estebanana -> RE: Questions about braces (Jul. 9 2017 2:31:05)
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I'm really interested in guitar building Poor boy. Hopefully this interest will soon pass. quote:
and was asking myself why some guitars have pointed braces and why some have parabolic braces. How do these different braces actually effect the top? Not necessarily sonically but rather structurally. The sonic part and the structural part are married. The bridge is being moved by the string, which does not output a heck of a lot of energy. The bridge moves multidirectionally, and transmits that energy and movement to the top. The top has to be made so that it can receive as much of that energy without wasting it or damping it. The energy is low, bridge/string output is low. The hard part is making the top thin, enough so it does not damp the low output of the bridge/string yet strong enough to withstand the tension of the strings. ( as an aside one of odd paradoxes of string tension is that high tension strings put out more energy, but it is also possible in some cases that they choke back the ability of the top to receive energy- a lower tension string will allow the top to move more openly of freely, but provide less energy. It's an interesting situation, but a separate topic.) Mass, density, and weight all play a part in damping of the top- Once all that is settled out the size a placement of the braces come into play - The tall and thin profile braces, like parabolic braces tend to favor higher frequency transmission, the lower flatter triangular braces not as much. Higher profile braces stiffen the top more microgram for microgram than a lower flatter brace. But there is not grand chart of reciprocity between lower braces and taller braces. So it might take more lower flatter braces to stiffen the top as much as one tall parabolic brace, and if weight is your concern, one slightly heavier brace might be better than three lighter less stiff braces. Parabolic shaped braces also concentrate the stiffness at the center of the brace, where as the lower profile braces tend to be more like archery bows, they flex evenly along the length. Parabolic braces are designed to more of less be rigid and function to stiffen the center area of the top. Lower flatter braces in general are easier to work with for flamenco designs, although one could brace a flamenco guitar with parabolic braces. Most of the time however parabolic braces work well for designs that are more about clear singing voice than the flamenco growl. The brace placement is fairly important, there are a couple basic understandings about placement if you want to speak of flamenco voice vs. other kinds of voices. Although this is not set in stone, it's just general theory, feel free to run with it in actual building: The Torres 'kite' set up is a design that is created to make low flat braces cross the grain at a steep angle, it provides a lot of cross grain support. If you put a pin point in the middle of the 12th or 13th fret and use that as a point to run diagonal lines through the top, (go look at a Torres drawing online ) you see the braces land on those diagonals and distribute stiffness and possibly some arch, to the top. The braces run at an oblique angle to the long grain of the top. That angle tends to stiffen the top across the grain, that is side to side across top. It also stiffens the top with or along the grain, but not as much. Using parabolic braces in that same way would render the top VERY stiff and probably take two fewer braces to do the same job, but the focus of the most stiffness would be more centrally located under the bridge area. The centrally located stiffness will make a different kind of voice, than the lower flatter braces with the triangular section. The next placement idea is effecting cross grain stiffness - If you move the pin point at the 12th fret up to fret 1 or 2 and draw the same diagonal lines down the face the oblique angle that the braces placed on those lines will be much less. Placing the braces more parallel to the long grain direction generally, important to say generally, makes the cross grain stiffness drop a bit. That is important to pay attention to because making that cross grain stiffness looser will really effect the voice. Looser cross grain stiffness favors the cross di pole modes of the top. Meaning the side to side action of the bridge has less tension to pin in down and make it move more equally in all directions. Less cross grain stiffness allows the bridge to move more like a 'See - Saw' on a kids playground. The bridge swings said to side and that emphasizes the 'growly' qualities in voice. There are reasons why, but it's a long diversion to get into why. Moving the point that the braces shoot down from diagonally creates a more open movement cross grain wise. Add flatter or parabolic braces to that scenario and then the predictable things happen. Parabolic braces bolster up the center stiffness, Because they are usually inflexible in their own center and flatter braces distribute stiffness more evenly and with more 'inherent spring' like the bow. Both braces placed more parallel to the long grain will allow for more open movement across the grain. And braces placed more parallel to the grain stiffen the top along the grain more directly than steeply angled braces. Stiffness along the grain is important because the strings are under tension of course which is using the saddle like a tiny levering device to push and pull the top into a distorted posture. If you choose parabolic braces with all the strength in the center area, you can make the most distortable area very impervious to distortion, that area is the top between the sound hole and bridge. Provided the lateral brace under the sound hole lower edge is bearing the load. Tops distort in ways that are very complex, and it would take a lot of time to explain how and why, and how it makes guitar sound. Most makers tend to agree a wee bit of distortion is going to happen and it is ok, some others think distortion is a mortal sin. That is how it becomes religious, because most of the information is anecdotal. I tried to show a little bit abut why structure and sound are married - if they were not then you could over build the guitar and any old structure would make it work. See the structure of a Smallman type guitar. It's over built like a super stiff speaker cabinet, and it has a delicate membrane that the bridge/string unit moves. The Torres guitar is based on an integration of structure and sound. The Smallman design is also, but in a radically different way. The engineering only matters to a certain extent, and engineers hate this (Richard and the fellow he quoted are two notable exceptions LOL and have read the Foro's own Kevin Richards express similar ideas.) . There is a genre of architecture called 'vernacular architecture' it means regional conditions call for special solutions to buildings and structures, and are old traditions that were not designed by architects or engineers. In Morocco a vernacular treatment of a house would be a special hole cut in the roof with panels of wood that direct the afternoon breeze down into the house to cool it. It is learned by trial and error over hundreds of years. A system is worked out, it becomes a regional practice. The guitar was developed the same way, there was a problem between the structure being to fragile and folding up under string tension, and of being built too heavily built and not picking up as much energy input from the bridge as it could gather. The guitar makers worked as 'vernacular engineers' to solve those problems and what we get today is a zone or parameters of size and strength of braces and structure that we know works. But it gets too weird really for striaght up engineering to solve problems after a certain point. One example is that we know that there is a difference between the concept of stiff heavy ribs that don't move or allow the perimeter of the rim of the too to move in anyway thus creating a perfect torque free attachment point for the top. Tis means the top will not syphon energy into the ribs and bounce around on top of this perfeclty hard structure. vs. The ribs are very thin and attached to the top by glue blocks, which anecdotally are supposed to make the rim less stiff than robust solid liners, and the ribs being thin absorb or filter some energy form the top thus stealing some energy and making the top less efficient. The problem is that the thin ribs do transmit some sound, and in fact do filter the sound and have a lot of bearing on voice for complex acoustical reasons. But measuring these things empirically is almost impossible. Just the solid liner vs. glue block issue is a mind bender to test, and to my knowledge it's not been done. But I can say anecdotally glue blocks once they are put in place and held in two directions by the rib and top make the structure REALLY F-ING stiff. So the braces whether they are parabolic or flatter and wider both work. The deal is that you get a style of guitar or you design a guitar that has the structural qualities that fit the kind of sound you want to make and you work in that zone. Flamenco guitars are a particular zone, there are established standards for what works and you dive an work with those parameters, You can use engineering or acoustical programs or methods to thin the top or tune the top, but you're still working is a vernacular zone. I call the process vernacular engineering. Square beams, rectangular beams, I beams are all interesting to analyze in section for strength, but modulating the beam for stiffness or flexibility and placement is a vernacular activity done by feel. Now, that should start a grand religious war! ( I have a little typo fixing to do and some tweaking to the language, but I'll fix it later.) quote:
And another thing, what difference does it make when a guitar has 3 back braces or 4? Not too damned much.
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