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I have just completed #5 and so far I have just thicknessed my tops according to the plans. My question is: what method do you use to determine the thickness of your tops? I have been through the archives until my eyes can't focus. Thanx, Vic.

4.5.7

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MK5acoustics.com

Wes- What does 4.5.7 mean?

Vic, when I made my first, I just used what the guys at Santa Cruz Guitar Co. shared with me and thicknessed to .110 for an OM.

From there I went to a method I picked up from Kent Everett's voicing DVD where you hold the top by the edges and wiggle it back and forth in front of you until it starts to warble like a saw.

Last week I picked up the things to make a deflection testing rig (dowels 18" apart at their centers). For what I'm after, I'll thickness to a .2" to .25" deflection with a 5 lb. weight initially, and adjust from there as my ears see fit.

I use the 18" deflection rig with a 5 lb weight. I also generally go for 0.20" to 0.25" deflection, maybe less for smaller guitars. I also look for a good tap. Last top I did was for a dread. It was Lutz spruce and I ended up at 0.108" thick with 0.215" deflection..

Some others all at 18" with 5lb weight - obviously I haven't nailed a good number yet. With these I was also using 2, 3, and 4 lb weights so I could calculate MOE:

Size O stiff Euro top 0.095" thick, 0.312" deflection - this one turned out quite loose but was made for silk and steel strings.

Size OM Sitka 0.105" thick, 0.160" deflection, box on this one sounds overbuilt but haven't completed the guitar yet.

Another dred, very stiff red spruce 0.097" thick, 0.265" deflection. This one is going on a '37 D28 copy I plan to make as soon as I can get all the information together.

EDIT: I should add that I'm trying to work out Trevor's method as well - all in the quest to understand as well as I can, as soon as I can, what the right thickness is for each top. And, again thanks to Trevor, I am looking at the backs as well.

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Steve Smith
"Music is what feelings sound like"

Sorry I was at my local sports watering hole when I posted ...the 4.5-7 is the equation in the Gore and Gillet books that address the thickness of tops, at #5 you are probably beginning to find out there is a lot of confusing information, bs, out there about top thickness, everything from warbling sounds to echoing of tops as you tap tune them ..Like Todd Stock says the first five are woodworking projects if you are interested in furthering your guitar building education , IMHO, I recommend getting the books , not inexpensive but more than worth the investment.......
equation 4.5-7 is a spreadsheet formula set up in excel for thicknessing tops and backs using measurements and frequencies obtained from the wood...no guessing .......

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MK5acoustics.com

No guessing as long as you believe the product of the equation gets you to the tone you're after.

My intent isn't to be pejorative, but there are arguments for and against any method. You could look at deflection testing and argue it loses it's relevance the second you glue the first brace. Trevor's book was a mile over my head. I just didn't know how to make any kind of meaning out of this:

So I also don't really understand how that equation can spit out the target for the unique sound you're after.

Again, my intent isn't to to be argumentative, but I genuinely don't understand the global application of the equation.

I agree its seems at first overwhelming!!! took me several months to make sense of it ...it you read , reread , then break it down into small elements it begins to make sense....much of the math is just proving the facts he makes, and quite frankly is just for "geeks" the actual math put into the spreadsheet although lengthly is not that difficult, one of the things that you have to get over , and it's tough, is converting everything into si units, since we are taught fractional measurement in the US and the rest of the world uses the metric system...
I am building my 15th and 16th , both commission builds and although both have different woods, one is euro spruce with EIR back and sides,the other is Sitka with bubinga back and sides the measured frequencies are almost identical , which is what we are after , consistency.. I have also been using carbon fiber bracing on the last three and these two using falcate bracing and a modified sweep x bracing ......

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MK5acoustics.com

James, There a parameter called the "vibrational stiffness variable" and can be adjusted to your preferred sound and provide you consistency from top to top. A larger number will stiffen it up and a lower value will soften it.
also that's the wrong formula for target thickness. Look at section 4.5.
If I remember right there's a spread of about 20 points between a classical and steel string and of course backs have a different value too.
This is the "knob" that allows you to dial n the sound your after.
It's really about consistency not Trevors sound.

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Jim Watts
http://jameswattsguitars.com

I would like to use the G/G formula, but the wood I get is seldom flat, a prerequisite for the formula.
So I use deflection testing as well. 18" spread with 5lb weight. Pretty much the same numbers as Steve, but I go more flexible for smaller guitars.

To start your own database, take your Sitka to .110.
Then do deflection testing on that piece and record the data. Build guitar, try to qualify whether the top should have been stiffer or looser judging by sound/response. Adjust accordingly. Repeat....
Or, use Steve's deflection numbers and do the same thing. Yours ears should be able to tell you. If its mushy and balloony sounding, it's too loose. If its thin and tinny it's too tight. But of course, your brace work is another variable but that's a different story...

At the end of the day .110 is a safe starting number. The majority of the tops, across spruce species that I've done of late have all been just under that. It's probably better to be just a bit to thick than too thin. Most of the builders I know that thickness to dimension as opposed to deflection, go to 2.8mm or .110",which, according to my calipers is the same dang number....

You don't have to!

When writing the book I had a few choices:
1) Just give the user result with a qualitative explanation of how I got there
2) Give the user result with all the back-up maths in an appendix
3) Derive the user result in the main text

I chose 3), because if I'd done 1) I'd be up against the usual argument of "where did that come from? It's just someone's opinion" If I'd done 2), I'd have driven anyone trying to follow the argument mad, flicking between the text and countless appendices. And that was in spite of advice given by Steven Hawking (paraphrasing) "the quantity of book sales is inversely proportional to the number of mathematical formulae". More Hawking quotes here.

So James, the bit that is of interest to you, the user, taking the example you've shown, is the formula in the box (1/sqrt(k*m)), and all the preceding stuff is about why and how that is right. As a user, you just need to know the equation in the box, and leave the rest to the geeks who may want to discuss it with me!

If you read the chapter summaries (at the end of each "mathematical" chapter) there's a qualitative version of what's in the preceding chapter. Maybe read that first then head for the user formulae.

Regarding deflection testing, the main problem with just using stiffness (the result from a straight deflection test) is that there is no simple way to relate it to plate mass and you need to know how the stiffness relates to the mass to get a reasonable prediction of the resonant modes. At the end of the day, what I'm doing is just a way of doing what the old masters did, after years of experience: listening for a tap tone of a certain frequency that is "right" for the guitar you're about to build from the piece of wood in your hands.

I think the other guys have clarified the other issues (thank you, gents).


Certainly back wood can be a PITA to get flat enough, but I've never had any trouble finding flat top wood.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

The guess work involved in building a guitar will only be eliminated when you remove the inconsistency present in the materials it is made of...!

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http://www.PeakeGuitars.com

Hey guys. Just got in from my wife's first triathlon. Seven hours all told of standing around in the sun watching a very interesting sub-culture. So, I'm very interested in reading your responses with greater thoughtfulness but did want to say very quickly that the picture was just a random one I took one night to say, "Oi. My head hurts!"

I've been able to go back through section 4.5 and believe I have a clearer understanding now. As Jim said earlier, you can tweak the vibrational stiffness variable until you find the ballpark you want to play in. That makes a lot more sense to me than what I understood in my initial posts. A lot more sense.

A quick question about finding m.

Page 4-59 reads, "The density p is calculated by weighing the sample and dividing the mass by the product of the three linear dimensions."

Is m just the weight in grams?

Express the weight in Kilograms (grams/1000) and the length , width and thickness in metres (mm/1000)
and you will come up with the density in KG/M3 which is the standard unit for density

Steve Smith said:I use the 18" deflection rig with a 5 lb weight. I also generally go for 0.20" to 0.25" deflection, maybe less for smaller guitars. I also look for a good tap. Last top I did was for a dread. It was Lutz spruce and I ended up at 0.108" thick with 0.215" deflection..

Thats interesting. I actually do the opposite. I use deflection testing like many of us Somogyi students. With a smaller guitar, I actually thickness to a greater deflection (thinner). My feeling, is with a "smaller drum head", you need a less stiff top. I have a range of deflection targets. My SJ 12 has the lowest deflection and my parlor has the most.

Bottomline, keep great records. I have an excel spreadsheet with the deflection data for every top I have ever made. Based on the end results of the guitar you can tweak your deflection goals for future tops.

For my set up, key phrase MY SET UP, my deflections with a 5# weight are around 250 for an SJ 12 string, but I might go as far as 350 for a parlor. With Shanes Lutz the SJ. tops are around 0.095. Parlors around 0.085. L-OOs around 0.09

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Andy Z.
http://www.lazydogguitars.com

Thanks for that, Andy. I really appreciate the openness of sharing.

So I just saw this one again and Andy is right - more deflection for smaller bodies guitars

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Steve Smith
"Music is what feelings sound like"

Newbie question,
When testing for deflection, what shape is the plate? Square? Cut to guitar shape? Soundhole cut in or not, etc?

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Doug

Don't let fear or common sense stop you from trying to build something

I do mine with rosette in but sound hole not cut out, full panel usually 17-18 x 22"...

I do mine as a rectangle so I can calculate density easily.

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Steve Smith
"Music is what feelings sound like"

here is my simple jig for checking bridge rotation , loaded and unloaded

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MK5acoustics.com

Maths...

while building you first 1/2 dozen,

flex it
twist it
tap it
remove a bit more wood
tap and listen
then...

flex it again
twist it again
tap it some more
remove a bit more wood
tap and listen some more
then
tap it again

lay it on your amp and watch it jump around to various "sounds" while
sprinkling sand on it and see the patterns

oh and did me mention to
flex it
twist it
tap it some more
remove a bit more wood
tap and listen
then
tap it again and again and again and listen to what she be saying.

Fondle and caress it like she your sweetie. Das right!
Talk to and ask her what she capable of and tell her what you need her to do. Then listen to what she has to say...what she be capable of and what you want may be two different things....talk with your wood dude.


Duh big idea here is to burn all that info into your cellular consciousness
so by the time your on your other half dozen you won't need to waist time on maths.

Is how me build em.

or as Obi Wan said "trust your feelings" ~ but thats not part of our cultural upbringing....and thats ok...sooner or later someone like LMI or Stumack gonna come out with a gadget for measuring tops and you won't hasta do it the old fashion way or use the maths. Yup, every body needs a gadget to do anything now~a days, right!

Maybe go have a chat with your local shaman.

Ya...trust you feelings...simple eh!

Maths!

However for the AR's reading this, you can now order my new book

"Looferin by the Numbers"
76 formulas needed to build a successful guitar.
just stuff $20. into an envelop C/O dis here web site and
Lance will be glad to send you your own copy.

Personal autographed copy's enclose an extra $5
Order now.


blessings
Padma



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Audiences and dispensations on Thursdays ~ by appointment only.


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Wes....

How much for one of those gadgets? Beats my cumbersome fixture all to Hamilton...