees.
Tone bar spacing (C to C) is 1 7/16 @ X-braces, 1 3/8 @ kerfing.
X-braces and tone bars are 0.325 wide.
Bass X minimum = 0.320
Bass X peak = 0.435, 6 5/16 from center of the X crossing
Treble X minimum = 0.300
Treble X peak = 0.565, 5 15/16 from center of the X crossing
Upper tone bar minimum = 0.355
Upper tone bar peak 1 = 0.532, 2 15/16 from the X
Upper tone bar peak 2 = 0.530, 8 3/8 from the X
Lower tone bar minimum = 0.310
Lower tone bar peak 1 = 0.460, 2 3/4 from the X
Lower tone bar peak 2 = 0.500, 8 3/8 from the X
The four short braces (generally called fans) are 1/4 wide, with a peak that is about 1/4 tall. The fans are about 1/16 tall where they intersect the X-braces. The tone bars are about 0.110 tall where they intersect the X-braces.



John

C to C distance from the upper tone bar intersection to the X crossing is 5 1/2. C to C distances from the X to the fans are 2 5/8 and 5 inches.
The angle between the X-brace and the upper tone bar is 62 degrees. Double-check this at the kerfing, where the C to C distance from the X-brace to the upper tone bar should be 1 1/2.

I have found that soundhole locations on 1930's Martins are very consistent. I measure the minimum distance from the brace to the soundhole, which is usually around 0.300". On D-18 61033, that measurement is 0.287.

John.

[Tim Benware]

Excellent, thanks Dave.

[Jim Ebert]

Thanks guys, that's some good specific info on something that I suspected to be the case. I will keep it for future reference.
Jim

[Herman]

Well Dave, I looked at the specs. Two things:
1. The distance of the upper tonebar to the X-joint mention 5.5". That seems quite far to me.
2. The angle of the tonebars: 62 degree with 1.5" between tonebar and X-brace at the rim. Quite short.

Did they put the tonebars so much more across the top than now?
Herman

[Dave Bagwill]

Herman - can you draw it out and post it so we can see?