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I have been trying to work out how to accuratly work out the pitch of the back and top of my recently completed No 1 instrument . I have read quite a few previous posts and understand that the ideal is to have the back a semitone higher than the top. Do I need to remove the strings before attempting to tap the plates? Also do I need to block the soundhole while tapping?
Another question related to tuning the plates. I understand most modify the top bracing and or graduate the thickness of the top plate in order to tune it to the semitone lower than the back. I was wondering if working with the back bracing was a worthwhile alternative to get the same result? Would thinning the back drop it's pitch?
Thanks in advance
Craig.

The easiest way to determine the top and back plate pitches on a finished guitar is with a signal generator by generating Chladni patterns. You simply find the frequency at which the top and back each produce a big circle in the lower bout. The strings should be left on and the sound hole should be open. You can damp the strings by putting a folded paper towel under them on the fingerboard. when finding the back modes the guitar should be at least 6" or so above the bench to avoid "blocking" the soundhole.

If you have good ears, you can also find the pitches by tapping in the center of the lower bout of the top and the back. This is not as easy because when you tap the loudest and lowest tone will be the air cavity mode. The pitch you want is the first one above that and is is not all that easy to hear with the air mode drowning it out. If you plug the soundhole, it will be easier to hear the pitches of the plates themselves, but plugging the hole changes the pitches a little so that's not quite as accurate.

My experience is that in order to get the plates within a semitone, you usually have to make the top stiffer than it might otherwise be and make the back more flexible than usual.


On my guitars, the top is usually between 185 and 190 Hz and the back is usually between 195 and 200 Hz. (not sure what notes those are)

Mark

Great post Mark! CLAP, CLAP, CLAP, CLAP!

This has worked very well for me too. My pitches are about 30 Hz higher, which shows a higher stiffness to mass ratio, but I don't know what that means in terms of tone. I was pleased with the results after lowering the back pitch on two finished guitars to within a semitone of the top.

I found that it's easier to retain the dome on the back if it's built with an x-brace in place of the two lower transverse ones.

Alan Carruth has all the big toys on this. Maybe he'll stop by.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

Pardon my ignorance, but why are you doing that?
What does this tell me as a builder and why is it a good thing to be in that tonal range?

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Mitch

Mitch

When the top and back main resonances are within a semitone it changes the way these resonances interact with each other and the main air mode of the soundbox. Instead of the top and back resonances producing distinct, narrow, peaks in the output frequency spectra of the guitar, the two merge to produce one bigger, broader, peak. This merging, or coupling, helps add a "fullness" to the lower end response of the guitar. The coupling will also cause the main air mode to lower in frequency a little, further supporting the lowest note on the guitar. In this way, it is possible to get a strong, full. low end response without having to make the top so flexible that the trebles loose their power and fullness.

Where the heck is Al anyway..... this is really his terrain.

Mark

Thanks Mark - thats a great explanation...

So how do you engineer this in the build process? I saw this:



but if you are doing this at the end, with the strings attached, then the ship has sailed on this guitar and you are just making notes for the next one? How do you adjust on one that is already put together?

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Mitch

This has always intreagued me.

this tuning.......

do you do it before you fit the top or after?

if after, before binding, & finish?

and how does fitting a bridge change that note?

and then fitting pins & a bone saddle?

and does intonation adjustment by filing the saddle change the note by a Hz or two.........

Sorry if this sounds like I'm a cynic..........


BUT!!!

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Mitch

Ya, once the guitar is done you can only change things so much. It is too late to thin the plates or make the top stiffer, but it is possible to lower the back frequency by reaching in and shaving the lower bout braces. Quite a bit of change in back frequency can be achieved this way.

Also, see my next post....

Mark

Mark,

Would you get the same coupling if the back was a semi-tone lower than the top rather than a semi-tone higher?

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

I have been attempting to do this on my last couple and have some observations. I find that attempting to match the free plates to the same tone is very hard for me, I know sort of where I want them stiffness wise and then join them to the body. Once on the body I block the soundhole and tap in the lower bouts and then usually they are fairly close. Almost always the back is higher and I use the random orbital sander to bring it down (I leave a little extra thickness knowing i will have to do this). Another thing I have noticed involves tapping the body with the soundhole open. On Ervin's presentation that was available to watch he talked about tapping the top and hearing the sustain of the tap which, I believe, he said was coming from the coupling with the back. He muted the back to show that this sustain disappearead when the back was muted. When matching the top and back, that sustain seems to get better and better until they are matched, when it becomes a wolf note and is almost nonexistant. This is what I think is happening, because it happens when to my ears they seem to be the same (I have been using my ears up until now). This is the best trick I have come up with to know when they are spot on, although I don't fully trust it. I will say too that the epoxy fill will alter the pitch. I have been doing 2 fills, leveling, and then tuning again before a 50/50 spit coat. I don't think there is any magic, though, in an exact semitone, so pretty close is probably still really good. The idea is that if they are matched pre-bridge then adding the bridge will lower the top a half semitone and the intial playing in of the top will drop a bit further to reach the desired difference in pitch. I have been tapping but am a few days away now from being set up to test the Chladni patterns and I hope it will give me a more accurate way to measure these small differences. Thanks Mark for your answer, every little bit I learn about this makes the picture clearer.

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

I do this "tuning" throughout the entire building process. If you have an idea of where you want things to end up, and you measure things along the way, you can, through trial and error, over the course of several guitars, get a handle on how the various parts should respond in order to get them to all come together and produce the desired result.

Example: If I measure how stiff a back is before it's attached to the rims and then, when the guitar is done, the back is stiffer than i wanted it to be. I know that next time I need to make the back a little more flexible.

Some things will change resonance frequencies quite a bit and some will not. Gluing on the bridge lowers the top fundamental a lot but it doesn't change the other top and back frequencies much at all. I know that if I want the top fundamental to be a semitone below the back after the bridge is glued on, then it needs to be about a full tone above the back before the bridge goes on.

Applying the finish doesn't have any significant effect on frequencies. Compensating the saddle does virtually nothing to change the resonant frequencies of the soundbox, but changing bridge pins might by virtue of their mass. Changing tuners can effect the frequency of the neck mode and thereby change the way it interacts with the resonances of the body, etc, etc.....

By measuring this stuff along the way, and when the guitar is done, one can get a handle on what effects change and what doesn't.

Mark

Burton

It sounds like you have a good handle on all this.

The sustain of the tap is a good indicator of having the top, back and air modes working together. I use that as well.

I think you will like having the ability to "see" it through the Chladni patterns. You will be able to easily and quickly pinpoint the frequencies and make lasting records of them in a notebook if you wish to do so.

Mark

Dave

Yes. you would get the same coupling effect with the back lower than the top. That doesn't mean that, all other things being equal, the guitar would sound the same as it would if the back was a semitone higher. Having the back that low might have some other effect on the tone in ways that have nothing to do with coupling.

Mark

Gee, Mark, you're a better explainer than I am....

There's no 'magic' to the semitone interval, it just works well. As Mark says, the top and back work together to move air through the soundhole at low frequencies. The more closely they are matched in pitch, the more effectively they pump air. At some point they get so effective that the top just sucks all of the energy right out of the string and dumps it into the air; you get a 'wolf' note. A semitone apart either way will get you a pretty effective coupling but avoid the danger of that 'thuddy' note. I have made a few that had the back lower than the top, and they sounded OK. The only issue there is that generally the top pitch 'plays down' over time: the top loosens up and the 'main top' mode pitch drops. It can also happen that the 'main back' pitch will rise when you put the strings on, since the back is under a bit of tension. The combination of a falling top pitch and a rising back pitch can mean that the back, which was below the top to begin with, will end up matching it, and the wolf pops out of the woodwork.

With some practice you can learn to hear the 'main top' pitch, and some others as well, by tapping. Hold the guitar up by putting a few fingers through the strings and pinching the end of the fingerboard between them and your thumb. The idea is to block the soundhole with your hand, but not to touch the guitar top except above the fingerboard, where it's pretty stiff and not moving much. The reason for blocking the soundhole is that the 'main air' resonance, an octave or so below the 'main top' pitch, will mask the sound of the top. Tap the center of the bridge, and listen carefully. If you can get a good impression of the pitch it sometimes helps to hum it until you can pluck a string and check it out that way. I can often also hear the 'cross dipole' pitch by tapping on the end of the bridge, and sometimes get the 'long dipole' by tapping in the lower bout halfway between the bridge and the end block. For these two it can help to _very lightly_ touch the bridge in the center while you tap, since that reinforces the central node line of these two modes. You can usually hear a couple of different back modes by tapping in the center of the lower and upper bouts while holding the guitar up in the same way.

F#=184.9 Hz. G=195.9 Hz G#= 207.6 Hz. That's a common range for the 'main top' and 'main back' modes, although they can run as high as A#=233 Hz and as low as F=174.6 Hz. Generally I feel that guitars with the highest 'main top' pitches are more 'forward' and have more 'seperation', but can lack 'fullness' or 'warmth', but that's just my feeling. You have to think about the structural stiffness as well: those low ones seem to fold up fast.

BTW, the easy way to get the 'main air' pitch is to lay the guitar down and pinch the low E string between your thumb and finger down near the nut. Pluck the string near ther bridge: you'll get a dull pitched 'thunk' because of the damping of your fingers. Slide the pinch point up and down whilst plucking away, and note where the 'plunk' is loudest: that's the 'main air' pitch.

For the 'neck mode': hold the guitar up by pinching the neck between your thumb anf finger, up near the first fret. Tap on thw back of the headstock lightly with a finger tip, and listen with your ear near the headstock surface. Try not to get one of those sharp string ends in your ear. Usually this 'neck mode' pitch is well below that of the 'main air' mode, but it can be high enough to match if the neck is stiff and/or short, and the head and machines are light. When the 'neck' mode and the 'main air' mode match you can get a particularly strong and 'dark' bass tone.

Alan Carruth wrote: "Try not to get one of those sharp string ends in your ear."

Ouch! Alan, How do you know that???

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Steve Walden
Aspiring Builder,
Bartlesville, OK

This is turning into one of the coolest and most useful threads I've seen in a while. For the first time, some of this is starting to click in my head. Whether the quality of the explanations or the the fact that I've finally heard it enough times to start registering, I don't know. Thanks! Now, if only I can learn to use it to my advantage!

In fact, IMO, this thread is quickly becoming worthy of "Tutorial Archive" status.

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Wes McMillian
Santo, TX
http://www.wesmcmillian.com

A big thank you to all who have posted replys and in particular Mark and Al for their detailed explanations. Like westex93 I think I am finally beginning to get my head around this part of the building. However I'm not sure I will live long enough for my ears to be able to hear the things that I need to hear with regards to the pitches of the plates and diapole's. I will keep trying and will certainly try some of the methods suggested above. I think most of my problem is that all my back bracing was only 1/4 inch and consequently probably ended up lower in pitch than I needed it . I am concerned that by continuing to try to lower the top by scalloping the lower X braces I will loosen it too much causing other issues.
Another question I would like to ask is in relation to string tension and the way that heavier gauges would affect the ability of the top to move with more rotational force on the bridge . Will this also factor in to the equasion for the top tap tone?

Thanks again

Craig.

I am testing by using a loudspeaker and a software tone generator. At the resonance point the plate vibrates noticeable stronger. I just lightly touch the bridge with the back of the nails and they rattle when it vibrates the most.

What Alan said about the difference between a stiff plate and a looser one seems to apply to what Ive experienced with the 3 tops ive voiced so far. The lower you go the fuller and rounder the sound gets, but at the expense of sustain on the high E string and at the expense of overtones.

Here is a random playing Sound Clip of my second guitar, the top is very loose approaching the bottom of the "permited" range. It has a very warm sound, fantastic bass, but a bit dry on the overtones part, and doesn't have much treble ring. I guess it is what makes a flamenco player happy.

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Build log

is it possible to detect the neck mode before it is attached to the body? and is there a way to alter it ? Jody

Jody asked:
"is it possible to detect the neck mode before it is attached to the body? and is there a way to alter it ? "

OK, we're into a nomenclature problem here: the 'neck mode' involves a lot more than just the neck.

In a way you can think of the whole guitar as a sort of funny shaped wood bar. The whole thing can vibrate in different ways, just the way a wooden ruler does. If you support a 12" ruler at two points, about 2-1/2" in from the ends, and tap it in the middle, it will vibrate freely, with the middle going 'down' as the ends go 'up'. On the guitar, the stationary points (nodes) are somewhere around the nut or first fret, and a line roughly across the wide part of he lower bout. The upper block area is an 'antinode': it moves a lot. If I'm thinking in 'technoid' terms, I call this the 'first corpus', or C1 mode, and that's how I lable it on my data sheets.

We use the shorthand of 'neck mode' to talk about this because the neck is bending a lot, and so the stiffness of the neck, along with the mass of the headsock and tuners, has a lot to do with the pitch of this mode. It's a misnomer, of course; the body of the guitar is bending too.

In fact, that's how the 'neck mode' effects the sound. Most guitars have a top that is domed up a little, if only because of the pull of the strings on the bridge. When the guitar is bending 'upward', that is, the head and tail blocks are moving 'away' from the player, and the body moving 'toward' them, there is a bit of compression along the length of the top. This causes the top to arch upward a little, and sucks some air into the soundhole. When the motion is reversed the air is pushed out by the top flattening out.

This is the same sort of air motion that happens in the 'main air' resonant mode. In fact, not only does the neck push on the air through the top, but the air pushes the neck as well: if you push outward on the top the way the air inside does when the pressure is high, it causes the neck to tip upward. If the 'air mode' and 'neck mode' are close in pitch they will couple with each other, in much the way the back and top do when they are close in pitch.

When you get the neck mode 'tuned' to the air mode, the result in the spectrum of the guitar is that the one tall 'air mode' peak becomes two that are rather lower. The total area under the curve, which is sort of the 'total available horsepower', is usually greater, and the response is more even. Subjectively I hear this as a 'dark' and 'full' bass response.

Sadly, since the entire guitar is involved in this, there is no way to predict what the 'neck mode' pitch will be from any measurement you can make of the neck by itself. I can tell you that the 'neck mode' is generally lower in pitch than the 'main air' mode, unless the neck itself is very stiff, or short, or the headstock and tuners are quite light. I seldom get this couple on 14-fret steel strings with full sized, sealed gear tuners, and can often get at least close on classicals with a cedar neck, particularly if the guitar has a short scale. In fact, on one classical with a 630 scale I had to add 15grams of lead to the headstock to get the neck mode _down_ to the pitch of the 'main air' resonance. I would not have bothered except that guitar had such a 'bright' sound that it really needed it.

I'll note in closing that I try hard to set off terms that could be misleading by apostrophising them. Thus things like 'neck mode' and 'main air mode' refer to more complicated things that don't really bear explaining every time you bring them up. My posts are long enoungh and confusing enough as is, without wasting a lot of bandwdth on the 'C-1 corpus bending mode' or the 'lower frequency peak of the bass reflex couple in which most of the energy of vibration is in pressure changes within the body of the guitar and flow thorugh the hole'. I apologize if this adds to the confusion. If you see me use a term in that way, and want a clarification, by all means ask.

Alan , thank you for taking the time to explain this ! Jody

Alan (I keep wanting to say Mr. C because you are an excellent Instructor) for about the last 2-3 years I have paid very close attention to what you have published on the web.

I do not Know if you have covered this but I do not remember having this AH-HA moment before ( I have an Al Carruth-isms archive I should check);

I have always thought that 12 fret slot-head instruments had the fullest sound(subjective). For some reason It clicked this time when reading your explanation of the neck mode.

Is it possible there is a two fold reason for this both having to do with the neck mode?

#1 The obvious is the shorter lighter neck is more likely to couple with the top mode as stated.

#2 On the 12 fret neck the neck block is closer to the nodal point thereby reducing the damping effect upon the neck mode ?

This is of course on top of the difference in the bridge's position in the lower bout.

I keep trying to find myself an explanation of why I also prefer mahogany tops on them once they play in. Have you done any testing of mahog tops in that direction (before and after playing in)?

I would also like to start a thread (Related) about what I think is happening the smaller dulcimer in my avatar.

I have concentration probs so please forgive me if this has all been covered?

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K.O. asked, in relation to the 'better 'sound of 12-fret guitars:

"Is it possible there is a two fold reason for this both having to do with the neck mode?

#1 The obvious is the shorter lighter neck is more likely to couple with the top mode as stated.

#2 On the 12 fret neck the neck block is closer to the nodal point thereby reducing the damping effect upon the neck mode ?"

I'd say the first is most likely. I can't see how the location of the neck block would alter the damping, sprictly speaking. We may be using the term differently, of course. Anyway, in either case, it's near the _antinode_.

"This is of course on top of the difference in the bridge's position in the lower bout."

IMO the bridge position on the lower bout is something of a red herring. You want the bridge to be on the 'acoustic center' of the top, and you determine that by how you make the bracing. It might be easier to get it at the wide point, but even a fairly large shift from that can be acommodated perfectly well. Ever look at the bridge location on a fiddle?

"I keep trying to find myself an explanation of why I also prefer mahogany tops on them once they play in. Have you done any testing of mahog tops in that direction (before and after playing in)?"

No. Mahogany is denser than most soft woods, and ought to give a bit less power, as the top will tend to be heavier. But, of course, there's mahogany and mahogany; like all woods it varies a lot. It would tend to have a relatively high cross grain stiffness compared to spruce. It probably has it's own 'sound', and if you like that, that's what matters.

Thanks again to all who have replied .This has helped me a lot.
Alex I think I might give it a try with the software tone generator and loud speaker . Where and how do you put the speaker on the guitar? By the way I think your second guitar sounds very impressive. I never thought that nylon strings could sound so good What are the specs for it? How did you record it?

Craig.