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| Down Force On Archtop Bridge http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=53971 |
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| Author: | Ken Nagy [ Sun Feb 21, 2021 12:45 pm ] |
| Post subject: | Down Force On Archtop Bridge |
I drew up an arch top design the other day and was shocked at how huge the bridge is. Figuring some things out, my arch top with about 90 pounds of string tension puts about 24 pounds of pressure on the top. That's about the same as a violin with heavy strings. My bridge has 14 TIMES the surface area on the feet. So where a violin might have 24 pounds of pressure on its 1cm feet, my bridge has less than 2 pounds, spread out over a huge area. I wondered what the effect would be if the surface area of the feet was reduced. Has anyone tried it? How about thinning the bridge? I wonder about a lot of things. |
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| Author: | Ken Nagy [ Mon Feb 22, 2021 9:29 am ] |
| Post subject: | Re: Down Force On Archtop Bridge |
I was thinking that the smaller footprint might transfer more sound to the body. That can never be bad, can it? It also moves around some. The end pin is off to the bass side a bit. I think that instead of lining the hole up with the neck, I put it on some line I drew up to the centerline of the top. I can't even see where the center of that top is; the grain lines are pretty tight, and the join isn't apparent. The neck was probably lined up on an angle. I can see it now, but apparently I was blind when I set it. It works better looking from the end. Looking from the top I can't see the angle. My first one. With such a large footprint it doesn't have much force to hold it in place. It doesn't move a lot, but once a month or so I have to move it some. As for thinning it, The bridge does seem to be massive to me. The top looks thick. I planned on thinning it more, but never did. The bottom looks like it could support the strings of a double bass; easily. I know that in thinning the bridge on a violin, the tone can come out more. I've only done this on about 6 of the latest instruments, so I'm no where's near an expert; but I have heard the difference between an uncut bridge, and one the is trimmed down. I would imagine it would be the same on guitars? I don't even know WHY it works, which makes is harder. Is it that the extra wood is damping some of the sound? Attachment: IMG_0286.jpg
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| Author: | Alan Carruth [ Mon Feb 22, 2021 10:57 am ] |
| Post subject: | Re: Down Force On Archtop Bridge |
Where to start.... The numbers you give imply a break angle at the bridge of about 15 degrees. You probably can get away with less, which would reduce the down bearing load. Benedetto shows about a five degree break angle on his plan, and something on that order seems to work well. The only reason to use more than that is to keep the strings from rolling or sliding sideways when you pluck them, and with notches on the saddle top that's not happening. In my break angle experiment I used a 'low' angle of about 6 degrees on a classical guitar, and it was sufficient. The 'normal' angle of 25 degrees didn't work any better. So long as the string stays in contact with the saddle top all through it's vibration cycle it will transfer all of the sound to the bridge, so there's no benefit to going to any higher break angle than you need to keep things in contact. I used to use a 'hook' type tailpiece on my arch tops. It was 'L' shaped, with the lower arm of the 'L' bearing down on the tail block, to provide a pivot point below the edge of the top. The line of the strings points from the top of the saddle to the pivot point, so with that low pivot they would virtually go through the top at some point above the lower edge. The idea, of course, was 'more down bearing = more sound'. I ran a few experiments on one of them, using shims to alter the pivot point and checking the effect on the sound. As I moved the pivot further down from the edge the sound suddenly died at one point. I'm not sure why the effect was so pronounced and sudden. I was not actually measuring the output at that time, since I had no computer, so I can't say what changed exactly. It could have been linked to the top thickness and arch shape, or maybe there was some more esoteric geometry involved. The compression force of the strings against the tail block would cause the top to pop up, and it may be that there is some sort of balance involved there, for example. But there was no doubt about the effect. These days I go for a minimal break angle in the 5-6 degree range. Archtops lack a sound post to help keep the top from collapsing, and I've seen too many with the bridges cranked all the way up to keep the action usable as the top sinks slowly into the box. There are ways to shape and graduate the arch that help with that, but an ounce of prevention is worth a pound of cure, and minimizing the force that has to be dealt with makes a lot of sense. Mention of the sound post brings us to the function of the violin bridge. Bowing the string pushes it across the top, more or less parallel to the plane of the top of the rim, and the contact of the bow damps 'vertical' string motion strongly. The top only really moves air to make sound in a loudspeaker-like 'vertical' motion, which bowed strings don't provide. The sound post acts to provide a stationary pivot point on the top near the treble foot of the bridge. This converts the bridge into a bell crank, with horizontal motion at the top being converted to a vertical force on the bass foot of the bridge. That is, of course, directly above the bass bar, which carries that force out along the top to produce the vertical motion that makes sound. The top of the violin bridge is thin and light, so that it presents a fairly low impedance which comes reasonably close to that of the strings. This promotes the transfer of energy from the strings into the bridge. The bridge becomes thicker as you go down, until the feet are closer in mass and stiffness to the top at that point, again, making for a decent impedance match and good sound transfer. It serves a similar function in this respect to the bell on a horn, matching the high amplitude low impedance string/bore signal to the higher impedance of the top/air at a lower amplitude. The cuts in the violin bridge allow it to flex in a number of ways, producing resonances at various frequencies. It's a tuneable filter, enhancing some frequencies and cutting down others. Trimming the bridge alters the mass and stiffness of the various parts, changing the filter characteristic, and helping to shape the overall sound of the fiddle. Problems with this can cause 'wolf' notes. The arch top bridge is, by comparison, pretty simple. It needs to have enough mass and stiffness to provide a strong impedance mismatch with the string at all frequencies so that the string will 'know' how long it is, and what note to make. Since the guitar string is not bowed a guitar 'wolf' typically manifests as a 'short' note, one that's twice as powerful for half as long. A massive and rigid bridge helps keep that from happening. On the other hand, a lighter weight bridge tends to produce a more 'treble' or 'bright' sound, so you can use bridge mass as a way to 'tune' the response. I've seen no advantage in changing the flexibility of an arch top guitar bridge, although it can be artistic. I'm not sure that the contact area of the bridge of an arch top with the top makes a lot of difference. You certainly do want to see that the bridge feet are well fitted. I have not done a lot of experimenting on this, but my experience mirrors the consensus view that a single foot is better on an archtop, rather than the two-footed bridge of a fiddle. The tail piece on an archtop can also get into the act. D'Aquisto said, in a talk at a GAL convention long ago, that he could get pretty much the sound he wanted by changing the tailpieice. It's mass and length, and the way it's pivoted, all make a difference. Arch tops give you a lot of knobs you can twist to adjust the sound after it's all together, and it would take a long time to explore the whole space. |
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| Author: | John Arnold [ Mon Feb 22, 2021 11:30 am ] |
| Post subject: | Re: Down Force On Archtop Bridge |
Dont conflate force and pressure. Pressure is force per unit area. Reducing the footprint of the bridge increases the pressure, but the force remains the same. It is the force that keeps the bridge from sliding around on the top. Sent from my SM-G950U1 using Tapatalk |
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| Author: | Ken Nagy [ Mon Feb 22, 2021 12:20 pm ] |
| Post subject: | Re: Down Force On Archtop Bridge |
John, force, not pressure? I was thinking just the opposite. Interesting. Alan, I see what you are saying about the difference between violins and guitars. The main thing I've been thinking of trying for is more treble. It isn't bad, but though the high notes can ring nicely at times, throw a low note in the mix, and they can disappear. At least to my old ears. The angle is about 15 degrees. The arch is pretty high, maybe even higher than it was at first. It certainly shows no signs of sinking. My adjustable bridge was all the way down! I might try some heavier strings on it. With the ones on it now, it sounds like a classical crossed with an arch top through a pickup. It has the arch top sound, but not the same attack on the strings. Of course I've never used a pick. The light nickel/bronze ones I have weren't that much brighter on the trebles, and the basses certainly don't have the richness of the Ambra 900's. If I could dial in some more treble; something I never did on a stereo; the 900's would be perfect. Maybe it's just my ears? A bridge with a solid base, not with two feet? So that would take the pressure down another 50% or so. So spreading the force out works better? What about something like rosewood compared to cherry? Does the wood structure of the bridge show through? The bridge on the Staufer I'm making is absolutely tiny. All it must do is attach the strings to the belly. That one would probably even flex with the body. That is a whole different animal. Now I have some ideas. |
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| Author: | John Arnold [ Mon Feb 22, 2021 3:53 pm ] |
| Post subject: | Re: Down Force On Archtop Bridge |
https://www.britannica.com/science/friction Sent from my SM-G950U1 using Tapatalk |
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| Author: | Chris Pile [ Mon Feb 22, 2021 6:54 pm ] |
| Post subject: | Re: Down Force On Archtop Bridge |
Quote: D'Aquisto said, in a talk at a GAL convention long ago, that he could get pretty much the sound he wanted by changing the tailpieice. It's mass and length, and the way it's pivoted, all make a difference. There's a cat with an enormous amount of knowledge who died too early and couldn't pass on the knowledge he'd learned on his own and from D'Angelico. I had the pleasure of lunch with him and his son at an ASIA convention, and we barely engaged in shop talk at all. We mostly discussed the news, and what we'd seen and heard in the display hall. I missed my chance to really forge a friendship with him. |
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| Author: | Ken Nagy [ Mon Feb 22, 2021 7:26 pm ] |
| Post subject: | Re: Down Force On Archtop Bridge |
The only thing I do with tailpieces is to try to get them,at a 2:1 ratio to the overhang, and balance them in the middle. Some way to maybe make the strings more lively? |
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| Author: | Alan Carruth [ Wed Feb 24, 2021 3:36 pm ] |
| Post subject: | Re: Down Force On Archtop Bridge |
I made my first two archtop nylon string guitars with high arched and thin tops. They certainly had treble, but not 'nice' treble. Then I found out that you have to gauge the arch height to the top thickness not the length of the box. That really helped. Arched tops have high stiffness impedance anyway, which favors the high frequencies in the sound. They need to have some mass to get the balance back. It's certainly worth while to mess around with that as a variable, though. Poster adhesive is your friend. As the bridge goes up and down it pushes on the tailpiece. In most cases the primary vibration mode of the tailpiece has it swinging up and down, hinged on the pivot down at the bottom end. The absolute mass of the tailpiece, and it's center of moment, go along way toward determining the pitch of that mode. I was trying to figure out a way to improve the bass on my first arch classical. Tuning the 'neck mode' to match the 'main air' resonant pitch would have worked nicely, but there was no way I could do that at that point, since it was too low and I could not make it stiffer. The tailpiece mode, OTOH, was too high in pitch, but not out of range, so I started loading it with poster adhesive until I got it down within range. I worked, but it took a big wad of the stuff. I cut a piece of sheet lead to fit, struck it on the tailpiece with double stick tape, and took it to show my friend Aaron Green. He was out at that moment but his shop partner at the time was in, so when Aaron got back Steve was playing the guitar. Aaron noticed the added weight and asked about it, so I just reached over and pulled it off while Steve kept playing. Aaron exclaimed: "Who turned off the speaker!". The shorter the back strings are between the bridge and the tailpiece the tighter the couple between the top and the tailpiece mode. So you can see that there are several tings you can do with the tailpiece just in matching that one mode. There are probably a couple more that would be well worth looking into. As I say, arch tops give you a lot of dials to twist. |
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| Author: | Chris Pile [ Wed Feb 24, 2021 5:38 pm ] |
| Post subject: | Re: Down Force On Archtop Bridge |
I need to read Benedetto's book again.... |
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