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I'm in the midst of my fastest and smoothest build so far.
This is the build where I did a glitter pattern and tuned the top up.

I'd like to tune the back to the top.

Problem is, the top's free, un attached fundamental pitch is 233hz (which seems a bit high, but I assume it will go down once I attach it to the rims.) The back, which is Ovankol, quite flamed, very pretty and REALLY STINKIN' dense is at 300 right now after my first two tries at thinning braces.

At least I think it is at 300. The other possible frequency, using fft analysis is at 180. There are no peaks in the 200s.

Is this a normal finding? If so, what should I do to get the back down another 60hz? Scrape it a bit thinner? Continue taking mass off the braces (which are already getting quite short.)?

I'm at a loss and really want to keep rocking and rolling on this build.

Thanks in advance for any advice anyone can lend.

Dave

Gee, I hadn't thought about Chladni patterns in years.....

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"Act your age, not your shoe size" - Prince

Dave,

I don't find that matching the free plates means they will match when glued up. I don't have a real scientific way of knowing what the plate will do when it is glued to the rim. I would be happy with the 233 as the ring+ but it should definitely drop when glued up. With the back I guess based on feel and adjust later to match the top.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

Dave, I don't understand why the top fundamental frequency would go down after the top is glued to the sides. This will increase the fundamental frequency as the top is now constrained and in a stiffer configuration.
I do fft's on my tops and they always increase after gluing it to the rim, like I would expect.

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Jim Watts
http://jameswattsguitars.com

Jim is probably correct there. I think of the top main frequency as the ring+ frequency on the unbraced plate but that is not scientifically what it is (I think) and it doesn't help when I talk like I know what I am talking about when I don't! Or at least it doesn't help when I use scientific terms in an incorrect way.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

You may want to reconsider that. Typically, people that are tuning them get them close but you want them at least a half step apart. If they're "the same" you can get wolf tones and other artifacts. Basically you want them close enough in pitch so that they reinforce each other but far enough apart so that the width of the resonance is wider and the peak lower than it would be if they were the same.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I go for having the back and top plates at the same resonant frequency after closing the box. The top then drops about a half-step when the bridge is glued to the top, which is where I want to end up. The top often drops a bit more after a few weeks under string tension. Then I might bring the back down to get back to that half-step difference.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

Sounds like a good plan. Do you find that the string tension itself changes the top frequency by any significant amount?

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Andy,

Oddly enough, I haven't. After putting on the strings and tuning to pitch, the change I see seems to be more related to the guitar settling in with the new tension. It appears to be more noticeable during the first few weeks, which is when I hear the most change in tone.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

Check out this video. He doesn't speak about frequencies, but illustrate really well the relation between the top and the back. Check out at 1:59 of the video.

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Francis Richer, Montréal
Les Guitares F&M Guitars

I'm counting on the bridge dropping the pitch a half step, so am not worried about wolf tones.
Still curious how to get the fundamental pitch down 60hz though.

Mission Accomplished.

I took a scraper to the back and thinned it enough that shaving on the braces was a viable option.

I'm tuned up and gluing pruflings.

Dave

Pat,
Sorry if this is dumb question but how do you adjust the back after the box has been glued ? Are you scraping braces through the sound hole ?

I walked down the aisle at Guitar Center the other day tapping the acoustic guitar tops and backs. Seems like most were around a third apart, some the same, one was a fifth apart.

That sounds typical, about the guitars at GC.

Not a dumb question at all. Usually I get the backs down by shaving the back braces, using a small spokeshave with its "ears" removed.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

The ring+ mode of the free plate is typically 40-70 Hz higher than the main (0,0) mode of the glued top. This is well documented and very consistent. I think Jim is looking at a different mode on the free plate, perhaps the ring mode.

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

I don't think I'm confused. The ring-and-a-half mode is not the fundamental frequency of the top. It's another mode.

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Jim Watts
http://jameswattsguitars.com

Hi Jim, sorry I didn't say you were confused or meant to imply it, nor did I say the ring+ mode is the fundamental frequency of the top. What I'm saying is that the confusion lies in what mode Dave is looking at. I'm convinced he is looking at the ring+ mode and is a very significant mode that those of us who do chladni patterns look at. The ring+ mode is one of the largest amplitude modes of the free plate and understanding the affect its shape and frequency has on the sound of the finished guitar is a major component of free plate tuning. The frequency of the main (0,0) mode of the top typically is 40-70 Hz lower than the frequency of this mode. There is lots of literature on this stuff.

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

I think your probably right Randy.

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Jim Watts
http://jameswattsguitars.com

My nomenclature was off.
The mode I was looking at in the fft scans was the ring+, which showed up as the highest peak.

Dave

Here are some typical numbers for my guitars:

Braced freeplates:
Top ring+ = 250 Hz
Back fundamental, 250 Hz

Finished guitar fundamentals:
Top 190Hz
Back 200 Hz
Body air fundamental 98 - 101Hz

The trick with the back is to tune the unbraced plate before bracing. I look for a ring+ at about 110 Hz. Often it looks line three horizontal lines because the lower two don't close into a perfect ring+. If you use standard back bracing, where 3 or 4 braces run straight across, the braces will do almost nothing to change the lengthwise stiffness. That's why you need to get it right by adjusting the thickness of the back. Once the braces are on, you can carve them as needed to get the cross grain stiffness in balance with the lengthwise stiffness. Keep an eye on the cross dipole while you carve the back braces. If it's blown open at the corners, the bracing is still too stiff.

Mark

Mark,
How do you measure the body air fundamental?
Thanks

I use my signal generator to measure the air mode. Support the body on foam blocks so that the back can vibrate freely then hold the speaker near, but not directly over, the sound hole. When you get the right frequency, you will know it, the body will vibrate like crazy. On most guitars it's between G and A (97 to 110HZ).

You can also find it by humming into the hole. You will know when you're humming the right pitch. A simple guitar tuner will tell you the note.

M