Official Luthiers Forum!

Ok, so I'm trying to actually tune a top to a specific pitch. The freq of the free plate according to my software was about 192hz. I expected that to rise after glue up, but to my surprise it dropped to about 141. Anyone have any idea what happened? Will that number change when the back goes on?
Thanks

Meddling, I don't have an answer for you but somethings not right, this doesn't pass the haha test, as you're going against physics here.
So, I would go back and see if I'm testing exactly the same way, same equipment ect..
192 hz sounds awfully high for a free plate top fundamental frequency.

_________________
Jim Watts
http://jameswattsguitars.com

I agree with Jim's assessment.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Jim, I think you and meddlingfool are confusing two different modes on the free plate. Meddlingfool is looking at the ring-and-a half mode or ring+ (the #6 mode), which is the mode most people look at to tune the free plate. When glued up, the (0,0) mode of the top is about 40-70 Hz lower as observed by meddlingfool. This is well documented and has been published by Al Curruth as well as by many, many others. Starting with a ring-and a-half mode on the free plate at 192 Hz is pretty darn low. Typically you'd want it between 220-250 Hz. Unfortunately, your reading of 141 Hz for the (0,0) mode of the top is probably correct and way too low.

When looking at the back free plate, you are looking at the #4 mode, so this one jumps about 40 Hz when glued to the rim.

_________________
Randy Muth
RS Muth Guitars Website
RS Muth Guitars Blog
Facebook Fan Page

He states "software" not a Chaldni pattern so I'm assuming an fft. I'm in full agreement with you, if this is the ring and a half mode it's way to low IMO but his numbers could make sense.

_________________
Jim Watts
http://jameswattsguitars.com

Thanks for the replies folks.
First bear in mind that I have never tried this before, so I may very well be going about it all wrong, so I'll say how I did it.
Yes I am using FFT, iAnalyzer on my iPhone.
To get the resonance I just hung the top on my finger through the soundhole and bonked with my finger. Maybe not too precise, but it read within 1hz, usually less over multiple taps. Ten knocks averaged out to give the number.
I rough shaped the braces until I had them what I felt to be about right, with what I thought was a bit more meat left on. This measured out to around 192hz.
Then I glued the top to the sides and after drying knocked again. That's when I got the 140hz. However it was still in the mold which I think was part of the problem. When I took it out it measured at 170hz. Which unfortunately is still lower than I want, especially if it will drop even more when the bridge goes on.
I don't suppose the pitch will miraculously rise when I glue the back on?
Seriously though, is there a method of raising the pitch without resorting to rebracing?

And another question.
Why does shaving the braces (removing wood) lower the pitch, while adding a bridge (adding wood) also lower the pitch?

Well I'm no expert but here are my thoughts.

Increasing stiffness (assuming the same mass) raises the pitch. Reducing mass (assuming the same stiffness) raises the pitch. And the reciprocal of these statments also holds true. Decreasing stiffness lowers the pitch (assuming the same mass). Increasing mass lowers the pitch (assuming the same stiffness).

When you shave a brace you reduce mass (raising pitch) and decrease stiffness (lowering pitch). When shaving braces the reduction in mass is more or less linear (meaning there is a linear relationship between the amount of wood removed and the reduction in mass). However, when shaving the brace height the reduction in stiffness is not linear, it's exponential (remember the cube rule?). So the affect of the reduction in stiffness outweighs the affect of the decrease in mass and the pitch drops.

_________________
Formerly known as Adaboy.......

OK, but still the same answer. You're still looking at the resonances exhibited by the various vibration modes using impulse testing with FFT analysis. The ring+ mode will have the largest amplitude in that region and most assuredly corresponds to the 192 Hz frequency meddling is seeing.

_________________
Randy Muth
RS Muth Guitars Website
RS Muth Guitars Blog
Facebook Fan Page

Meddling, resonance is a function of the spring constant (stiffness) / mass. Not that exact formula, but that's the relationship. Stiffness can be effected really quickly due to the "cube effect" on thickness. You can significantly reduce the stiffness without changing the weight in the same proportion, the result is a lower resonance, Also adding weight will lower the resonance. So, shaving the braces reduces the spring constant and adding a bridge increase the weight ( it also increases the spring constant btw). So if the bridge is dropping the resonance it's wieght is a bigger factor than the stiffness it brings to the system. I don't believe all guitars follow the "bridge drops the resonance" model, classicals may be different as they have lighter and longer bridges, just guessing here though.
That's probably clear as mud, but I hope it helps.
This is the reason I said you were going agaist physics in my earlier post, by constraining the edges of the top (gluing it to the rim) you have stiffened up the top making for a larger spring constant while keeping the wieght of the top constant.

_________________
Jim Watts
http://jameswattsguitars.com

Just gluing the plate on the rims will give you a lower reading. When you put the binding on everything changes again, it will raise the resonant frequency a BUNCH. I don't know how to predict just how much it will change tho...

_________________
The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

Well, go figure. I forged ahead and built the box. After gluing in the soundhole braces the top's final resonance was 175hz. The back free was 222. Assembled, knocking on the top with the body upright on the bench yields 208, knocking on the back yields 205.
What this will mean in the endgame, I have absolutely no idea.