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Greetings,
I have a guitar that is my first experiment in tuning the plates to a specific pitch. On the closed box, I was counting on the bridge, saddle, and pins to drop the freq a semitone below the back, but this did not happen. Now they are very nearly identical to each other, within a hertz or two. Unfortunately, the pitch is just around F3, lower than I had meant, but so be it. If it were higher, I would just shave the top braces to drop the pitch to the target semi-tone below the back interval, but as the pitch is already lower than the 'magic' F#, I'm reluctant to do so. I was thinking I had three choices...
1)Sand the bridge wings down to remove mass (I did put thicker bridge wings on it in an effort to drop the pitch) and thus raise the tops pitch (hopefully). If this did work, the top would be above the back in pitch, for good or ill I've no idea. It could also have the opposite effect by making things more flexible and thus lowering the pitch, stymied here...
2)Sand the top braces to lower the pitch of the top to the 'correct' relationship to the back, and accept that the overall freq spectrum is lower than ideal for a dreadnaught, for good or for ill I know not.
3)Somehow raise the pitch of the back, how I would accomplish this I have no idea...
So, if anyone has any thoughts on how to get around this little problem, I am all ears!
Thanks in advance....

Can you flatten the top of the 2 lower back braces and glue additional brace material to stiffen these two braces? (the back braces in the lower bout) If so, this would stiffen the back plate and likely raise the pitch above where you want it. You could then remove height from these two braces a little at a time till you have the relationship you want between the top and back plates.

Also, the main top frequency may drop a little after it's played in.

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Formerly known as Adaboy.......

Similar to Darryl's suggestion, add a 5th back brace--either adjacent to #3 or #4, in between, or below #4 (if you can reach that far).

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David Malicky

I'd wondered about that, but I think that may be just out of reach for my repair skills and that I'd end up demolishing an otherwise nice guitar....

First off, how does it sound?

If it's a new guitar, I'd wait a few months to see if the top's resonating frequency drops. Then go from there.

Pat

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

The main top resonant frequency will likely drop a bit after it is played in. You may not have to do anything.

Chuck

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Changing the freq of each brace, can lead to a lot of frustration. and impatience. Shoot for a middle ground , were they will all have a slightly different pitch. Perhaps playing the instrument for a while will change the fundamental freqs.

Another vote for waiting a few months. Some well respected builders try to tune both plates to the same frequency knowing that the top frequency will normally drop by a semitone during the initial break in. This includes the frequency master, Al Carruth, if I'm not mistaken.

Aaron

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Aaron Craig

Play it hard for a month and then proceed if necessary.
Tom

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A person who has never made a mistake has never made anything!!!

It's nice to have a perfect goal in mind but rarely does it work out exactly as you planned. For my thinking, they only need to be close, there is no magic in that perfect semitone apart. If you are not getting a wolf note at the pitch they meet at I would say you shouldn't have to alter anything unless it really sounds bad or you are getting that wolf note. I would think that the top would drop a bit as it is played in. I should know, but I haven't done the tests on my own guitars to back up Al's assertations that it does. I don't really doubt Al, but I should do my own tests as well.

I always aim to try to get the top and back in "tune" with each other and to have a nice even set of Chladni patterns but it just doesn't always work out. I can't even say that the guitars where it does not work out are really any worse than the ones where it does. Over time maybe they will show themselves to be but I don't always get to see them again. I should maytbe be frustrated that I don;t see patterns or results from the method I follow but really as long as they keep sounding good I am not that bothered. I am also not going to stop keeping data and looking for patterns though.

Can we see a picture of the guitar? I am also curious about the magic F#, I haven't heard of that as a goal before.

As an answer to your original question though, if I was going to remedy it by altering something I would glue long grain patches over the back cross grain reinforcement. I'm not positive, but I would expect that to raise the back frequency a bit and also not be super hard to do. That is assuming the top is already too loose. If not, I would perimeter sand the top and if it is already finished and that is not a possibility I would work on the x scallops.

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

I was also wondering about the "magic F#".......where did your hear or read about this?

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Formerly known as Adaboy.......

About the F#, I couldn't even begin to say the huge variety of different places I've read that...just goes to show how the net streams different things to different people via different searches. But I've been seeing that for years. F#top, G back...
I am actually getting some snappies of this guitar so I might be able to post some soon, if I can figure out how.
In the meantime, in a startling break from character, I've decided to do Nothing At All, and to stop meddling with this one. After futzing around with an extra brace on the back, putty weights on the top etc., I realized that nothing much was changing the noise coming out of the box. So I've decided to listen to John for once, and Let It Be. Unless that was Ringo...

Having the top and back the same frequency will not necessarily lead to a wolf note. I have built guitars like that and they were not a problem at all. I built one with the top and back at 190 Hz and the main body air mode at 95Hz, exactly 1 octave below the top and back. No wolf note.

I say finish the guitar and string it up. If there's a problem, deal with it then.

Mark

Actually, Sir Paul (though not a knight at the time the song was written).

Aaron

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Aaron Craig

Never could remember which moptop did what...

The F# top seems to be from the classical tradition--Google shows some hits. It is in the middle of the typical range of 'main top' resonances for steel strings as well (185 Hz, though I've heard some great dreads that are at 170 Hz, between E and F.)

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David Malicky

I think leaving it alone, at least for a couple months is correct. OTOH you could glue a nickel to each bridge wing

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

that also puzzles me. i once heard Jose Romanillos mocking this F# thing, it was something like "don´t waist your time with F#-ing this or that, just build the guitar the best you know". worked for him, i´d say.

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member of the guild of professional dilettantes

Miguel, many people tend to mock the attempts of basic resonance measuring in guitars, but really it is just basic stuff. You don't even need any high tech methods, you can simply feel the main resonance points when playing the guitar.
Most guitars (probably 95%) from 300$ to 20000$, will measure somewhere in between and around F# - G#. It's not an indicator of quality tone, just a rough indicator of general stiffness, and a rough indicator that the instrument has a normal guitar voice. Stiffer plates/bracing tend to high tuning, producing a brighter treble. Romanillos' tops are in this camp rather, even if he doesn't searches for a particular high pitch. His methods lead to a certain result, that is all.

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