Double Top Constr: Translating it into Lay Language

[BKENNA]

Double Top Construction: Translating it into Lay Language

Dear Luthier, Builder Tec, Dealer, Knowledgeable Guitar person~

I have read some things that essentially talked about “Double Top” construction and I have encountered a couple short essays on the subject but I have not yet found it explained in “Regular / Street folks” language.

If anyone can do that many thanks to you!

I think many of us would appreciate getting up to speed on understanding what Double Top Construction is, it’s History and its argued Good and Bad attributes?

WOW! Thank you everyone that has / is / does/ & will yet contribute. This post has been fantastic. I am elated with the really high quality, thought out responses. Thanks a million everyone, All the responses are educating and eye opening!


Thanks

Cheers,

B.

[RustyZombie]

A double top is two thin pieces of wood sandwiched around an artificial material called nomex. The nomex gives the top greater strength, allowing for thinner tops and less bracing. This results in a more resonant and what some feel is a better sounding top. The wood in the top still allows it to keep its "woody" tone, though it will have some tonal differences from a traditional solid top.

[billgennaro]

This is an excerpt from Ervin Somogyi's great book "The Responsive Guitar":

"Double tops are based on the I-Beam principle in that the load-bearing work of any structural beam is done by its upper and lower layers only, and that the material in between (the “neutral axis”) exists merely to connect the stress-bearing top and bottom sections. A double top consists of two thin skins of spruce separated by a middle layer of balsa wood or very lightweight space-age honeycombed plastic. Such faces are very lightweight for their stiffness. The upside is that one obtains superior stiffness to weight material. The downside is that there’s no room whatever for sanding, shaping, voicing, thicknessing, or error in making the plate. What you initially make is what you get because you can’t shave, scrape, sand, tinker with, taper, or even easily repair a top once its made in this fashion."

Hope this helps.

[charles Tauber]

Originally Posted by billgennaro

the load-bearing work of any structural beam is done by its upper and lower layers only...

This statement is a simplification and not entirely true.


Consider a piece of steel bar that is square in cross section. Support it at its two ends and then hang a weight from the middle. The steel "beam" will sag (bend/deflect) in the middle under the weight.

Consider a piece of steel that is the of the same size as the one above, but it is hollow - i.e. a square tube. Support it at its two ends and then hang the same weight from its middle. The hollow beam will sag only very slightly more than the solid beam, but the hollow beam will weigh a fraction of the amount of the solid one. Its strength/stiffness to weight ratio is much higher - it weighs much less, but deflects only slightly more than the solid beam.

What this illustrates is that most - not all - of the resistance to deflection is a result of having material towards the extremities of the beam's shape - upper and lower surfaces. If one wants to reduce weight while keeping the resistance to bending/deflection nearly the same, one way to do it is to put most of the material towards the extremities. One example of that is the "I" beam. The illustration below shows the distribution of stresses in the I beam, going from zero at the "center" of the beam - the neutral axis - to a maximum at the extremities of the beam. By putting the material where the stresses are highest, one maximizes the strength/stiffness to weight ratio.



"Response" in a guitar is a function of weight (mass) and stiffness. If one desires to make a guitar top that is as light as possible (minimum mass) while making it as stiff as possible, placing the material where it is needed - towards the inside and outside surfaces of the guitar top - best accomplishes that. Filling the space in between, creating a three-layer top, with a lightweight material in the middle layer keeps the mass down when compared to a single piece of wood that is the same density throughout its cross section. That's the theory. In practice, opinions vary on how successful is the resulting sound (i.e. whether or not people like the sound produced).

[ljguitar]

Originally Posted by BlackmoresNight

A double top is two thin pieces of wood sandwiched around an artificial material called nomex. The nomex gives the top greater strength, allowing for thinner tops and less bracing. This results in a more resonant and what some feel is a better sounding top. The wood in the top still allows it to keep its "woody" tone, though it will have some tonal differences from a traditional solid top.

Hi BN...

I believe all the double-top acoustic 6 string models I've played have all been two pieces of thin wood glued one on each side of Nomex honeycomb core/bracing made of a paper derivative. Perhaps the paper in the honeycomb is comprised of synthetic fibers (paper can be made from cotton, wood pulp, plant fiber and perhaps even synthetic fiber.)

I may be wrong, but that is what I recall from chatting with three different double top builders (Charles Fox, Paul Woolson, Tim McKnight). It would be interesting & helpful if a luthier were to wander into this discussion and lend some info from a professional point of view.

I played the McKnight Diamond double top, double back (and perhaps doubled sides as well) and the guitar had the quickest responsiveness I've ever experienced, and the most explosive dynamics I've ever encountered. It was like playing a family car with a racing motor in it. Zero to full-bore in a split second.

Once I adjusted to the potential of that kind of responsiveness, it was a lot of fun to play. The dynamic range was incredible. Actually I loved everything except the double back (which is different from either a doubled top or doubles sides…which are also different from each other).

[RustyZombie]

Originally Posted by ljguitar

Hi BN...

I believe all the double-top acoustic 6 string models I've played have all been two pieces of thin wood glued one on each side of Nomex honeycomb core/bracing made of a paper derivative. Perhaps the paper in the honeycomb is comprised of synthetic fibers (paper can be made from cotton, wood pulp, plant fiber and perhaps even synthetic fiber.)

I may be wrong, but that is what I recall from chatting with three different double top builders (Charles Fox, Paul Woolson, Tim McKnight). It would be interesting & helpful if a luthier were to wander into this discussion and lend some info from a professional point of view.

I played the McKnight Diamond double top, double back (and perhaps doubled sides as well) and the guitar had the quickest responsiveness I've ever experienced, and the most explosive dynamics I've ever encountered. It was like playing a family car with a racing motor in it. Zero to full-bore in a split second.

Once I adjusted to the potential of that kind of responsiveness, it was a lot of fun to play. The dynamic range was incredible. Actually I loved everything except the double back (which is different from either a doubled top or doubles sides…which are also different from each other).

I read your post a few times, because it sounds like you are saying something different than me. But I think we are saying the same thing in regards to the double top.

Is the double back you are referring to is where the outer back is stationary and the inner back is what acts as a reflector?

[BKENNA]

Wow, thanks everyone I really, rally appreciate the sharing.

Cheers,


B

[Tom West]

Originally Posted by charles Tauber

What this illustrates is that most - not all - of the resistance to deflection is a result of having material towards the extremities of the beam's shape - upper and lower surfaces. If one wants to reduce weight while keeping the resistance to bending/deflection nearly the same, one way to do it is to put most of the material towards the extremities. One example of that is the "I" beam. The illustration below shows the distribution of stresses in the I beam, going from zero at the "center" of the beam - the neutral axis - to a maximum at the extremities of the beam. By putting the material where the stresses are highest, one maximizes the strength/stiffness to weight ratio.

Charles: The distribution of tension and compression is shown as a linear function. Not so sure this is true for an I beam. My strength of materials classes were 50 years ago...........!!! So I may be dead wrong.
Tom

[ljguitar]

Originally Posted by BlackmoresNight

I read your post a few times, because it sounds like you are saying something different than me. But I think we are saying the same thing in regards to the double top.

Is the double back you are referring to is where the outer back is stationary and the inner back is what acts as a reflector?

Hi BN...

Tim McKnight's double back is a back which vibrates sympathetically housed inside the body and spaced from the outside back. The inside back is still free to interact with the top and is not dampened even if the guitar is held against the player's chest/stomach. (I hope that was clear)

I'm not sure there is only a single way of building doubled tops. If I have my chronology correct, they started in the classical guitar realm (as did side ports), and have migrated to the acoustic 6 string world.

I've even seen mention of experimental double topped instruments which were built without internal bracing, but internal bracing is used not only for strength but for directing sound to various parts of the top.

I think repairing double topped guitars could be a challenge. That sandwich has really really thin exterior sheets.

[RustyZombie]

Originally Posted by ljguitar

Hi BN...

Tim McKnight's double back is a back which vibrates sympathetically housed inside the body and spaced from the outside back. The inside back is still free to interact with the top and is not dampened even if the guitar is held against the player's chest/stomach. (I hope that was clear)

I'm not sure there is only a single way of building doubled tops. If I have my chronology correct, they started in the classical guitar realm (as did side ports), and have migrated to the acoustic 6 string world.

I've even seen mention of experimental double topped instruments which were built without internal bracing, but internal bracing is used not only for strength but for directing sound to various parts of the top.

I think repairing double topped guitars could be a challenge. That sandwich has really really thin exterior sheets.

That is what I thought the double back was, thank you.

[ljguitar]

Originally Posted by BlackmoresNight

That is what I thought the double back was, thank you.

Hi BN...

That is what Tim McKnight's double back is.

[rick-slo]

A few earlier threads on the topic to read:

http://www.acousticguitarforum.com/f...d.php?t=154859

http://www.acousticguitarforum.com/f...d.php?t=334421

http://www.acousticguitarforum.com/f...d.php?t=168233

[softballbryan]

I want one with a slice of red cedar and engelman spruce please.

[Tim McKnight]

Just a quick correction ... we call ours a "Hollow Back" not a double back. Larry explained how it works quite well so no need to repeat that (thanks Larry).

Double tops are a different animal to build. The goal is to save as much weight as possible so you have to be VERY careful how much adhesive you use when gluing the sandwich together. You don't apply the adhesive to each plate or you defeat the weight reduction goal and add too much weight and mass. You have to apply glue to just the Nomex and that is where the difficulty begins. The gluing surface of Nomex is the same width as the thickness of the edge of a sheet of copier paper. Have you tried to glue the edge of a piece of paper lately? Good luck.

[billgennaro]

Originally Posted by charles Tauber

This statement is a simplification and not entirely true.


Consider a piece of steel bar that is square in cross section. Support it at its two ends and then hang a weight from the middle. The steel "beam" will sag (bend/deflect) in the middle under the weight.

Consider a piece of steel that is the of the same size as the one above, but it is hollow - i.e. a square tube. Support it at its two ends and then hang the same weight from its middle. The hollow beam will sag only very slightly more than the solid beam, but the hollow beam will weigh a fraction of the amount of the solid one. Its strength/stiffness to weight ratio is much higher - it weighs much less, but deflects only slightly more than the solid beam.

What this illustrates is that most - not all - of the resistance to deflection is a result of having material towards the extremities of the beam's shape - upper and lower surfaces. If one wants to reduce weight while keeping the resistance to bending/deflection nearly the same, one way to do it is to put most of the material towards the extremities. One example of that is the "I" beam. The illustration below shows the distribution of stresses in the I beam, going from zero at the "center" of the beam - the neutral axis - to a maximum at the extremities of the beam. By putting the material where the stresses are highest, one maximizes the strength/stiffness to weight ratio.



"Response" in a guitar is a function of weight (mass) and stiffness. If one desires to make a guitar top that is as light as possible (minimum mass) while making it as stiff as possible, placing the material where it is needed - towards the inside and outside surfaces of the guitar top - best accomplishes that. Filling the space in between, creating a three-layer top, with a lightweight material in the middle layer keeps the mass down when compared to a single piece of wood that is the same density throughout its cross section. That's the theory. In practice, opinions vary on how successful is the resulting sound (i.e. whether or not people like the sound produced).

Of course, the OP did ask for "layman's language".