Stephen Perry

Method for determining arching height?

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In light of late discussions, specifically the secret was a concept one, I have the opportunity to set the arching height on two different instruments, both based on the Betts outline. The backs are arched, but nowhere near finished.  The fronts were first-pass cut.  Edges taken to 5 mm, but no channel width set, and the arch height not set.  One is in that rough condition and one I have set the long arch a bit high, and have roughed out the general arch shape.  The long arch has three sections, with the joining points determined by compass construction from points on the upper and lower bout vesica pisces points marked.   I selected points that were further toward the ends than I usually would to get a longer center run.  My inflection points appear to correspond to those eyeballed from the Strad poster of the Betts.  I also selected the long radius for the center run based upon that poster.  The cross arching I set using David Beard's description of 2/3 run/1/2 rise, using two iterations.  The arching looks good, will need work at the level of the corners.  My intent is to marry the 2/3 - 1/2 system with the Francois Denis vaulting description, see whether I can make the two fly together.  The arching is currently 16.3 mm, leaving room for me to progressively and selectively lower it to a "suitable" height.  

My question is, what could I look for to determine a suitable height?  I have the wood density in notes in a box somewhere in my storage unit.  Not really accessible, but I could determine the density if that's key.  The speed of sound - well.  I am not set up to determine that, but these tops ping within the center of the wood I have, and ring pretty nicely.  Decent tops.  I know I selected them to match the backs, but cannot remember how (they're from years ago - projects stalled after my motorcycle wreck).   I'm sure it's in my stored notes, along with other things I'd really like to have!

So what do I do to set the height?  My intent was to float things down to about 16 mm progressively and carefully to accommodate Denis concepts, the usual modifications around the F holes, straight line segments (which seem quite important to me, but are another subject), and now to merge with the 2/3 - 1/2 run - rise system presented by Beard.  

My use of the run - rise system springs from examination of plates I quickly arched and showed to a well-known maker, who really liked them.  I'm not totally married to it, but the result was the most pleasing and satisfying of anything I had done using templates, and this top I am working on is giving me the same warm fuzzy happy feel.  

Thank you for any suggestions.

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1 hour ago, JacksonMaberry said:

I was taught to go higher for lighter wood, lower for denser wood as a rough and ready guideline. 

... and there are those who say the exact opposite, some of them very reputable makers.

My own personal opinion is that there is a huge range of arching heights that are "suitable", with a similar wide range of densities that work as well.  Also my opinion (at the moment) is that lower arching = more midrange power, and lower density = more midrange power, with all else being equal, with the additional consideration that lower and denser = lower B mode frequencies, if that matters for anything.

15.5 - 16.5mm  heights I think are the easiest to deal with, but the most important thing is the SHAPE of the arching, not so much the height.

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16 minutes ago, Don Noon said:

... and there are those who say the exact opposite, some of them very reputable makers.

My own personal opinion is that there is a huge range of arching heights that are "suitable", with a similar wide range of densities that work as well.  Also my opinion (at the moment) is that lower arching = more midrange power, and lower density = more midrange power, with all else being equal, with the additional consideration that lower and denser = lower B mode frequencies, if that matters for anything.

15.5 - 16.5mm  heights I think are the easiest to deal with, but the most important thing is the SHAPE of the arching, not so much the height.

Agreed, I am one of those.

Not to put me on the list of those reputables :), but only one of those who say the opposite.

 

At least for the range from 14 to 17 mm, I have to admit that I have not explored much the heights beyond these numbers and maybe the story could change, who knows.

 
I also agree that it is more a matter of shape than of height, and more of tone than of power.

 

 

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1 minute ago, Davide Sora said:

I also agree that it is more a matter of shape than of height, and more of tone than of power.

My comments about more midrange power are based on a FFT plot mindset... but it also means a different tone.  More midrange power I would characterize as a more crude-sounding, aggressive tone.

I have gotten some decent results as low as 14.5 mm top arch height, not so good with an experiment slightly below 14mm, and totally unusable below 10 mm.  The highest has been 18.1mm, and that worked reasonably well., but I am just sticking with the 15.5-16 range for now, where I have had the best results.  For the back, I haven't experimented much.

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1 minute ago, Don Noon said:

My comments about more midrange power are based on a FFT plot mindset... but it also means a different tone.  More midrange power I would characterize as a more crude-sounding, aggressive tone.

I have gotten some decent results as low as 14.5 mm top arch height, not so good with an experiment slightly below 14mm, and totally unusable below 10 mm.  The highest has been 18.1mm, and that worked reasonably well., but I am just sticking with the 15.5-16 range for now, where I have had the best results.  For the back, I haven't experimented much.

My comment about the power was not directed to your observations (which I share) but in a more general way, I apologize for expressing myself clumsily.

I also progressively moved away from low archings (I tried up to 13.5 mm) at least my violins I like more tonally with heights of around 16 / 16.5 mm, but in fact it is a very personal matter when we end up talking about tone.
 

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38 minutes ago, Davide Sora said:

Agreed, I am one of those.

Not to put me on the list of those reputables :), but only one of those who say the opposite.

 

At least for the range from 14 to 17 mm, I have to admit that I have not explored much the heights beyond these numbers and maybe the story could change, who knows.

 
I also agree that it is more a matter of shape than of height, and more of tone than of power.

 

 

I'm glad you and Don made this observation. I hold nothing as sacred, and will always bow to better information or techniques when they present themselves. 

I'm 100% in agreement about the shapes of the arching being a much more important consideration, tonally, than height. 

The way the (clearly erroneous) rule I gave above was explained to me made sense at the time, but in retrospect seems a bit like folk wisdom. The reasoning was that a higher arch is somehow more stiff, in order to accommodate naturally less stiff wood. 

Anyway, I posted with no intent to mislead. Happy to learn something new!

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All makes sense to me.  Thank you.  I will let these arches drift into that 15.5 to 16 mm range and concentrate on refining this arching with fairly wide channels.  Then I will get back to a narrow channel version.  See what differences come up.

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Hope some questions from a non-maker about measuring arching heights are ok.

What distances are used for arching height measurement?  I assume that one point is the highest point in the plate arching.  But what do you measure down to?:  The surface that the plate would be sitting on if it were detached and lying on that surface? (In other words, is the thickness of the plate included in the arching height?) The outside surface of the plate at its lowest point? (You're not including plate thickness.)  If you are not including the plate thickness in the arching height, where on the plate surface do you measure down to?:  Where the plate is thickest, probably at the edges, or more inward, in the channel, where the plate would be thinner? 

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 Arching height does include plate thickness.    Arching height is normally set before the inside is hollowed, so you just measure the thickness at the highest point.   After the plate has been carved....yes, you would measure to the surface that the plate would be sitting on.

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7 hours ago, Brad H said:

 After the plate has been carved....yes, you would measure to the surface that the plate would be sitting on.

Things can get unclear when you consider that the top is often slightly bent to glue down to a short top block, and then there's distortion as the whole body is bent over time from string tension.  Then there isn't a well-defined surface to measure to.

Strad posters occasionally will list arching height as measured across the arch (referred to the C-bout rib surface) and along the arch (referred to the endblock surface), and can differ by 1 mm or more.  I generally prefer the cross arch dimension, as it seems less likely to undergo as large a distortion as the long arch.

In new making, assuming you start with a flat surface, it's easy to define arching height after the external arch is carved.  After the inside is hollowed out, and glued to the body, and varnished... it can move around to something different.

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To my experience arching height of the top has a much bigger influence on the result than the back. Regardless of the shape the arching, the height can counterbalance to some degree a bit weaker type of spruce. (but you should know which type of wood you reject from the beginning) But then for sound quality the shape is what makes the sound for both top and back.

Don's figures are basically the standard measurements. I recommend to make maybe 2 or 3 experimental top archings for one and the same body to find out the differences as real experience and from there you will certainly develop your own standards what to do when. The earlier you do it the better for the rest of your career. Talking about it doesn't help much because words cant describe what you hear.

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Thanks.  I've figured out how to do good measurements once completed, using all three of my arms!  

I expect this top will be fine with a standard arch height, being dead center normal spruce.  I have some very light spruce, too, which will be fun.  

The back and ribs for the two Betts influenced instruments I selected to be as close as I could find for density and ring.  Intended to do two different archings.  Mulling over how to do number 2.  Will take me a while to finish the first, regardless.

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