Guitar bridge as a brace

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blackalex1952
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Guitar bridge as a brace

Post by blackalex1952 » Sat Sep 12, 2020 9:47 pm

On standard X braced guitars, as an example, the bridge consists of a bridge plate, a bridge, and sandwiched in between both is a section of soundboard defined by the area of the bridge. The bridge acts as a brace between the two X braces south of the X and the amount of bridge that contacts these two braces serves to activate the soundboard via the bracing, creating both an up and down vertical movement and a rotating movement which is measurable using both a deflection guage and a bridge rotation beam. Trevor Gore and Gerard Gilet, in "the books" recommend a rotation of 2 degrees. But there seems to be little discussion regarding the amount of leverage exerted upon the X braces and the cross grain stiffening of the soundboard which I believe would have an effect on the cross grain modes of the soundboard's movement. Somoygi has referred in his books to the amount of bridge acting at the bridge ends on the braces, but only so much as to stimulate speculation. Also, there has been no discussion to my knowledge regarding the area that the bridge occupies, TG and GG refer to a lightweight bridge in 'the books' of around 25 grams due to the effect on the inertia that extra mass will impose on the soundboard which influences the attack of an instrument and its projection.
Looking for comments, theories, speculation, experience etc on the above statements?
-Cheers! Ross
"Everything I say on the topic is based solely upon inexperience and assumption!"

johnparchem
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Re: Guitar bridge as a brace

Post by johnparchem » Sat Sep 12, 2020 11:52 pm

Adding the bridge to my falcate braced classical guitars without a lower transverse brace raises the top frequency (T(1,1)2) nearly 10 Hz. My bridges are generally around 18 grams. So the bridge in this case has added enough stiffness to the top to more than make up for the added mass. I haven't used the mathematical tools that are in the Gore/Gilet book to calculate what is happening, but I believe the falcate bracing design with a lack of braces crossing the centerline and a thin classical top relative to the steel string really needs the cross bracing that the bridge provides.

Adding the bridge drops the top frequency (T(1,1)2) on my steel string guitars by about 5 Hz. Interesting in that I use the same bracing pattern on my steel string guitars and classical guitars. The major difference is that my classical guitars have taller primary braces (10 mm vs 8 ) and a thinner top relative to the steel string. I use the Gore/Gilet book target thickness equations with the suggested vibrational frequency values for my classical and steel string guitars. The braces are higher on the classical to target higher (T(1,1)2) frequencies (190 Hz - 200 Hz vs 170 Hz - 180 Hz).

blackalex1952
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Re: Guitar bridge as a brace

Post by blackalex1952 » Sun Sep 13, 2020 3:31 pm

Thanks for your comments John. One thing I forgot to include in my speculation is that the bridge has quite a lot to do with the impedance/admittance of the string energy into the soundboard and the amount of energy reflected back into the strings to create sustain. This balance could possibly be expressed in a design as a ratio, even an ideal one to aim for, along with soundboard deflection and bridge rotation measurements. I had a quick look at the plans in the books of a falcate design and noticed two things immediately, having not explored the falcate design myself, so far. Firstly, the bridge connects to the bracing by crossing all four braces, thereby activating them with its movement. It is also, at it's leading edge connected to the lateral tertiary brace by the bridge itself, whereas the bridge plate connects the falcate braces as well underneath the soundboard but the bridge plate doesn't connect to the lateral braces. I recall a discussion where it was pointed out that the falcate design isolated the cross and long dipoles more and allowed for more independent adjustment of those frequencies relative to an X braced design...I would hope that someone chimes in on this subject alone, as these things are levers for subtle tonal refinement and the development of our understanding and quality of build as we become more experienced. That's if my memory serves me correctly? I may have that one wrong...I'll have to ponder this more now!!!-cheers, Ross
"Everything I say on the topic is based solely upon inexperience and assumption!"

Dave M
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Re: Guitar bridge as a brace

Post by Dave M » Tue Sep 15, 2020 1:29 am

Adding another question to this. I had got used to making short bridges for my classicals - around 150 rather than the more traditional 190 mm or so. However I have moved to fan bracing based on Torres' SE114 and the short bridge did not intersect the last but one fan braces.

I really wasn't sure if this could cause problems either structurally or sonicly and in the end plumped for safety and made the longer bridge with it's (very slightly) greater weight.

Anybody's thoughts on this would be interesting.
------------------
Dave

blackalex1952
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Re: Guitar bridge as a brace

Post by blackalex1952 » Tue Sep 15, 2020 10:22 pm

I borrowed the Somoygi book at one stage and he talked about the amount of the bridge touching the braces and the coupling...but now I cant remember what was actually there, but at the time it got me thinking back then about the bridge activating the braces...I think if the outside braces didn't couple with the bridge it was wise to lengthen it. Cheers ! Ross
"Everything I say on the topic is based solely upon inexperience and assumption!"

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