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I have encountered problems on my fan fret baritone with the G2 note going sharp around +10 cents and F2 flat -10 cents. The coupled body resonance is 92Hz F#2 and needs to be moved. The guitar has a sound port that when blocked reduces the resonance to F2 87Hz, I have partially and fully blocked it and it moves things around, so I can see what needs to happen, but I think that raising the frequency would be better. I want to move the frequency by adding stiffness to the back and or top. I tried changing frequency by adding weight to the top or back but this did little to improve it so I think that raising the frequency will be better.

I have done lots of reading in the Gore book and learned plenty from it and from trying things with this problem but am not sure where to go to improve things. The owner has great pitch hearing so is quite bothered by its errors

I am frustrated and Ideas from anyone who has encountered this type of problem will be greatly appreciated.

Sorry for the rambling description.



Fred

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Fred Tellier
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What frequency is the tallest peak that is probably your main top resonance?

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Formerly known as Adaboy.......

Yes it is the main coupled top resonance and is at 167Hz, the one that seems to be the issue is the coupled body resonance at 92Hz, which is F#2.

Fred

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Fred Tellier
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The 92 Hz resonance is the 'main air' part of the 'bass reflex couple'. The most effective way to move it would be to enlarge the port. You can get an idea of how powerful a given change in size will be by blocking part of it: if you reduce the size by, say, a couple of square centimeters, and the pitch drops by two Hz, then enlarging it by 2cm^2 should raise it by something like that amount.

One issue with doing it that way is that it will usually narrow the bandwidth of the 'air' resonance. This will make the top more active at that pitch, and if it's still close to a played note you could have an even worse problem. It's likely thart almost anything you do to raise that pitch will also lower the Q value, so it might not make much difference how you do it.

Where's the 'neck mode? If that's somewhere near high enough in pitch, and there's something you could do to raise it, like swapping out heavier machines for lighter ones, that could help. If you get the 'neck mode' to match the 'air mode' pitch, the two will couple, and you'll end up with two lower peaks spread over a wider bandwidth, which could help the issue. I'm not too sanguine: from the look of the chart you posted I'd think the 'neck' pitch is somewhere between 60-70 Hz, which is where it often is, and that's probably too far off to get it to work.

Al, I have reduced the port area by about 1/2 and it lowered the frequency by 1 hz and did help the intonation problem by about 3 cents. Would raising the back frequency with a extra brace change much? As to the neck resonance here is a plot with higher resolution and it shows a couple lower frequency peaks, would these be the neck resonance. Is there a way to isolate the neck frequency by taping on it?



Thanks
Fred

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Fred Tellier
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Hi Fred,
I am on my 4th guitar using these tuning principles now
The section in the book which I find I refer to most is the Graphs of Fig 2.4-5 on page 2-39 which shows the effects of making changes to the various parameters.
I would not be making changes to the soundboard or back stiffness here to change the air resonance.
You don't say what your back resonance T(1,1)3 is, how it relates to the top is important.

How big are your soundhole and soundport at the moment?

At the moment your air resonance is sitting exactly on F#, I would be a bit surprised that it is affecting F and G.
I would probably be trying to get it to sit at 90 by reducing soundhole or soundport size

The back coupled resonance is 217 Hz, it is a little hard to see the exact value from the charts I posted. As stated above I have reduced the sound port area and got the resonance down to 90 and it helped but not enough. I feel that the top and back frequencies are a little too low and increasing the stiffness of either or both will bring up the value of the box resonance. I thought that changing the port/ sound hole size will not do enough.

Here is a plot with the sound hole reduced to get 90 Hz top 161 Hz and back 206 Hz

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Fred Tellier
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90 Hz is midway between F and F# so should not be giving you tuning problems, especially on the G.
Perhaps there is another issue?

Jeff that is exactly what I am trying to figure out, I have done all the same research and am baffled what is happening. That is why I was hoping that someone here has seen something similar or has other ideas. I like Al's idea on enlarging the port but am hoping to avoid major surgery for small gains. The improvement with the closed port is about 5 cents so not enough to solve the problem and I am afraid going larger will only give the same results.

Fred

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Fred Tellier
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You said this is a baritone, tuned B-B?
Does the tuning problem show on both the 7th fret on the 6th string and the 3rd fret on the 5th string
ie G2 ?

Tuned C to C using heavy strings, and yes it is on both strings, that is why I am looking for a resonance issue.

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Fred Tellier
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Just to be sure you've covered the basics first, you did change the strings for fresh ones, as well as trying a different manufacturer and/or type of string, right?

Yes Mario, all the basics like strings have been covered several times. This is a body resonance driven problem.

Fred

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Fred Tellier
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I don't have a solution. A few things I'd try, if you haven't already:

You're tuned C-C, so your open 5th string is the out of tune F? The seventh fret harmonic of the 6th string, and the open 2nd string are an octave up from the out of tune G. There's another G at the seventh fret harmonic of the first string. What happens when you tune down to B-B? Or up to C#-C#?

What happens when you damp all the strings except the one you're measuring?

Does compensating the nut on the 5th string improve the picture?

What happens when you put a couple of paper towels inside the instrument (lower the Q, and alter the pitches of the air resonances)?

How does clamping temporary bracing on the outside of the back, the top change things?

If those experiments didn't provide enough information to get me thoroughly confused, I'd try adding a tornavoz, maybe look into astrology.

I'm showing my ignorance here......but I thought the string frequencies would move toward the resonant frequency instead of moving away from that frequency. Am I misunderstanding your first post?

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Formerly known as Adaboy.......

To quote Trevor Gore's book section 4.7.2 page 4-99 "Fretted notes above the body resonance will play sharp and those below will play flat"

Fred

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Fred Tellier
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Probably the best you can do is minimize the effect by locating the resonance exactly between scale notes
You have the choice of raising it to 95hz between F# and G by increasing soundhole size or
Drop it to 90 Hz between F and F# by reducing the size of soundhole or port

I'd be inclined to go down since it's a baritone

Modifying soundboard or back stiffness will have a minor effect on the air resonance, but not the 2-3 Hz you will need.

You are measuring by tapping the bridge, and your microphone is located at least 3 feet away?

From what I can see on the chart it's hard to pick out the 'neck' resonance, which normally won't show unless it's actually coupling with something. It's easy to hear though.

Hold the guitar up by pinching the neck at about the first fret between your thumb and finger, so that your finger damps the strings. Get your ear as close as you can to the top of the headstock surface without getting stuck with a string end, and tap on the back of the headstock with a finger tip. You should hear a low pitch, often around 60 Hz or so. To be certain, you can add some mass to the headstock and see if the pitch drops.

Sometimes you can fake an added brace by making an arched piece to match the curve of the back or top, and sticking it on with double-stick tape. It won't be as stiff as a real brace that's glued on, but it will have some of the effect, and give you an idea of whether adding one might help.

I just had another thought about this. 10 cents flat on F2? Doesn't that mean that the fundamental is off by about .5 Hz? So it's beating once every two seconds? I think that would sound pretty good to any ear. If the higher partials are 10 cents flat, that could get ugly. But if that's the case, shouldn't you be looking at resonances in those ranges?

Fred, In your very first paragraph in this thread you stated that you wanted "to raise the main air by adding stiffness to the top or back." I agree that this may be solution you are looking for. In a recent build I had a main air which initially was waaaay too low for my intended purpose; a flamenca blanca. After many iterations, reading and attempts for an easy fix, I finally ended up taking the back off and increasing the height of the back braces substantially. This solved my problem and raised the main air resonance to 98, from the initial 88 with which I was sorely disappointed and had resulted in a very bassy response and lack of clarity.

As Al Carruth describes in one of his posts, you can make a brace and tape it on to the top or back to see how it affects the main air. This is what I ended up doing to determine if I was headed in the right direction. The substantiating data from the temporary 'taped on braces' on the back was enough to convince me to proceed from there.

Many here have a great deal more knowledge of this subject than I do and I am just sharing what worked for me. YMMV

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Peter

Thanks for the info guys, it is basically where I was heading also, but it is great to get others ideas. The guitar is back with its owner for a week with the sound port blocked enough for the body resonance to be at 90 Hz. The G2 is still around 5 cents plus and the F2 about the same amount minus. If this will make it sound OK to his ear we will take take Al's idea and raise the frequency by way of sound port enlargement and I will add some stiffness to the back.

Thanks for the input
Fred

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Fred Tellier
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Fred, if you tune the guitar 10, 20, 50, or a whole pitch sharp does the degree of the problem or fret position change? Is the intonation problem affected at all octaves? Would not shifting the tuning by some number of cents prove or disprove that the problem is related to the air resonance frequency and qualify how far it needs to be shifted? I'm just trying to understand...

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David Radlin
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Hi Fred, One thing I am having difficulty in understanding is if you now have the air pitched at 90hz which is midway between F and F#, why it is affecting G at all?
What is F# doing?
Your air at 90Hz is 8hz below G and should not be causing tuning issues with it at all.

Dave the problem is at the 2nd octave in the area between the F and G.

Jeff the plus on the G reduced and the F# is now gone a few cents sharp the - on the F also is less.

Peter the raising the frequency is probably where we will go next. We have been doing non surgical things 1st.

I do not have the guitar with me to make any exact readings at this time. The movement of the resonance from 92 to 90 has had a good affect on the issue but not enough to make things totally correct. As said above I gave the guitar back to see if Paul can live with the amount that it is still off. We discussed where we will go next and it will be the larger port and extra back bracing but 1st he needs to see if he can live with the amount of error that we have now. Even if we go between F#2 and G2 there will still probably be an error around that area of the octave. Paul the owner is a very good Luthier and a more experienced set up guy than I am and we are both scratching our heads on this.

This is such an interesting problem and has been a great education with the research and experiments I have done, but I am glad to not have the guitar available for a few days so I can clear my head and get a rest from it.

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Fred Tellier
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Fred, if you tune the guitar up by 10, 20, 30 cents or whatever, does the intonation problem disappear.... I mean just as a trial to prove your suspicions?

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David Radlin
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