Australian/New Zealand Luthiers Forum

Ah, right! You did mention that. I'll drag my 100Hz numbers out as soon as I can.

You have all probably seen this problem: areas where the wood is missing between the grain lines, giving parts of the top a kind of washboard look. One has to ask why the wood is missing. Torn out through planing the wrong way? Scoured out with too coarse a grade of paper? Eroded out by continually...

John, the D's are all a function of ω, so how are you handling that? In your last post you must have them evaluated at a particular frequency?

Well, John, I think you're there, or very, very nearly. Well done! Sorry about that last bit. Not sure why that didn't get into the book! Some of the residual differences may be computing "differences". For example, my computer (Toshiba Tecra running XP) gives: αat (alpha at) = 2512.222087 αab = 233...

When we calculate the Di's for each increment, do we also need to do the same with the Ys... I just checked my spreadsheets: I used COMPLEX to get the D's, IMSUM to evaluate the determinants, IMDIV for the y's, IMPRODUCT for the p's; and there is a value for each of these parameters for each freque...

I just nested the imsums and imsubs because I was trying save space to keep everything on the same page so that I could try to keep up with it. I put it all on one sheet, but was out to column DG! Given that the resultant air pressure is still in a complex form (rectangular x+jy - for 100Hz it is a...

Yes, history has not done us any favours with the nomenclature. Really, the i or j operator (depending on whether you are an electrical engineer or not) should be a sign like "+" or "-", at least for engineering applications. Anybody who teaches that j is the SQRT(-1) has no understanding of the con...

I just checked my spreadsheets again because I couldn't recall any nested IM functions. I had a bunch of columns hidden (not noticed before, with current half speed brain!). So the determinants are a bunch of IMPRODUCTs in separate columns and then these are IMSUMed and that's how I avoided nesting ...

Dominic wrote:PS, I hope my questions are clear or I am doomed

Probably doomed!

I think my last post might have answered most of your questions. If it didn't, have another go!

I worked out the method to pass the complex equation to the IM functions called COMPLEX funnily enough. I ran the IMABS function and got the same answers as the manually calculated cells (but you're right, a lot easier to manipulate). Which IM functions do you use? I assume from above that you use ...

The mass per unit length is the mass per metre of a string. So say a bit of guitar string weighs 5 grams and is half a metre long, the mass per unit length is 0.005/0.5 = 0.01 kg/metre. Should I be converting to Kgms, metres, and Hz? What sort of number should I get for k. As in how many digits, 0.4...

I'll do a bit of research on them this evening and have a bit of a play. I just manually solved them for the magnitude (hypotenuse?): You need the phase angle as well, rather than just a conversion to a scalar. Not sure if you've got this in or not. The IM functions take care of this without having...

johnosb wrote:Asoundhole radius change seems a bit odd 44mm radius sets T(1,1)2 to about 155 Hz but at 50mm it shoots up to about 180Hz. I had it set to 44.7mm to match your area and volume of the 'air pipe'.

Hmmm. Missed out a 2 changing diameter to radius?

Os, sorry for not getting back earlier. It's hard to see where you're going wrong (or where the book is incorrect!!). But to get a plot like you're getting, you must be pretty close. So a few quick comments/questions: 1) the D's need to be calculated for each frequency increment (5 Hz, 10 Hz etc). I...

One of my customers goes into his local music store [possibly due to the fact I am building him a new guitar] and plays all their top shelf guitars: Martin, Taylor, Maton etc. salesman asks waddaya think? Customer says there’re all crap ... This is what I call the "conspiracy against guitarists". T...

I am pretty sure I read in Trevor's book that balsa needs to be used with horizontal grain. Although I don't remember the reason. The medullary rays seem offer better shear resistance (slipping of one plane through the thickness over the other). Instead I do not recall where in the book Trevor talk...

Quick question Trevor. Have you done any emperical studies to determine whether the "average" player can tell the difference between your work and a decent guitar from, say Martin or Cole Clark? I'm not challenging the legitimacy of your work, which I acknowledge, but wonder HOW MUCh of a differenc...

Does my head in, all this math, and I sometimes wonder if it's truly needed. I mean, if you go into a guitar shop, and pick up 10-20 acoustics, you'll like the sound of them to different degrees, and some you'll just hate, and if lucky you'll find one you love. And that epitomises the problem. If y...

Mark, one thing you could try is capo-ing very close (almost on) the fret, which minimises the string deflection whilst still allowing you to clamp pretty hard. Whether this works for you depends on whether or not you now feel crowded out of the "nut" position. Next option is a really good capo with...

Hi Os, Well done for having a go at the 4-DOF model! 1/ The omega and omega squared value in the displacement denominator I assumed to be 2 Pi F where F is the model's input frequency. Correct. 2/ The denominator (Dt, Ds, Db and Da) is in the form z=x+jy and the solution is z=sqrt(x2+y2) where x = m...

The "bulge" is a lot smaller than the curvature given by 5m radius; but, anyway, a Jim Williams type binding router (two point support) or a vertical axis binding machine (as per the book) will still work fine. A "rolling pin" binding router, if you have the nerve to use one, might be somewhat more ...

Where did you source the pulley for the strings to run over? I guess a Meccano set would be a good source for such stuff? Try a place that specialises in racing dinghy fittings. Whitworths is typically the go around here, https://www.whitworths.com.au . If you're building a rig like mine, you will ...

kiwigeo wrote:My brother is the mathematician in my family...an honours degree in maths and currently teaching maths in Bhutan

Well, it sounds like you both share an interest in mountains...

Can we have the equation please? :mrgreen: You mean you can't do it in your head? :wink: OK. What you're looking for is the included angle between the slope of the tangent to the back dome and the sides at the edge. The largest variation in slope will be between where the guitar is narrowest and wh...

liam_fnq wrote:If I built a guitar with say, a 15' radius back, would the angle that the back meets the sides be the same all the way around.

No. The angle that the tangent to the domed back makes with the sides will vary by ~1.5 degrees if the sides are more-or-less constant height all round (i.e. no big wedge).