60 replies to this topic

[Tom R]

What is the best method for measuring the arch height on a plate that has not been hollowed out yet?

I have been measuring the height by placing my caliper on the top of the arch and the unhollowed inside, so my arch height is basically the thickness of the unhollowed plate. Is this an accurate way of measuring?

[robertdo]

I don't know if it's accurate but this is basically what I usually do. Probably a more precise way would be to use a device like the one we used to measure people height.
Now this height will be the one of the free plate, but no doubt the number will change once the plate is glued on the garland, then when the sound post is in place and after some playing.
In fact I don't really know which it is on the posters from the Strad.

[Bruce Carlson]

What is the best method for measuring the arch height on a plate that has not been hollowed out yet?

I have been measuring the height by placing my caliper on the top of the arch and the unhollowed inside, so my arch height is basically the thickness of the unhollowed plate. Is this an accurate way of measuring?

The measuring method you describe during construction is essentially your arching height, but as Robertdo explains it could change or move slightly during the later phases of construction, bassbar installation, gluing the belly to the ribs, soundpost tension and finally when strung up to pitch and under string tension. It's easy enough to check along the way if you like to see how or if it changes but a small amount more or less of arching height is not going to make or break an instrument.

I don't know if it's accurate but this is basically what I usually do. Probably a more precise way would be to use a device like the one we used to measure people height.
Now this height will be the one of the free plate, but no doubt the number will change once the plate is glued on the garland, then when the sound post is in place and after some playing.
In fact I don't really know which it is on the posters from the Strad.

Many of the measurements on the Strad posters are likely taken with the instrument strung up to pitch and, for me at least, is only an "indicator" of how the arching is, not an absolute.

Bruce

[Tom R]

I don't know if it's accurate but this is basically what I usually do. Probably a more precise way would be to use a device like the one we used to measure people height.
Now this height will be the one of the free plate, but no doubt the number will change once the plate is glued on the garland, then when the sound post is in place and after some playing.
In fact I don't really know which it is on the posters from the Strad.

Glad to know I wasn't doing it wrong. I have done a ton of reading and research on arching but I have never seen any mention on measuring the arch height. It's obviously a very important aspect and I just find the lack of info strange. I was pretty sure the way I was doing it was accurate but I wasn't sure if the height should be measured from the top of ledge or from the bottom.

Since we're on the subject of arches- I'm working on a strad model and I am planning on using a height of 15mm for the top and 16 for the back. Any thoughts or suggestions on these heights?

[Christopher Jacoby]

The top arch should be higher than the back. Not that you can't do it the opposite way and get good results-- but Strad tops are generally taller.
And if you need to take the arch height of a hollowed plate, lay it on a flat surface, lay a straightedge across the heighest point of the arch (perpendicular to the center seam), then measure down from the straightedge to the flat surface on either c bout and take the average of the two measurements.

[robertdo]

there might also be a relation between the arching heights and the ribs heights. In turn there might also be a relation between these and the thickness of the plates, meaning a relation with the internal volume of the violin.

[fiddlewallop]

I just checked the arch height on my back plate last night, only to discover that it's 16 1/2 mm instead of 15 mm. 1 1/2 mm over what I originally intended it to be. That's a bummer, but it will be interesting to see what kind of Frankenstein baby I end up with here.

The top plate is a solid 15 mm, so it will def look a little "off" when viewed from the side, I'd imagine. Not sure what effect it will have on sound.

[Ernie Martel]

What is the best method for measuring the arch height on a plate that has not been hollowed out yet?

I have been measuring the height by placing my caliper on the top of the arch and the unhollowed inside, so my arch height is basically the thickness of the unhollowed plate. Is this an accurate way of measuring?

I've been doing the same as you...but interesting changes proceed as Robert & Bruce point out...Good question.

-Ernie

[Bruce Carlson]

there might also be a relation between the arching heights and the ribs heights. In turn there might also be a relation between these and the thickness of the plates, meaning a relation with the internal volume of the violin.

A very general rule of thumb is: arching height back + rib height + arching height top = about 60 mm to 62 mm total body height. Half of the height is rib height. I say it is a general rule because it's an average and you probably will find that instruments in real life are always different.

Bruce

[Oded Kishony]

there might also be a relation between the arching heights and the ribs heights. In turn there might also be a relation between these and the thickness of the plates, meaning a relation with the internal volume of the violin.

Bruce has already mentioned that the sum of back and top arches equals rib height but would you enumerate the internal volume and graduation relationships?

Thanks,
Oded

[robertdo]

Bruce has already mentioned that the sum of back and top arches equals rib height but would you enumerate the internal volume and graduation relationships?

Thanks,
Oded

I had read this article (I think I posted a link at one point) where some people had measured the internal volume of several Stradivarius violins and found it to be quite constant. Something that would make sense if there is some kind of relation between height of ribs and arching height. As for the graduation let's say that at a fixed external height and shape, a variation in the thickness would have an impact (although very limited) on the internal volume (thicker meaning less internal volume. Again, all else being equal). But compared to the variation of height of the ribs this effect would be very small.

I had started this thread on Deciding arching height and I had taken the measured ribs heights (neck and end pin) and top and back and after adding had found this to be around 90-92 (equivalent to what Bruce said, simply I added both ribs heights)
I wondered why/how the makers had chosen this as a rule of thumb.

Just to make sure there is misunderstanding, when I talk about internal volume I talk about the internal air volume of course.

[Tom R]

A very general rule of thumb is: arching height back + rib height + arching height top = about 60 mm to 62 mm total body height. Half of the height is rib height. I say it is a general rule because it's an average and you probably will find that instruments in real life are always different.

Bruce

That is some very helpful info, thanks. Would you happen to know if the 60-62mm is on a fully strung instrument? I am still pretty new to violin making so I'm trying to figure out how much the top arch might drop once the tension is applied.

[Torbjörn Zethelius]

I am still pretty new to violin making so I'm trying to figure out how much the top arch might drop once the tension is applied.

Or, you might ask how much the top arch rises once the tension is applied. It depends on the type of arch, the stiffness of the wood, and the graduations. But as a general estimation, I'd say about 2 mm.

[Ernie Martel]

Or, you might ask how much the top arch rises once the tension is applied. It depends on the type of arch, the stiffness of the wood, and the graduations. But as a general estimation, I'd say about 2 mm.

What is the school of thought about fingerboard projection of 27mm? Does this mean that necks set alittle high are OK?...or is the 27mm dimension with the strings on and up to tension?...

[Bruce Carlson]

That is some very helpful info, thanks. Would you happen to know if the 60-62mm is on a fully strung instrument? I am still pretty new to violin making so I'm trying to figure out how much the top arch might drop once the tension is applied.

Hi Tom,

It's a rule of thumb at its best and not meant in any way to be absolute. The belly is not going to go down a lot with a snug soundpost and a healthy bassbar but if it goes down slightly it may be compensated by the back moving outwards so in the end, you may not see any significant difference in the total measurement.

This is roughly what happens on the "Cannon", Paganini's Guarneri 'del Gesù' of 1743. The shape of the violin totally unstrung was measured and then the shape was measured a second time once tuned to pitch. The method used was optical interferometry. The amount of change (deformation) + or - was then calculated in a computer from the two images.

On the belly, max downward deformation was in the chest area between the upper eyes of the soundholes and the bridge (about 0.3 mm). The only area of greater downward movement was the area between the inner edge of the bass soundhole and the bass foot of the bridge (slightly less than 0.5 mm). Obviously the bassbar side deforms more than the soundpost side because of the way the belly is supported in the chest area (bassbar - horizontal beam versus a more rigid soundpost - vertical column). The max rising deformation on the belly was in the lower bout on the treble side (about 0.3 mm) followed by the upper bout on the treble side (about 0.2 mm).

Consequently, on the back, the outward or rising deformation was at maximum in and around the soundpost area (about 0.1 mm)and was virtually nil in the area directly below the bassbar (obvious).

These are not big changes but you must also bear in mind that this particular Guarneri is a very stiff violin due to relatively robust thicknesses.

Bruce

[Bruce Carlson]

Or, you might ask how much the top arch rises once the tension is applied. It depends on the type of arch, the stiffness of the wood, and the graduations. But as a general estimation, I'd say about 2 mm.

Torbjörn, the instrument is bending or deforming due to the string tension. The "apparent" rise of the bridge is actually the lowering of the neck and fingerboard assembly as the belly is compressed longitudinally and the back stretches or elongates slightly on the same longitudinal axis. This allows the neck to fall. In many violins it is about 1/2 to 3/4 of a millimeter when the line of the fingerboard projection is measured at the bridge position.

Bruce

[nathan slobodkin]

Bruce, Reading your post about the changes to an instrument when it's strung up I agree that the change in the neck pitch is from longitudinal compression of the top but most new instruments need a longer sound post after a fairly short time and usually another within the first couple of years.This for sure means that one or both of the plates must be bending outwards and if it's the back you wonder what could make it bend further than the soundpost pushed it especially since the longitudinal stretching should provide at least some flattening (and upward) force. Likewise have always assumed that if all other factors are equal a higher top would be stronger and more resistant to downward pressure but am not clear about how back arch hight affects resistance to sound post pressure. Physics anyone?

[Bruce Carlson]

Bruce, Reading your post about the changes to an instrument when it's strung up I agree that the change in the neck pitch is from longitudinal compression of the top but most new instruments need a longer sound post after a fairly short time and usually another within the first couple of years.This for sure means that one or both of the plates must be bending outwards and if it's the back you wonder what could make it bend further than the soundpost pushed it especially since the longitudinal stretching should provide at least some flattening (and upward) force. Likewise have always assumed that if all other factors are equal a higher top would be stronger and more resistant to downward pressure but am not clear about how back arch hight affects resistance to sound post pressure. Physics anyone?

A number of things are happening with a new instrument that have slowly, over time, resolved themselves in an older instrument. Even older instruments are not in perfect static equilibrium but the changes in a decently designed instrument structure become less and less perceptible over time. In other words, greater changes are going to take place in new instruments that have never been subject to the forces generated by the string tension.

Some of these are:

Wood compression, especially the spruce top, where the soundpost rests against the belly but in time this can be seen both on the top and on the back. Moving the soundpost a lot, even ideally without tension, but over time, will burnish or compress these areas as well.

The belly is bending around the soundpost mostly because of the downward force of the nearby treble bridge foot. This too, in time, creates a small dome-shaped area, more easily visible on cellos.

The back also deforms laterally (from c-bout to c-bout) in an asymmetrical manner as it too bends around the soundpost, moving outwards. This can be seen in older instruments where the highest point in the lateral arch, and sometimes in the longitudinal arch, has gradually deformed to become the highest point of the back arch. On unusually thin backs you can see, on the outside of the instrument, exactly where the soundpost is resting (scary ).

Although I'm not a physicist these appear to be the contributing factors necessitating a longer soundpost as new instruments settle in.

Much of how this is seen or visualized, depends upon your fixed point of observation.

Bruce

[Torbjörn Zethelius]

Torbjörn, the instrument is bending or deforming due to the string tension. The "apparent" rise of the bridge is actually the lowering of the neck and fingerboard assembly as the belly is compressed longitudinally and the back stretches or elongates slightly on the same longitudinal axis. This allows the neck to fall. In many violins it is about 1/2 to 3/4 of a millimeter when the line of the fingerboard projection is measured at the bridge position.

Bruce

Bruce,

The string tension causes the violin body to bend. Sort of like a banana. But not as exaggerated of course. This "crumbling" of the belly forces the arch to rise. Or, with too thin graduations, the center area may collapse. There are some immediate changes as you say, when tensioning the strings for the first time, but there is also creep which happens over long time.

[robertdo]

Is it not the reason why the top plate has a flat area in the middle? I am not a physicist so what I will say might be complete nonsense, but given the way a violin is strung, with the bridge between top block and lower block, and because the top plate is glued to both, the string action would tend to press the top plate inward. If the top plate was shape like the back plate, I believe it would tend to bulge upward, the strings and the bridge trying to push it back. And that would no be good. But being flat in the middle, it would mean only the lower and upper bout would bulge a little bit while both pressing forces would cancel each other in the flat central area, reducing greatly the deformation there.