3 dead C's on 3 different strings?

Construction and repair of Classical Guitar and related instruments
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Re: 3 dead C's on 3 different strings?

Post by GuitarsWeB » Mon Jan 21, 2019 8:09 pm

On a related note....suppose a pitch is short in one place and not in another? E.g., the B on the top string, 7th fret is short, but when played on 12th fret of the 2nd string its perfectly fine. Is there a "typical" cause for that?
I that case, I would say the string.

ernandez R
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Re: 3 dead C's on 3 different strings?

Post by ernandez R » Tue Jan 22, 2019 6:31 am

Alan Carruth wrote:
Mon Jan 21, 2019 7:44 pm
Trevor probably covered this, but I'll reiterate.

Keep in mind that a string works best when the ends are 'fixed': tied to something that is really massive and stiff so that they don't move. The problem is that if the bridge on the guitar doesn't move no energy will get out of the string to drive the top, and you won't hear it. You get a 'short' note when:
1) it's easy to move the bridge where the string is tied to it at that pitch, and
2) the guitar can extract the energy from that string quickly. These are related, of course, but not identical.

All of this is a function of the resonant structure of the guitar. Every worthwhile guitar has a number of 'resonances' at different pitches; structured ways of moving that are easy to drive and more or less well defined in pitch. Strings are the same way, as you can easily prove. Just touch a string at some point that divides it into equal parts and pluck it. If you touch it at the 12th fret you've divided it in half, and it vibrates an octave higher than it's fundamental pitch, which is twice the frequency. If you touch it at the 7th fret you've divided it into thirds, and get the interval of a 12th; an octave and a fifth, which is three times the fundamental pitch, and so on. Strings are pretty much one-dimensional structures, so the overtone series they produce is simple. Guitars are more complicated, so they vibrate in ways that are more complicated, and harder to predict as well. We often talk about these different ways of vibrating as 'modes', and sometimes refer to them as 'top', 'back' or 'air' modes, depending on what's moving the most. This is vastly oversimplified, but handy; none of these movements is 'just' in the top, or back, or air. The higher you go in pitch the more of them there are, and the more complicated things get.

The lowest pitched useful mode on most guitars is called the 'main air' mode, where the air flows in and out of the hole at a certain pitch. It's a 'Helmholtz mode', analogous to what you get when you blow across the mouth of a wine bottle. In this case, though, the top (and the back usually as well!) get into the act. Air moving in and out changes the pressure in the box, and pushes on the top, so that moves. It's easy for the string to get this going, and the guitar is very effective at turning it into sound, so it's a common 'short' note, often down around G on the low E string. There's another similar mode, often called the 'main top' mode, about an octave higher, where most of the action is in the top. It can be a problem as well, but not usually so much as the 'air' mode.

The next thing up in pitch is sometimes called a 'cross dipole' mode: the bridge is rocking sideways, with the treble side going 'down' while the bass side moves 'up'. Since the center of the bridge doesn't move the two middle strings can't push this one very easily, but the high and low Es can, and the A and B strings to some extent. This one often tends to come in at around B~247 Hz, right about the pitch of the open B string, so that can be a 'short' note too, but only on that string. The pitch is too low for the high E, and too high for a fundamental on the low E or A, although it might affect an overtone on those strings.

The next resonance up is often a 'long dipole' of the top, where the bridge is rocking forward and aft as the upper and lower parts of the top move out of phase with each other, similarly to the 'cross dipole'. This can be a strong resonance, and also an effective sound producer, but it's hard for the string to drive it, so it's not usually a 'short' note. It often happens at around F~349 Hz; the first fret on the open E. Ironically, the high E can't drive it as well as the Fs on the low E or D strings. Those Fs are lower in pitch, but the tension is changing twice per cycle, or at multiples of that, and they can rock the bridge by pulling on the top of the saddle in step with the motion of the top. The F on the high E pulls it too often. Since the bridge and top don't move much in that 'rocking' direction this doesn't result in 'short' notes (usually) but can 'enrich' the timbre of those notes. There are also some more complicated things that can go on with this one.

And so it goes. IMO it's precisely the complex nature of the way the guitar responds that makes it the instrument that we love. At the same time, the more effort we put into making it responsive and powerful, the more likely it is to cross over into behavior that is problematic. We're always trying to find that balance point where the 'great' instruments are. It's tricky.
I want to print this out and read it every night before falling asleep, not because it is slumber inducing, but because I want it to perkulate I my subconscience so as I'm laying out sound bars and fugureing out thicknesses I'll it will always be resonating with the conscience self.

Was laying out sound bars on my tops today and had decided to configure them under the bridge differently and was thinking about, or wondering really, if the bridge moved differently from end to end and how they interacted. So I marked the underneath of the top plate with a uppercase E and a lowercase e I'm their respective locations. Then I started to adjust the sound bar locations. Then not for any reason I thought I should run the sound bar directly under the e corner of the bridge all the way forward as far as the top of the sound hole just short of the forward brace with the idea that the high frequencies would like to be transferred to the smaller high frequency vibrating part of the top. Dog knows what it will sound like but I've the other two tops to compair it to which was one of the main reasons I wanted to build three at a time.
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I hate sanding wood or anything else for that matter I just happen to be good at it...

Alan Carruth
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Re: 3 dead C's on 3 different strings?

Post by Alan Carruth » Tue Jan 22, 2019 8:45 pm

The notion that there is a 'bass side' and 'treble side' to the top in terms of sound production is a fallacy. At low pitches the top tends to vibrate as a unit in the lower bout, but as you go up in frequency it breaks up into smaller and smaller vibrating parts, each of which is moving out of phase with (in the opposite direction to at any given time) the areas next to it. These areas are of different sizes, and move with different amplitudes, so they don't quite cancel each other out, and you get some sound. What's interesting is that the sound produced can radiate out as a 'beam' at higher frequencies. Low pitched sounds tend to go out about equally in all directions, but higher pitched ones tend more to be coming off the top and out of the hole toward the audience. This is why a guitar sounds so much different in front than it does to the player.

Asymmetric bracing mostly seems to vary the directional aspect of the sound. For example, the sound from the 'cross dipole' mode I spoke of tends to cancel out along the center line of the top on a normal Torres type of top. On a top with an angled waist bar the 'null' points off to one side because of the different vibrating areas of the two sides of the top. Again, this is 'way more complicated than such a simple description seems. The point is that an asymmetric top will sound different from a symmetric one, but that's not automatically 'better'. I used to make my guitars with angled waist bars, butin the end I think that the symmetric top works better, all else equal. Just my opinion.

I'v always thought that the critical area of the top for stiffness is between the bridge and the sound hole. When the top starts to cave in there you lose the guitar. The two most successful bracing schemes for guitar tops, the Torres 'fan' and the 'X' brace system that Martin adopted both concentrate bracing in that area. I don't think that's an accident. It may have come about accidentally ("Hey, what if I..."), but it's persistence is not accidental. It's fun to try new stuff, but keep in mind that most experiments fail.

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