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Thread: Mandolin top arch differences?

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    Default Mandolin top arch differences?

    I am asking this of the builders on this forum as I have seen quite a few mandolins that have a higher or lower arch than others and would like to know what difference, if any, does the heigth of the arch do to the tone of a mandolin? Another question about the arch is: Did the one and only Gibson A-5 Loar signed mandolin have the same arch as the Loar F-5s` of that era? I have seen two A models that were supposed to be exact copies of the A-5 and those mandolins didn`t seem to have as high of an arch as the Loars that I have seen...

    Willie

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    Registered User sunburst's Avatar
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    Default Re: Mandolin top arch differences?

    Generally, all other things being equal, a higher arch has more overall stiffness than a lower arch. The carved arch serves to increase the top's (or back's) stiffness to mass ratio, so a higher arch will support the bridge with less "give". To make a big generalization, something I usually don't like to do, within the normal limits of carved arched tops, the higher stiffer arch will tend more toward treble and have a "brighter" sound, again all other things being equal.
    When carving, some of us use a higher arch for lighter, less stiff woods and a lower arch for stiffer heavier woods to help balance things out both structurally and tonally.
    I have not seen the A5 in hand, but from what I've read and been told my assumption is that they used the same pattern carver that they used for oval hole A mandolins, and that would mean the arch would be similar to other A-styles. Perhaps those more familiar with that particular mandolin can confirm or refute that assumption.

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    NY Naturalist BradKlein's Avatar
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    Default Re: Mandolin top arch differences?

    Luthiers - Is there a rule of thumb among makers of archtop instruments - violin family as well as mandolin family - about the relationship between the back arching and top arching of an instrument?
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    Registered User sunburst's Avatar
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    Default Re: Mandolin top arch differences?

    In the violin world, arching is "everything". Builders don't discus graduations as much as mandolin makers, and they perhaps attach a little more importance to density of wood than mandolin makers. So... violin arching is much more standardized than mandolin arching. When we build mandolins, we can use any arch height and shape we darned well please, within reason... though some are perhaps not within reason, but I'll limit personal opinion here.
    As for a relationship between top and back arch, that's up to the individual builder. Personally, I use the stiffness and density of the wood as more of a guide to my arching, both top and back, than the relationship between the two. I don't vary back arches nearly as much as top arches, and I put attach more importance to a good relationship between the top and back in terms of density and stiffness. In other words, I choose woods that I think will work well together, arch the plates according to the characteristics of the chosen wood, and build the darned thing.
    I guess the answer is: no, there is no standard rule of thumb, at least not among mandolin builders. Beyond our early learning years, we tend to stick with what works for us regardless of what others do, and what others do can be significantly different.

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    Registered User fscotte's Avatar
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    Default Re: Mandolin top arch differences?

    Dudenbostel told me the Griffith A has a "long and low" arch.

    I pay a 110% attention to top arching. I found through deflection testing that higher arches flex less at the center than lower arches. Even when you try and compensate for the higher arch by reducing thickness, the center remains insanely stiff. So careful consideration for the contour of the arching is likely the most important aspect of the top.

    I think higher arches tend to be brighter because they are stiffer, especially at the center as mentioned. You can try and make the tonebars as small as possible, even reduce overall thickness to dangerous levels, but if its too archy, trying to get the center to flex can be difficult.

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    Registered User bernabe's Avatar
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    Default Re: Mandolin top arch differences?

    Quote Originally Posted by fscotte View Post
    I pay a 110% attention to top arching.
    Yes. As for the ranges of spruce species, I like keeping arch height under 16mm, regardless and more often closer to 15mm and graduate accordingly by how much wood I leave at the end of the day. I also like a longitudinal profile to be gradual and as "straight" as possible and still be an arch, if that makes sense. i.e no dramatic ramping up from the tailpiece or drop down towards to FB. I think it helps the top move better and resist longitudinal compression [possible deformation down the road]. I then get the center thickness where I want it and leave other areas a little on the heavy side, then graduate while strung up, particularly the general area in front of the FB extension and behind bridge in front of tailpiece. The re-curve on the sides and around apertures I stay to standard numbers and dont think it has as much impact as other areas of the top in controlling stiffness. Sand here and there till the top is where I want while monitoring with a Hacklinger. There's tonal characteristics [highs] I recognize when the top is close, despite what the overall sound is like. If the tone bars are thick right up to the head block area, then that will effect things despite the arch. I like shaping bars like Nugget based on a picture Jim Hillburn posted once with the bars thinner and flattening out appx. an inch or so toward the ends which allows me to get better results by graduating from the outside of these areas without the bars controlling whether I can change things as much. I then work the back for lows and warmth, and maybe a little more on the top and back to pull it together. This can take some time and patience but Its been the most consistent way for me to get good results. I usually arch the back similar to the top height-wise and like my body [ribs] slightly deeper. Basically, Id rather go on the low end of the arch and leave more material. As John stated...we figure out what works for us individually and stick with it. High arches with thin tops haven't worked out for me as far as volume and tone I'm shooting for. But, it has for others perhaps. My $.02.

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    working for the mando.... Bluetickhound's Avatar
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    Default Re: Mandolin top arch differences?

    Riffing on this discussion.... If the top arch is on the higher side, does that necessitate compensating with a flatter back arch (or vice versa, high back/flatter top) to maintain air chamber volume?

    The point I'm driving at, as has been mentioned numerous times here, is that my limited understanding tells me that one aspect of a given build is affected by (and affects) the sum total of all the other aspects, to one degree or another.
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    Default Re: Mandolin top arch differences?

    Quote Originally Posted by Bluetickhound View Post
    Riffing on this discussion.... If the top arch is on the higher side, does that necessitate compensating with a flatter back arch (or vice versa, high back/flatter top) to maintain air chamber volume?
    Id rather not think that way when choosing arch heights. I want plates to do what I want and rather compensate internal volume with a slightly deeper body [rim]. As an alternative to that, I think I'd rather make a deeper back arch and a "normal" or not too high top arch than the other way around. The back isnt "pinned down" by downward string pressure, so you can get away with more with a higher back arch as far as movement and how thin you can get away with. Just my opinion.

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    Adrian Minarovic
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    Default Re: Mandolin top arch differences?

    The height of arch is not the only parameter here. The shape of he arch is more important IMO. The cross arch can be bulbous (circular shape - StewMac kits, Red Diamond and most modern Gibsons come to mind) or more parabolic (that's what I prefer, Old Gibsons were more like this). The long arch can be one of the above or can be long and flat (old Gibsons had often rather flat back arch, though most Loars less so). And then there is relationship of highest point of top and bridge position - makes quite a difference in sound if the bridge ir right on the top or 1" forward (this is case of Griffith A5 because of the body shape and typical arching shape for A, and some Loar F-5's but in those cases it's likely mostly result of distortion)
    Violin people tend to copy arching exactly after model. SO if they are copying "Alard" del Gesu then they copy arching of that particular violin to detail. Makers who use their "personal" model usualy use arch that they consider most succesful of those they tried... The makers who adjust arch height to properties of wood tend to use higher arch for stiffer denser wood and reduce thickness (though in violins the differences between tops are very minimal - few 10ths of mm at most) and lower arch for less dense or less stiff wood.
    Mandolin makers typically use one arch style for many years and work more with graduations of top and back. I personally don't think a small variation in top arch heigths really requires compensation in back arch or rim to keep the body volume constant.
    Adrian

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    Default Re: Mandolin top arch differences?

    Quote Originally Posted by Bluetickhound View Post
    ...If the top arch is on the higher side, does that necessitate compensating with a flatter back arch (or vice versa, high back/flatter top) to maintain air chamber volume?
    I used to do that, (lower or higher back arch opposite of the top arch) for that reason; to maintain a roughly similar internal volume. I've since decided that any difference in internal volume from a millimeter or so difference in arch height is trivial.

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    Registered User fscotte's Avatar
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    Default Re: Mandolin top arch differences?

    To put this into real "numbers", Don Macrostie makes his back plates 1.3 to 2.8 times LESS flexible than the top plate. He also uses higher arching in the back. Note that this is different than years before when he was building flexy top with flexy backs. I think that when he had the chance to start looking at more Loars, his philosophy changed. So to the point, the back can vary wildly from mando to mando.

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    Default Re: Mandolin top arch differences?

    Very interesting, in 1981 I had a mandolin custom built for me, it was the first one the fellow ever built and how I wish I would have had all of this info that you guys have forwarded on here...Now I just might get a new top installed on that mandolin...who knows?

    Thanks for all of the input.....

    Willie

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    Default Re: Mandolin top arch differences?

    I don't understand mandolin arches as much as violin arches, and I won't claim to be an expert on those. They generally seem symmetric from the edges in, in spite of having F holes cut in. Hadn't thought about back and top height. Watch the measurements on historic instruments - things swell and flatten.

    Violins might be illustrative. Think of the top sort of as two cups with a cylinder in between. That central cylinder gets supported by the "pillars" at the ends of the F holes. the shape right at the end of the F holes becomes very important.

    Let's make a violin arch. I've got the outline laid out with the viscus pisces, straight edge, compass, Cremonese ruler. The outline is the outline of the mold, of course, not the edge. Got to get an arch on it. The back and top I've got to the same measure as the starter rib height, 1" (32 mm). I take the wide spots of the upper and lower bouts, draw a line across, and use some triangles with the compass to find the inflection point at upper and lower longitudinal arch breaks. I have a couple of choices. My favorite that gave a look I like ended up making little divider dots on the top. Later saw these in the same spots under varnish on a Stradivari. One minor victory for the hack maker stealing from the greats!

    So I've got 16 mm at the center somewhat (insert long golden mean, ancient working method, phase of moon argument) and a radius on my long arch center section. This is some magic number. Or I happen to decide that an obscure working method for heights at various portions of a vault will do, perhaps my magic points relate to a magic point system. Anyway, I have and use a magic point system that gives me the elevation of points in various places. Beautiful system, highly elegant, I didn't make it up. I have my max height, I have my magic points, and I carve my central section as a arc of a circle with long radius. Then the two upper arcs from the magic points to the intersection of the arch with my mold edge.

    If I haven't already, then I cut a 5 mm thick edge shelf in 1/4 (for a working method) from the rough plate edge all around, except this is fudged here and there. So I have the long arch and the shelf, with a lump in between. I carve the lump down using the working method of 1/2 the fall in 2/3 the run, and drive this from the outside to the inside. Do some other tricks, and I have a nice Cremonese arch. Make the shelf narrower a bit, I get a flat stiff arch (less recurve area). Make the shelf wide, I get a flat high-looking arch. Make it wider yet and I have a drunk made goat humped monster.

    Now this system gets fudged or pulled around the magic points, and the exact curves in some area are manipulated by how the channel is cut into the shelf. This gives me perhaps a more obviously cylindrical central area to get more twisting, lets me stiffen or loosen the pillars (stiff = strong powerful, soft = easy play less power maybe, doesn't scare me to play).

    Or you can use a curtate cycloid. Or patterns from the oldsters. Or golden mean magic. Or whatever. The system I described first got me complements from people who actually know the subject.

    How are mandolin arches derived and worked up? I'm curious.
    Stephen Perry

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    Registered User Ivan Kelsall's Avatar
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    Default Re: Mandolin top arch differences?

    Willie & myself have been discussing this point via e-mail. I mentioned to him that my Weber "Fern" has a slightly higher 'arch' than my Ellis or Lebeda & put forward the idea that a higher arch = stiffer = more treble response. I was wondering if this had any bearing on why a particular brand / gauge of strings can sound good on my Ellis or Lebeda,but have "A" strings that cut like a razor on my Weber - the GHS A270's that i tried. Maybe it won't be the 'whole' reason,but i'm sure it has some input on the tone. Maybe the .016" gauge "A" strings are simply too strong toned for the Weber top, or, the heavier gauge + stiffer top simply don't work well together,
    Ivan
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    Default Re: Mandolin top arch differences?

    Ivan, that's an interesting question. I did a little bit of testing that gives some idea, and others have tried the same. If there's an optimum downloading on the bridge for a certain instrument, then the string tension should make a difference. An issue in testing is that the vibration of the top seems rather complex. I did have a low arch instrument that liked heavy strings better. I tried light medium and heavy on a couple of others. I couldn't refute that the string tension would be a primary aspect of response. The low arch liked high tension best. The medium tolerated high tension and was OK, but seemed to take way powerful pick action. The high arch rejected high tension. Lights didn't work on any. Mediums seemed the most tolerated. Not a statistically or observationally robust test!!!
    Stephen Perry

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    Default Re: Mandolin top arch differences?

    Another question that pops up is: Does a heavier gauge A string seem to make it sound more shrill than a lighter gauge string does since the heavy string puts more down pressure on the bridge and top?

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    Default Re: Mandolin top arch differences?

    Willie, the issue of course to course variations is quite interesting. I have not tried extra loading of a course, but I can adjust perceived volume and clarity of a specific course (with some impact on other courses - especially where I work on A or E courses and give the G and D a boost!) through bridge modification. That is quite easy (for me at least) to do and hear. There are also minor physically but distinct and generally good things that can be done in the body. Those are more tone quality and focus, reduction in noise. Oddly, much of that centers around limiting / reducing excited areas rather than increasing anything. For example, on a bit of a crunchy A course, often there are areas sapping or fighting towards the neck and tailblock more or less in line with the course. a very minor reduction of thickness (5 scrapes) in the key spots reduces the noise a great deal. I wish I could make a map, and I could in a general way, but the exceptions are 50% of the work!

    So the heavy strings may well do something useful, but that might merely mask issues more efficiently dealt with via file, blade, and scraper.

    Perhaps I should hold a video course on this kind of thing. It's a bit challenging to write about.
    Stephen Perry

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    Default Re: Mandolin top arch differences?

    Quote Originally Posted by stephen perry View Post
    viscus pisces
    I don't know if you're talking about fish stew or intersecting arcs, lol. :-)

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    Adrian Minarovic
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    Default Re: Mandolin top arch differences?

    We are skating on very thin ice here... I think no one really knows how exactly arch affects sound. But everyone knows it really does...
    I wouldn't talk about arch heigth alone again, I've seen good deal of instruments where arch was normal or even higher than average but it was far from stiff. One mandolin I have right now for setup has approx. 16mm arch but it is not a stiff arch, actually it had some serious problems that I corrected few years ago. The bridge sits farther from the tailpiece end than on typical mandolin (much like the Griffith A5) and the belly of the top is 1" behind bridge. The arch slopes down towards neck in rather straight line. This makes the top quite flexible and it reacts to strings easily but it cannot be driven too hard. Produces very nice tone but doesn't cut too much.
    Many old violins have very flexible arches of top that under tension of strings created "camel" arch with bulges at both bouts and lower area under bridge. There is theory amongst violin makers that classical violins dind't start with the "barrel" arch but it mostly developed slowly under tension over time.
    IMO to decide whether mandolin has stiff or soft arch you need to see both graduations and shape of arch (of course knowing stiffness of used wood would help). But typically the feel of strings under fingers tells a lot to experienced player...
    Adrian

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    Registered User fscotte's Avatar
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    Default Re: Mandolin top arch differences?

    The affects of an arch are well known aren't they? A very archy arch (isn't there a better word for this?) is stronger than a lower arch, if the material is the same. But it does seem there is a point that when an arch is too bulbous, there are diminishing returns in flexibility by making the wood thinner. Then at some point, too thin does make it less stiff, but at the same time more susceptible to deforming under pressure at the bridge.

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    Default Re: Mandolin top arch differences?

    How about the symmetry of the arch? Is it a bullseye contour? Oval? Amount of recurve? There's a lot to an arch other than height. Height to the eye can be so misleading.
    Stephen Perry

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    Default Re: Mandolin top arch differences?

    Hogo is very perceptive and observant, as usual. These instruments are under significant stress from string tension and do tend to deform, especially over time. I have had a couple tops begin to collapse because of the bulbous arching I used and then on top of that made the tops too thin. Ever since that experience I have made my tops with flatter sections between the bridge and the blocks but with "standard" height. So the bridge sits on a tighter arch (neck to tail) with flatter sections to the tail and to the neck.

    Flatter sections can carry more compression load than comparable thickness with more arch. The flatter sections also can flex more easily. These are the concepts in my mind as I am carving a top, and I know the top will deform into a very nice dome when the srting tension is applied. If you carve a nice dome in the first place you will get the deformation Adrian speaks of, where the top pushes up between the bridge and blocks, and the bridge sits in a low area. That is an extreme example but the deformation is happening, or at least trying to happen.

  29. #23
    Adrian Minarovic
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    Default Re: Mandolin top arch differences?

    There's quite a theory behind arches and their shapes in engineering. People often refer that higher arch is stronger but that is general rule when the arch is uniformly loaded. In such case, I believe, the strongest arch shape would be catenary curve (that's the shape that chain or rope creates when two ends are fixed), it is close to parabolic.
    BUT, arched tops are NOT uniformly loaded but there is more or less simple load at center and tailpiece and neck root. In this case the strongest would be simple triangle. The optimal height of the triangle would depend on the forces applied by tension of strings via tailpiece and neck and string break angle so teh forces are balanced. There's point when the triangle is too high and compression would overcome the bridge force or too low when the bridge force would deform the top down.
    BUT (last one:-)) mandolin arches are not triangular so every bit you stray away from the triangle you make the arch more flexible, recurve to one side and center arch to the other. The thickness of the wood and bracing is there to carry the load for many years. Now not only top is in equation but also thickness and shape of back as it takes lot of tension created by the strings (the whole mandolin is bending and thus back is stretched and top compressed) from the top. Also size of blocks - especially width of tailblock- can reduce the "twisting" of the ribs (and sinking recurve under tailpiece) due to the compression load.
    SO generaly like Michael wrote the more bulbous arch will be less stiff than "straighter" arch of same height and would require thicker grads to hold. And no matter what you do the forces on typical mandolin will push the arch behind the bridge out a little so keeping that in mind while carving is crucial to me.
    Doing other than typical arch can result in different deformations - I've seen mandolins that developed "camel arch" that were too flat in the center to start with and some mandolins that just plainly sunk in the whole recurve - too thin / deep recurve all around top.
    Adrian

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