Longer Scale Axes: Flat Top vs. Induced Arch vs. Carved Arch

[Ed Goist]

Considering longer scale 'fifth-tuned family' instruments (octave mandolin, bouzouki, mandocello, tenor guitar, cittern, etc.), I'd like to hear some comments about, and comparisons of, how the different types of tops affect the instrument's construction, tone, and overall character.

I'm particularly interested in thoughts about instruments with induced arch tops and backs, as this option seems to be mentioned here much less frequently than the other two.

Thanks.

[Jim Garber]

I thought that all "flattops" had some induced arch to them and that true flat tops would sink under string pressure. I suppose there are some instruments that have more of an induced arch but in general I thought they all did.

[Ed Goist]

I believe that most producers/builders do use a very slight radius on the top and back of 'flat tops'. Martin and Taylor us a 40' radius (I believe), while some custom builders like a radius of 25' on the top and back of their "flat top" guitars.

However, I know that some builders induce an arch as great as a 12' radius (top & back), and those instruments are often called "archtops".

[Audio Artillery]

Well it certainly takes a lot fewer trees to make a flat or arched top mandocello vs carving.

[Graham McDonald]

The difference is probably more a combination of soundboard structure and bridge type. There are really three ways you can approach building one of these big mandos:

1. a fully carved soundboard and floating bridge. Like a Gibson mandocello, with the option of a flat back.
2. A more or less flat soundboard with some degree of arch 'induced' by braces, still with a floating bridge. The amount of arch, the radius of the curve,is variable. I would be looking at 15' (usual guitar back arch), but others approach it differently.
3. A more or less flat soundboard with a fixed pin type bridge, 12 string guitar style, but smaller. Some arching or doming is built in with the braces, but often less than with example 2. A 24-25' radius, which is pretty standard for a guitar soundboard is common.

Style of construction often depends on the background of the builder.If they have previously make A and F style mandolins they are going to look at an OM or bouzouki differently than if their previous experience has been building guitars. Describing the sound or tone of these instruments really hard, mostly because there are few reference points. A mandolin gets compared to a Loar F or a snakehead A, a guitar to a pre-war Martin or Gibson. There aren't really the same instruments in the OM/zouk world as comparison points, and all of us who make them have a different sound in our heads as an ideal. A trawl through the CBOM forum will turn up numerous discussion over the years where admirable tonal qualities and how they are described has been debated, none particularly conclusive.

cheers

graham

[Tavy]

All I'll say is that I'm a big fan of induced arch tops - it's surprising just how far you bend these tops - and just how strong and stiff an induced arch can be. One thing that's often not mentioned is how much easier it is to bend across the grain than along it. One consequence of that is that unless you're using a cylinder top style profile, there's a limit to how much of an arch you can induce based on how much of a longitudinal bend you can get along the grain.

One other thing I've idly wondered about is profiling the edges of the two halves (rather like on a canted top, but less of a sudden change) so that when they're jointed the top naturally forms an arch-top profile. You would have to use super glue or something to joint the halves together in that case though...

[Jim Garber]

John: I have thought the same thing about joining two top halves with a slight angle.

Brian Dean used to make something he called wee- bit carved where he essentially did an induced arch but also did some carving of the top.

As far as bending across or with the grain, have you ever seen any Tilton parlor guitars? They often had the top grain running diagonally.

[otterly2k]

I have one of Brian's "wee-bit" OM's. My understanding is that it is carved-- that the arch is closer in degree to what is accomplished by an induced arch, but that the wood is not a flat piece as in an induced arch, but carved with some of the same concepts as a fully carved top-- slightly thicker in the middle and with a thinner area paralleling the rim. It does not have a recurve on the outside. So from the outside it looks much the same as an instrument with an induced arch top. Incidentally, the back is flat... with no discernable arch at all.

Perhaps Brian will chime in re: his hybrid concept. I remember he had ideas about why he suggested this combo... but wouldn't want to misrepresent his thoughts based on my poor memory. FWIW, this particular OM is spruce over curly cherry, oval hole, with a 20.5" scale. I'd say that it has very good projection, responding well to a lighter touch as well as heavy handed driving and that tonally it tends to accentuate the low midrange and bass ends... it's not as "sparkly" or "jangly" as some I've played, but has a rounded, refined tone. Brian described is as "creamy". I played another of Brian's that was "flat" (slight induced arch) back and front and it had a bit more power, but a less refined tone. (it also was walnut and longer scale).

I fully agree with Graham re: lack of standardization in the OM/zouk world making it hard to characterize how the differences manifest tonally. There are so many variables, it's hard to know what is responsible for what.

[Tavy]

As far as bending across or with the grain, have you ever seen any Tilton parlor guitars? They often had the top grain running diagonally.

What where they thinking of? Can't imagine what kind of bracing would be required to support diagonal grain!

[Jim Garber]

Does the direction of the grain really impact how the top is braced? I would think that if they did use x-bracing it wouldn't really matter -- or even ladder bracing. If the grain rain across the width then maybe ladder bracing would not work. I had a friend who had one of those -- I can check to see if he still has his and can see what the bracing would be like.

Some info on Tilton.

[Ed Goist]

It seems one of the "heroes" of induced archtop mandolin family instruments is Howe Orme.
* Howe-Orme Cylinder-Top Mandolin
* An Interview with Howe-Orme Guitar Collector Rick Turner
* 1890's Howe Orme
And a modern example of the style as posted here on the Cafe:
* NK Forster Guitars Mandolinetto

[Jim Garber]

I think those are not strictly induced arches -- I believe that the tops, esp the cylinder portion were steam-bent.

[Tavy]

Does the direction of the grain really impact how the top is braced? I would think that if they did use x-bracing it wouldn't really matter -- or even ladder bracing. If the grain rain across the width then maybe ladder bracing would not work. I had a friend who had one of those -- I can check to see if he still has his and can see what the bracing would be like.

It certainly should have an impact yes - particularly with ladder braced instruments you rely on the strength/stiffness along the grain to stop the whole thing from caving in

[Tavy]

It seems one of the "heroes" of induced archtop mandolin family instruments is Howe Orme.

Indeed, as soon as I first saw those I just thought "that's how it should be done" - not that I know anything

Here's my own modest effort in that direction.

I think those are not strictly induced arches -- I believe that the tops, esp the cylinder portion were steam-bent.

Not sure about that - there's certainly no actual need to use steam - you can get a pretty radical arch across the grain just with finger pressure and some shaped braces. Having said that, the Howe Ormes have a pretty sharp transition from the cylinder portion to the flat area, if you use a more subtle curve (pretty much the same profile as a carved top actually) then it bends real easy. For the record here's the profile I've been using:



7-8mm arch across the grain and about 2mm along the grain, sides obviously shaped to match. Top quite thick 3mm minimum, 4mm along the centre line. That's all for a mandolin though, so you'd have to scale up for larger instruments: I used a similar profile for my first build (a mandola) and a 7mm arch wasn't enough IMO.

And just for a laugh, here's how it sounds:

[nkforster]

I think those are not strictly induced arches -- I believe that the tops, esp the cylinder portion were steam-bent.

Cylinder tops are not steam bent. The strength of the top comes from the arch itself, so we can take the top considerably thinner than a standard instrument, so thin it easily conforms to a cylinder shaped former.

I've made many instruments in this style - mandolinetto, guitar and guitar bouzouki - its a superb way to build and very logical.



nigel

http://www.nkforsterguitars.com/nkfo.../the_book.html

[Bill Snyder]

But the top on the Howe-Orme mandolins tapered down at the neck and tail end so that they were no wider there than anywhere else on the sides.
The guitars did not.

[nkforster]

But the top on the Howe-Orme mandolins tapered down at the neck and tail end so that they were no wider there than anywhere else on the sides.
The guitars did not.

It all depends on wether the instrument has a floating bridge or a fixed bridge. With a floating bridge and tailpiece you make the top in an identical fashion to a fixed bridge(maybe make the arch in the cross brace near the tail a little shallower in the curved section) but the fitting is different - you pull it to ribs that are flat. You induce an arch. On a small instrument with a thin soundboard this is not as difficult as you may imagine.

For fixed bridge instruments like my guitars and guitar bouzouki's you fit the ribs to meet the curve of the soundboard. No tension is built in.

Both ways of working make very strong soundboards.

n

http://www.nkforsterguitars.com/nkfo.../the_book.html

[Bill Snyder]

I was not asking how YOU do it. I was simply commenting on the shape of the original.

[nkforster]

I was not asking how YOU do it. I was simply commenting on the shape of the original.

Well, they probably did it the same as me. Or rather I probably do it they same as they did. They main difference being I use an X brace rather than ladder braces.

http://www.nkforsterguitars.com/nkfo.../the_book.html

[Tavy]

Well, they probably did it the same as me. Or rather I probably do it they same as they did. They main difference being I use an X brace rather than ladder braces.

Nigel, if it's not a trade secret - what sort of arch height do you use on a guitar sized instrument?

[Bill Snyder]

Nigel that could be. I prepared a top a few years ago to make a Howe-Orme style mandolin and was told by two or three knowledgeable luthiers that they were pretty sure that they were steam bent. The bend is pretty dramatic on the Howe-Orme mandolins. Much more so than what yours appear to be in photos.

[Jim Garber]

Well, perhaps Nigel is correct. In the original patents I do not see any mention of the method of bending the top. I suppose it is still a matter of conjecture but I would think that the method would have been mentioned in the patent as well, certainly if they steambent the tops.

Then again it is possible that that mandolin tops might have been made differently as Bill notes.

[Bill Snyder]

Like I say, I don't know how they did it. I do know that Paul Hostetter and I believe Rick Turner have both stated that they think they were steam bent or pressed into shape.
Given their credentials and the fact they have both had several in hand I am inclined to think they might know what they are talking about.
Jim your posted photos really show the sharp transition of the hump very well.

[Jim Garber]

I was wondering who the luthiers were. I wonder what their reasoning for believing that is... it still sounds like we are just guessing how they were made and may never really know for sure.

[Bill Snyder]

So Ed, did we railroad your thread good enough?