Post by R the F on Jul 13, 2016 21:20:17 GMT
I went for a run this morning and decided I need to build another guitar toot sweet and preferably one that works. Now there were lots of things I actually liked about the one I’ve just “finished” – I’ll get there – but the bulging of the soundboard behind the bridge was not one of them. I have also inadvertently misled (I always read that word as /ˈmɑɪzld/) readers about what is actually happening beyond the bridge. I originally suggested the problem was that the braces, being so firmly dowelled and glued to the bridge, were pulling up and trying to break through the skin of the soundboard; it turns out that the opposite is true: the braces seem to be staying low while the soundboard itself is bellying up between the braces – and must be close to separating from them if my gluing isn’t perfect. The good thing about this is that, once it has accepted that it won’t come unstuck, it really can’t move very much further since the space between the braces is so limited. But even if it stays as it is, there’s always the potential that it will pull apart and, besides, it’s not aesthetically acceptable and certainly does not instil confidence that the luthier knows what he is doing. Not a good selling point. It’s also sinking very slightly in front of the bridge, which is part of the same problem.
Assuming I retained a very similar construction, how could I stabilise the bridge? I wondered as I strode through the fens. richm ’s link to the Bridge Doctor set me thinking about supporting the bridge from somewhere else within the box. Now my starting point here, from the time a couple of years ago when designing my first guitar, a carved archtop, was that imprecise use of language confuses people. We have this idea that the guitar box has to do two things: it has to resist the pull of the strings, which is no mean feat with 80 kilos or so to contend with; and it has to allow the soundboard to vibrate freely in sympathy with the vibrating strings. We see this as a problem because we don’t use language precisely enough; we think it must move but it mustn’t move and that sounds like a serious problem. But the truth is really that it must vibrate up and down with a maximum amplitude of, say, a millimetre but it mustn’t distort upward or downward by, say five or ten millimetres and stay distorted. The point I’m making is that “vibrate” and “distort” are not synonyms though they are both vaguely synonymous with “move” as in “it must move but it mustn’t move”. So we’re happy for the soundboard to move up and down and then revert to the original position but we’re not happy for it to move to a stressed position and stay moved.
When building the carved archtop, I tried to allow free vibration by not bracing the soundboard at all but I tried to prevent distortion by putting a strut all the way from the tail-block to the neck-block so that the box could not move and distort the soundboard under the pull of the strings. I also, of course, carved the top into an arch, which is a pretty difficult thing to distort; have a play with some corrugated iron if you don’t believe me. Here is a picture of the inside of my carved archtop to help relieve the tedium:
Of course, a carved archtop is a much simpler structure to deal with under stress than a so-called flat-top because the tail ends of the strings are fixed to the tail of the guitar and the bridge is held against the soundboard purely by the tension of the strings. In other words you just have to make sure the soundboard is strong enough to resist the downward pressure of the strings and this pressure must be tiny compared to the end-to-end pull that they are exerting. When I switched to making my first “flat-top”, I innocently enough thought that it would be easier to build “because you didn’t need to do all that carving” or some such. Now I know better. Carving requires a certain skill but at least there’s something to get your teeth into; to produce a “flat” soundboard which is reinforced to support 80 kilos of pull right from the middle, its weakest point, and then to ask it to sing as if it were free as a bird is like trying to chew tapioca; it’s just very hard to get your brain to clamp down on such a squidgy problem.
As I turned in to Cow Lane, I went back to the original solution which I had used with the archtops. Would it be possible to help the bridge to withstand the twisting torque by connecting it to some kind of strut spanning the space inside the guitar so that it could still vibrate but not distort? For example, a piece of Kevlar looped around a strut like the one in the picture above and also threaded through the back of the bridge to discourage it from pulling up? And at the front a post to prevent it from dropping? (My brain is always starved of blood when I'm running since my body knows my brain is not a vital organ. Remember, I am still striding through the fens.) This would still allow vibration of the soundboard because the strut is not built to be laterally rigid. But by the same token, I hear you say, it would allow distortion… So could the support come directly from the back of the box? Like a violin’s soundpost? But of course a violin is fed a constant stream of energy unlike the guitar which only gets a little pluck whose energy is so easily dissipated.
Back to the carved archtops. When I switched to “flat-tops”, I immediately decided to arch the tops – and this time with true arches (like Mr Sobell’s) in one dimension rather than doming the tops as is usually done. Remember the lesson of corrugated iron.
The guitar which instigated this ramble had a completely flat top which I was hoping, in vain, to tame. One thing I could certainly do is return to the arched top (and back) which seems to have served me well until now. It also looks jolly nice. Could I go even further down the corrugated iron route and adopt the soundboard arching of Howe Orme guitars? Tempting, but I’m worried that excessive arching will inhibit vibration. A simple solution may be to combine this arching with real thickening of the soundboard in the important areas rather than trying to add a strengthening plate or lattice bracing. And perhaps this rather severe arching would allow space for two lateral braces linking the top edge of the rim either side of the front and back of the bridge but not touching the soundboard itself. And I was just on the point of formulating the complete and perfect answer to the question when I arrived home in serious need of a shower – I was my own “person from Porlock” and I had spoiled everything.
Assuming I retained a very similar construction, how could I stabilise the bridge? I wondered as I strode through the fens. richm ’s link to the Bridge Doctor set me thinking about supporting the bridge from somewhere else within the box. Now my starting point here, from the time a couple of years ago when designing my first guitar, a carved archtop, was that imprecise use of language confuses people. We have this idea that the guitar box has to do two things: it has to resist the pull of the strings, which is no mean feat with 80 kilos or so to contend with; and it has to allow the soundboard to vibrate freely in sympathy with the vibrating strings. We see this as a problem because we don’t use language precisely enough; we think it must move but it mustn’t move and that sounds like a serious problem. But the truth is really that it must vibrate up and down with a maximum amplitude of, say, a millimetre but it mustn’t distort upward or downward by, say five or ten millimetres and stay distorted. The point I’m making is that “vibrate” and “distort” are not synonyms though they are both vaguely synonymous with “move” as in “it must move but it mustn’t move”. So we’re happy for the soundboard to move up and down and then revert to the original position but we’re not happy for it to move to a stressed position and stay moved.
When building the carved archtop, I tried to allow free vibration by not bracing the soundboard at all but I tried to prevent distortion by putting a strut all the way from the tail-block to the neck-block so that the box could not move and distort the soundboard under the pull of the strings. I also, of course, carved the top into an arch, which is a pretty difficult thing to distort; have a play with some corrugated iron if you don’t believe me. Here is a picture of the inside of my carved archtop to help relieve the tedium:
Of course, a carved archtop is a much simpler structure to deal with under stress than a so-called flat-top because the tail ends of the strings are fixed to the tail of the guitar and the bridge is held against the soundboard purely by the tension of the strings. In other words you just have to make sure the soundboard is strong enough to resist the downward pressure of the strings and this pressure must be tiny compared to the end-to-end pull that they are exerting. When I switched to making my first “flat-top”, I innocently enough thought that it would be easier to build “because you didn’t need to do all that carving” or some such. Now I know better. Carving requires a certain skill but at least there’s something to get your teeth into; to produce a “flat” soundboard which is reinforced to support 80 kilos of pull right from the middle, its weakest point, and then to ask it to sing as if it were free as a bird is like trying to chew tapioca; it’s just very hard to get your brain to clamp down on such a squidgy problem.
As I turned in to Cow Lane, I went back to the original solution which I had used with the archtops. Would it be possible to help the bridge to withstand the twisting torque by connecting it to some kind of strut spanning the space inside the guitar so that it could still vibrate but not distort? For example, a piece of Kevlar looped around a strut like the one in the picture above and also threaded through the back of the bridge to discourage it from pulling up? And at the front a post to prevent it from dropping? (My brain is always starved of blood when I'm running since my body knows my brain is not a vital organ. Remember, I am still striding through the fens.) This would still allow vibration of the soundboard because the strut is not built to be laterally rigid. But by the same token, I hear you say, it would allow distortion… So could the support come directly from the back of the box? Like a violin’s soundpost? But of course a violin is fed a constant stream of energy unlike the guitar which only gets a little pluck whose energy is so easily dissipated.
Back to the carved archtops. When I switched to “flat-tops”, I immediately decided to arch the tops – and this time with true arches (like Mr Sobell’s) in one dimension rather than doming the tops as is usually done. Remember the lesson of corrugated iron.
The guitar which instigated this ramble had a completely flat top which I was hoping, in vain, to tame. One thing I could certainly do is return to the arched top (and back) which seems to have served me well until now. It also looks jolly nice. Could I go even further down the corrugated iron route and adopt the soundboard arching of Howe Orme guitars? Tempting, but I’m worried that excessive arching will inhibit vibration. A simple solution may be to combine this arching with real thickening of the soundboard in the important areas rather than trying to add a strengthening plate or lattice bracing. And perhaps this rather severe arching would allow space for two lateral braces linking the top edge of the rim either side of the front and back of the bridge but not touching the soundboard itself. And I was just on the point of formulating the complete and perfect answer to the question when I arrived home in serious need of a shower – I was my own “person from Porlock” and I had spoiled everything.
Jo whoever he is.
