Bridge wood cut?

Construction and repair of Classical Guitar and related instruments
vesa
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Bridge wood cut?

Post by vesa » Thu Apr 11, 2019 8:48 am

Which kind of cut do you prefer to bridge wood?
Slabcut, quartersawn or in between?
And if in between, grain ¨leaning¨ towards soundhole or to endblock?

Vesa
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Antonio Marin nr. 813 1995 (Bouchet)
Vesa Kuokkanen 2016

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Michael.N.
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Re: Bridge wood cut?

Post by Michael.N. » Thu Apr 11, 2019 9:00 am

Quarter. Slab has a tendency to cup especially with big humidity swings and it's during the humidity swings that bridges can start flying off. It's just a safer cut.
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Alan Carruth
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Re: Bridge wood cut?

Post by Alan Carruth » Thu Apr 11, 2019 4:32 pm

I actually go for rift cut, with the ring lines at 45 degrees to the glue face. Try to avoid wood that came from close to the center of the tree, where the rings curve a lot; that's what causes cupping. Rift cut has the lowest cross grain stiffness (which doesn't matter in this application) and the highest shear strength (which matters a lot). I don't think it matters too much which way the rings lean.

Another thing that really helps avoid split outs of the bridge front is to cut the saddle slot at a back angle, as seen from the side. If the saddle bisects the break angle these is no net tipping force trying to break out the front of the slot. Even a bit of back angle helps a lot, as does avoiding high break angles. Once you get past about 15 degrees of break angle there is no benefit in tone, but the tipping force starts to rise fast.

khs
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Re: Bridge wood cut?

Post by khs » Fri Apr 12, 2019 1:40 am

In my opinion, quartersawn as close as possible might be recommendable. But within the range of +/-10 degree grain direction beyond the long axis of bridge may be permissable practically. Bridge is a critical point of sound generation in a guitar. Every detail regarding the bridge matters whatsoever trivial. So we can never be too cautious at choosing the matrial of, making and installing the bridge.

ernandez R
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Re: Bridge wood cut?

Post by ernandez R » Fri Apr 12, 2019 5:59 pm

Alan Carruth wrote:
Thu Apr 11, 2019 4:32 pm
I actually go for rift cut, with the ring lines at 45 degrees to the glue face. Try to avoid wood that came from close to the center of the tree, where the rings curve a lot; that's what causes cupping. Rift cut has the lowest cross grain stiffness (which doesn't matter in this application) and the highest shear strength (which matters a lot). I don't think it matters too much which way the rings lean.

Another thing that really helps avoid split outs of the bridge front is to cut the saddle slot at a back angle, as seen from the side. If the saddle bisects the break angle these is no net tipping force trying to break out the front of the slot. Even a bit of back angle helps a lot, as does avoiding high break angles. Once you get past about 15 degrees of break angle there is no benefit in tone, but the tipping force starts to rise fast.
Alin,

This saddle angle issue has been on my mind for a while.

In my airplane pilot world we say the horizontal component of lift is what turns the airplane: we get the aircraft in a bank first...

My thought along those lines is if we break the string energy into the saddle into component vectors, I'm having a hard time coming up with proper terms, can we direct said energy into the top in a more controlled manner, or rather in a direction that makes the top respond in a manor of our liking.

Something tells me you have already chased Miss Liddell and her furry friend down this hole.

HR
I hate sanding wood or anything else for that matter I just happen to be good at it...

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geoff-bristol
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Re: Bridge wood cut?

Post by geoff-bristol » Fri Apr 12, 2019 7:12 pm

Yes but the strings are attached to the tie block just 10mm away - the whole bridge is vibrating as one mass is it not ? I do not see how an angle on the saddle can have any great effect. If it were a floating bridge - with strings attached at end pin, that would make sence to me ?

printer2
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Re: Bridge wood cut?

Post by printer2 » Sat Apr 13, 2019 2:09 am

geoff-bristol wrote:
Fri Apr 12, 2019 7:12 pm
Yes but the strings are attached to the tie block just 10mm away - the whole bridge is vibrating as one mass is it not ? I do not see how an angle on the saddle can have any great effect. If it were a floating bridge - with strings attached at end pin, that would make sence to me ?
It is not how the bridge is vibrating but rather the splitting of the saddle slot.
Fred

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geoff-bristol
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Re: Bridge wood cut?

Post by geoff-bristol » Sat Apr 13, 2019 7:23 pm

printer2 wrote:
Sat Apr 13, 2019 2:09 am
geoff-bristol wrote:
Fri Apr 12, 2019 7:12 pm
Yes but the strings are attached to the tie block just 10mm away - the whole bridge is vibrating as one mass is it not ? I do not see how an angle on the saddle can have any great effect. If it were a floating bridge - with strings attached at end pin, that would make sence to me ?
It is not how the bridge is vibrating but rather the splitting of the saddle slot.
That makes sence - must pay more attention at the back :)

Alan Carruth
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Re: Bridge wood cut?

Post by Alan Carruth » Mon Apr 15, 2019 8:57 pm

Right.

The experiments I've done suggest that the height of the strings off the top makes a difference in the timbre, but the break angle does not, so long as there is 'enough' to keep the strings in contact. Whether or not the saddle has a back angle makes no difference in how the strings drive the top: you can assume that the saddle and bridge are 'rigid' for that purpose, moving as a unit. The back angle of the saddle only matters in determining whether the front of the bridge will break out.

bftobin
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Re: Bridge wood cut?

Post by bftobin » Wed Apr 17, 2019 4:38 pm

I seem to remember Ramanillos saying he increased the saddle height and tried more neck angle thinking it might drive the top more. The end result was more of a banjo-like sound. My bridges are usually just off quarter by about twenty or so degrees.

Alan Carruth
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Re: Bridge wood cut?

Post by Alan Carruth » Wed Apr 17, 2019 6:45 pm

I've done several experiments looking at the forces the vibrating string produces at the saddle top, and how changing the string height off the top alters the sound produced. As always, it takes a fair amount of effort to figure this stuff out, but not much time to describe the results.

The largest 'signal' that the string produces at the saddle top is a 'transverse' force: across the length of the string. Basically, if the string is moving 'up and down' relative to the plane of the top, it pulls the top 'up and down'. The string can move 'across', parallel to the plane of the top, as well, of course, but it's hard to move the top that way, and it doesn't produce much sound. The 'vertical' string motion makes the lower bout of the top move like a loudspeaker, which is very effective at making sound. This is why you try to push the string straight down in a 'rest' stroke; so that it will end up moving 'vertically' as far as possible. The actual force the string puts on the bridge to pull it up and down depends on the amplitude and where you plucked it: at most it would be something like 10% of the tension of the string 'up' or 'down' initially.

The string tension also changes as it vibrates; it's higher when the string is either 'up' or 'down' (or 'left' or 'right' if it's moving horizontally). Thus the tension change is a double the frequency of the transverse vibration; an octave higher. The relative amplitude of the two signals varies for different strings, but it averages something like 1/7 of the force of the 'transverse' signal. This twice-per-cycle tug on the top of the bridge rocks it forward, toward the nut, and causes the top to belly up behind the bridge and dip downward a little in front when the string is displaced. This is a less efficient way of making sound than the 'transverse' force for several reasons. One is that half the soundboard is moving 'up' while the other half moves 'down': some of the sound is cancelled out. Its also much harder to tip the bridge like that than it is to push it inward: luthiers build tops to resist that tipping motion because that's what breaks tops. This, along with the smaller force, means that this 'tension' signal doesn't produce much power compared with the 'transverse' force signal.

There is also a longitudinal compression wave in the string, which is produced whenever you pluck it anyplace but in the exact center of it's length (in other words, it's usually there). You can think of it as sort of like a pressure wave in a long, thin pipe. The pitch of this depends on the material and construction of the string; the tuning of the string hardly effects it at all. Normally, with nylon guitar strings, it's up around the 7th or 8th partial, but, obviously, it can vary. Because of the peculiar way it's generated the power of it depends in part on how it relates to the tuning of the string. It can 'come and go' if it's not lined up in pitch exactly with a string partial, and it matters whether it's closer to an odd or an even one. This string signal works on the bridge top like the 'tension' signal.

The higher the strings are off the top, the more leverage there is for the 'tension' and 'longitudinal' signals to drive the top, so raising the saddle puts more of those frequencies into the sound. In particular, you get more of the second and (sometimes) the fourth partial, and also more of the high pitched 'longitudinal' wave frequency. The second and fourth partials would probably add some 'fullness' but the longitudinal wave is often dissonant. It's also in a high frequency range where your hearing tends to be acute, so you pick it up. When it's very close in pitch to an odd numbered partial (often the seventh) it can 'couple' in strange ways with the other string vibrations, and cause things like buzzing on every note. And, of course, the seventh partial is dissonant anyway. I suspect that's the source of Romanillos' 'banjo-like' timbre.

In my tests, people listening to recordings of well-controlled mechanical plucks were unable to hear changes in break angle, but picked out changes in string height off the top virtually every time. I didn't ask them to describe the sounds; just to say whether they were 'the same' or 'different'.

Measurements of those plucks showed that there was not any more power in the sound with the tall saddle, nor did changing the break angle alter that. What changed with changes in string height off the top was the mix of frequencies in the signal, with, as I say, more of the second and (maybe) fourth partials, and the high-pitched 'longitudinal' wave, when the strings were higher off the top. The power in the string, and the mix of partials, is set by how and where it's plucked, and the amplitude, and the efficiency/effectiveness of the guitar in turning those string signals into sound seems to be pretty much fixed for a particular instrument as well. If there's more sound at one frequency there seems to be less at others, all else equal, so the power stays the same while the timbre changes with string height.

Of course, this is a pretty condensed account, but it hits the main points. As with any experimental finding, it's provisional: if somebody does a better experiment and finds something different we'll have learned something. At the moment, though, that's my story and I'm sticking with it.

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