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Ok Im a dummy and need some advice on deflection testing a soundboard. Its time for me to start working on getting all the sound possible out of my guitars. Ive read about deflection testing the soundboard for consistancy. The concept as i understand it is you take your soundboard and support it on both ends with rods. You then place a weight on the center and measure the amount that it bends down. This assures you get the same stiffness in your tops. My question is where is a good starting place? I understand that each person has to learn by trial and error and find the sweet spot for the way they build, but I dont know whats a good starting point. Im guessing the rods should be 20" apart? How heavy should the weight be and what is a good starting point for a deflection? I know it can vary with bracing patterns too, I am currently just using martin style scalloped braces on my builds so thats what Ill be using. Does any one also test deflection after they brace the soundboard? Any info would be much appreciated. Thanx everyone.

Hi Zeke,

A good starting point would be to take you top at about .110" (provided it's spruce) and just mesure the deflection, and take note. Using a standard Martin bracing pattern is also a good start since it should give you a sound you are 'used to'. It eliminates an extra variable your would have if you would use some experimentale bracing pattern. An important thing to do as well is to mesure the density of your soundboard wood. Different sounds will come from tops having different weight but the same stiffness (and vice versa).

As for mesuring techniques, although the technique you described is widely used, it has its flaws. I use it (I place the supports 18" appart, using a 1lb weight). But using tubes as supports makes the points of contact move down as the top deflects. So the deflection you mesure end up being more than the actual deflection of the top. On the other end, square points of contacts have their own problems; the top will rub against them as the top deflects, thus offering some resistance to the deflexion, thus screwing up your readings. The best option is definitaly to use tubes as supposrt, only always make sure to use the same tubes. And don't compare with others who don't use the same tubes as you do!

(I'm thinking about starting to mesure deflexion by clamping down one end of the soundboard, and placing a weight at the other end. I think this way would eliminate the flaws I mentioned previously. But I still have to do some trials before I can say it does...)

Regarding mesuring deflexion with braces on, I don't think it would be reliable since the to pwill not have a regular stiffness anymore since its braced.

Maybe others will have better opinions on the matter, but that is my 2 cents...

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

A good place to start is to search this forum ... a search of deflection in the title yields two pages of threads and lots of people asking just what you are. A "where to start " amount of deflection will obviously depend on the span and weight, but you'll get an feel from the research. (I believe there's at least on video tutorial as well)

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Dave
Milton, ON

These links might prove useful:
http://www.lessonsinlutherie.com/woodte ... icing.html
http://www.ukuleles.com/
http://dunwellguitar.com/DeflectionJig/ ... ionJig.htm

good luck!

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member of the guild of professional dilettantes

Thanks for the info, thoughts, and links. Im starting to sort of get an idea of the direction I need to head.