Australian/New Zealand Luthiers Forum

Unfortunately, I have no further explanations, just further information. A little googling, and I found that steel strings on guitars are made of the same material as piano wire . Which is spring steel . Specifically, SAE grade 1080 ( ASTM A228 ), which has an elastic modulus of 190 GPa. Or maybe 21...

Jurgen-- I know it's been four years since you posted this, but did you ever figure this out? I got the same results you did. I got a value for E that was nearly twice the usual value (411 vs. 210 GPa). Using your numbers for the E string: T = 8.07 * 9.80665 = 79.14 N ΔT = 79.14 * ((((135.7*325.25) ...

I have posted several spreadsheets here before. Panel thickness, fret positions, fretboard radius/width, neck shape, intonation compensation, 4DOF model, brace stress/sizing. I decided to combine these into one big spreadsheet file with 20 worksheets, and create connections between them, so when you...

edvinguitars.fi wrote: ↑

Tue Mar 05, 2024 6:04 am

Without weights ( except my weighty double laminated sides using purple heart in inner) I hit 96/178/226, by shawing off the back brace briefly, maybe 2-3mm

I think that's right on target. 95/180/227. In any case, it sounds great!

Thanks for sharing.

Greg

Well, with the caveat that I've never built a 12-string, I have these observations, based on my calculations: The panel is 60 to 70 % of the mass of the active area of the sound board. Mass is the #1 priority in a good sound board (other than not breaking, of course). Braces are 75% of the rigidity ...

Wow. Gorgeous. Amazing work. Did you have any trouble obtaining North American woods (Walnut, Sitka) in Finland? And Spanish Cedar comes from Central and South America, I think? Usually used on nylon-string guitars, its flexural modulus is a little on the low side, at 9.12 GPa. Have you found much d...

This forum (on "the book") used to be a lively place. Now it can be weeks between posts. I've posted some things that should be of interest to those who have the book, and, save for Trevor (thanks, Trevor!), I haven't gotten a single comment. So where have the "left-brained" people gone? Where can o...

Google Sheets is a bit slow on the 4DOF model (it's plenty fast for everything else). In the spreadsheet linked above, I only calculated every 1 Hz, 60 to 500, so 440 rows. This takes 9 seconds to recalc, so about 50 rows/sec, if for example, you change Kt. In a copy of this sheet I tried every 0.1 ...

I couldn't find any threads about this section--I guess no one has implemented these formulas? Here's a Google Sheet for neck cross sectional shape , as in section 4.6.7. Look at the sheet labeled "back". You may make a copy and modify it for your purposes. I wasn't sure how to smooth over the trans...

Can Google Sheets implement the 4DOF model? Yes, it can. It works pretty well, too. The graph appears to exactly match the output from Jim's Octave script (graph I posted earlier in this thread), and is pretty close to the book, Fig. 2.4-4. The differences appear to be in the dB values, but the freq...

OK, I made an improvement to the parabolic model. New formula is: y = M/2EI x(xlast - x) This gives just the relief of the parabola compared to a straight line from the nut to the last fret. So, similar to the circular formula, it's 0 at the nut and at the 18th fret. To get the right action at the 1...

OK this thread's pretty old, but I thought I'd add my implementation to the pile. Here's a compensation spreadsheet using Google sheets. Yes, you can see the formulas. You can make a copy (File -> Make a copy). I used the sample data for measured action from Woodsy23 (eight years ago!), and I got pr...

Thanks, Trevor. I get it. I will have to fit the model to the prototype if I want any accuracy. My hope is that when coming up with a completely new design, where I know what frequencies I want for the first three resonances, a model will at least give me a target top stiffness (brace dimensions/mas...

Thanks, Trevor. That makes sense. So, if I designed with a target stiffness K, and I could estimate the mass m of the top within the piston area (I guess, within the Chladni line of the monopole mode) including the bridge and bridge plate, I should not expect an uncoupled frequency f = SQRT(K/m)/2π,...

Or, asked another way, what is K-bar on page AII-7?

He says the equations have the form:
In which case, for the top,
But what which should be used to calculate MM or frequency, K or K-bar?

Greg

In the 4DOF model, on page AII-7. Equ. AII 2-21, we see the natural resonant frequency of uncoupled top with small damping: ω^2 = (K + κA^2) / m This is quite different than the relationship used for frequency, stiffness, and mass used when calculating the monopole mobility: ω^2 = K / m I'm a little...

There's not a lot of information in the book about sizing braces. There are some standard sizes given, such as a J-45 using 1/4 x 3/8" and 1/4 x 1/2" gabled. Or a CF+KBF falcate using 5x4 mm and 5x7 or 5x9 mm rectangles. The example stress calculation uses 6x8 mm and 8x15 mm gabled. And there's a di...

I don't have data on the spread within a species, or even within Spruces, but I used the averages from wood-database.com to chart about 200 species, both softwood and hardwood. And there is a pretty good fit to a trendline relating density and stiffness. And an even better fit relating density and t...

By the way, for people in North America, it appears that a good substitute for King William Pine when making falcate braces, is Black Willow . Native to the Eastern US, and is not expensive. I haven't tried it myself, but the stress calculations look good, and the US Forest Products Lab says that 73...

Ok, thanks. I wasn't sure if the transformed section method was an exact calculation or an approximation. If it is an approximation, then I wouldn't need to get the exact same answer. My method differed by 7%. My method seemed more realistic (actual dimensions used to build, paired with actual E), s...

However, when you're adding second moments of area together you have to use the parallel axis theorem, whether or not they are multiplied by the same E or different Es. The only time you can simple add two I's together is when their neutral axes are in the same plane (which is the transform you are...

Any engineers out there? I have a question about calculating stress when using a combination of materials. Specifically, I want to calculate the stress of a sound board when the panel and the braces are different materials (different species of wood). I've got a google spreadsheet here in which I've...

Nice to know they're doing a good job at PRT. I had seen one piece on their website that was measured when it was 21 mm thick. If they then slice it up to thinner pieces and put the same label on all of them, I suppose there could be small variations within the larger slab, which would explain the s...

I stopped using it and run REW, I still look at it sometimes, but it’s never clean like Trevor’s graphs and I have it spec’s to his settings I like Room EQ Wizard too. It's free and it runs on Windows, Mac, or Linux. Also supports the UMIK-1 USB measurement mic ($130), and can import the calibratio...

Here's a Google spreadsheet that shows how to calculate thickness using the book's equations. The thickness is in column T.

Hope this helps,

Greg