Double Tops

bertie

Soupman said:

No, it's £2200 and made in china.
I think there are some good traditional guitars for that money, so I'll pass on the laminate top.....

Jon Gomm and George Lowden don't seem to mind      but that's a different thing to this..........................

anyway was just interested to know if anyone had any real world experience of playing them, and how they differ.  

Tannin

@bertie, I know you don't like reading long posts so in case you missed it, yes, I have played a double top in the real world (briefly in a shop a couple of years ago) and it made no great impression on me for good or ill. If I hadn't known it was a double top I wouldn't have noticed anything much other than that it seemed slightly odd, not in a bad way. Would I buy one? Sure - if I played it and loved it. Same as any other guitar really.

bertie

@Tannin ;

LOL,  thanks  -  I do tend to nod off after two paragraphs         

Always "sceptical" of anything "not solid"  - but after reading the Lowden/Jon Gomm  special hybrid laminate thingy a couple of years back,  its regained the "I wonder what/if"   status in me

sev112

LastMantra said:

Tannin said:

BillDL said:

Is this the next big thing to annoy "solid top purists"?  

No. Classical guitars have done it for decades, and it is sometimes done for steel strings. 

The principle has been known and used in other applications for a very long time. It is the standard and best way to make the lightest possible structure of a given strength. Buildings and bridges and structural members of aeroplane wings all use the same basic idea, which starts from the knowledge that nearly all the stress on a beam is concentrated on the outer layers. Structures of various other shapes work the same way. If you have, for example, a 6 x 2 flooring beam and put a piano above it, the top few millimetres of the beam are heavily loaded in compression (which is some help but not much because wood is fairly weak in compression) and the bottom few millimetres are heavily loaded in tension. The wood in the 150-odd  mm area in between the top and the bottom doesn't do anything much other than hold the two working sections apart.

This is why skyscrapers are made using I beams: an I beam has lots of material at the top and the bottom where it is effective, and as little as possible between. But the same principle applies to other forms too, including flat sheets.

Racing dingies (small, very fast yachts) are routinely made using sandwich construction. When I was a growing up (many years ago now!) they were made from  the lightest possible woods, mostly Western Red Cedar (the same cedar we use for guitar tops). Ply was preferred for most tasks. It can be stronger for a given weight because you can use thin layers of a strong timber on the outside and a thicker middle layer of something softer and usually cheaper, and also because it's much better on the cross-grain. Everyday yachts were often made from fibreglass but fibreglass of the required strength is heavier than timber, so racing yachts always used wood. About this time  the smart builders figured out that they could make something lighter and stronger by having a thin layer of wood coated with two very thin layers of fibreglass. The timber filler is loaded vertically in compression (the top and the bottom "want" to squeeze closer together; the task of the filler is to resist that) and timber is stronger when compressed end-on rather than sideways, so the next step was to use end-grain timber. What was the lightest possible timber strong enough to hold the outer sheets apart when used in end-grain? It turned out to be Balsa, and fibreglass over end-grain Balsa became the standard racing yacht construction material. As time went by refinements were made: carbon fibre and Kevlar replaced glass fibre; the end-grain Balsa was replaced by expanded polystyrene foam. 

To guitars. If a light top is desired, wouldn't it make sense to do the same sort of thing? Couldn't we make a top just as strong but lighter and more responsive by using a sandwich? Yes indeed. It gets more complicated than a yacht because we don't just want strength and light weight, we also care about the vibrational qualities of the top. So the top and bottom layers are made of a suitable tonewood such as spruce, while the filler is carefully chosen to have good acoustic properties. Oh, and it is important to use as little glue as possible while still getting a good bond. 

Surely with an I beam it's the bit in the middle, the vertical bit, that gives it strength? The top and bottom bits are basically there to keep the middle bit in place and stop it twisting or bending in the other direction. 

A combination of both - the middle bit doesn’t need much actual strength but it is an important contribution. 

an I Beam (used as a Beam and not as a Column, where we use Universal Columns instead but which look like I Beams) consists of a web and 2 flanges (the end bits).  In many ways the web is purely there to provide as big as poss a distance between the 2 flanges, because it is that distance that creates the strength in the beam by converting the applied bending “moment” into low stress in the flanges by virtue of the lever arm of that distance.
but there are also needs for the web to have strength for example for the beam to resist shear (typically vertical) forces and to resist torsion

the majority of the mass of the web is too close to the neutral axis (see below) of the beam to build up any significant forces in-plane.  At the neutral axis itself there is zero stress

so optimal beam design is to optimise the distance between the 2 flanges and their thickness/width so that the stresses therein are kept within material failure stress, and which have as light a weight as possible.

we often see the use of composite beams, which only have one real flange made of concrete connected to a “deep” flange of steel.  The concrete takes the bending (moment) in/by compression which it is very good at.  And just the web is often enough to take the tension stresses induced the other side of the neural axis (the point where on one side it’s in tension and the other in compression)

this then gets more fun when we talk about a “plate” or “diaphragm” where we have the 3rd dimension to consider too and its contribution to resisting loads 

Tannin

Ahh, there is a proper engineer in the house!  

So is this the time to ask about lattice beams?  As I understand it, they don't work like plain beams or I beams or even I beams with holes for lightness, but are a bit of a law unto themselves. 

(OK, I'm a million miles off topic now, but I've already had my "be nice to @bertie" moment for the day, so why not?)

LastMantra

sev112 said:

LastMantra said:

Tannin said:

BillDL said:

Is this the next big thing to annoy "solid top purists"?  

No. Classical guitars have done it for decades, and it is sometimes done for steel strings. 

The principle has been known and used in other applications for a very long time. It is the standard and best way to make the lightest possible structure of a given strength. Buildings and bridges and structural members of aeroplane wings all use the same basic idea, which starts from the knowledge that nearly all the stress on a beam is concentrated on the outer layers. Structures of various other shapes work the same way. If you have, for example, a 6 x 2 flooring beam and put a piano above it, the top few millimetres of the beam are heavily loaded in compression (which is some help but not much because wood is fairly weak in compression) and the bottom few millimetres are heavily loaded in tension. The wood in the 150-odd  mm area in between the top and the bottom doesn't do anything much other than hold the two working sections apart.

This is why skyscrapers are made using I beams: an I beam has lots of material at the top and the bottom where it is effective, and as little as possible between. But the same principle applies to other forms too, including flat sheets.

Racing dingies (small, very fast yachts) are routinely made using sandwich construction. When I was a growing up (many years ago now!) they were made from  the lightest possible woods, mostly Western Red Cedar (the same cedar we use for guitar tops). Ply was preferred for most tasks. It can be stronger for a given weight because you can use thin layers of a strong timber on the outside and a thicker middle layer of something softer and usually cheaper, and also because it's much better on the cross-grain. Everyday yachts were often made from fibreglass but fibreglass of the required strength is heavier than timber, so racing yachts always used wood. About this time  the smart builders figured out that they could make something lighter and stronger by having a thin layer of wood coated with two very thin layers of fibreglass. The timber filler is loaded vertically in compression (the top and the bottom "want" to squeeze closer together; the task of the filler is to resist that) and timber is stronger when compressed end-on rather than sideways, so the next step was to use end-grain timber. What was the lightest possible timber strong enough to hold the outer sheets apart when used in end-grain? It turned out to be Balsa, and fibreglass over end-grain Balsa became the standard racing yacht construction material. As time went by refinements were made: carbon fibre and Kevlar replaced glass fibre; the end-grain Balsa was replaced by expanded polystyrene foam. 

To guitars. If a light top is desired, wouldn't it make sense to do the same sort of thing? Couldn't we make a top just as strong but lighter and more responsive by using a sandwich? Yes indeed. It gets more complicated than a yacht because we don't just want strength and light weight, we also care about the vibrational qualities of the top. So the top and bottom layers are made of a suitable tonewood such as spruce, while the filler is carefully chosen to have good acoustic properties. Oh, and it is important to use as little glue as possible while still getting a good bond. 

Surely with an I beam it's the bit in the middle, the vertical bit, that gives it strength? The top and bottom bits are basically there to keep the middle bit in place and stop it twisting or bending in the other direction. 

A combination of both - the middle bit doesn’t need much actual strength but it is an important contribution. 

an I Beam (used as a Beam and not as a Column, where we use Universal Columns instead but which look like I Beams) consists of a web and 2 flanges (the end bits).  In many ways the web is purely there to provide as big as poss a distance between the 2 flanges, because it is that distance that creates the strength in the beam by converting the applied bending “moment” into low stress in the flanges by virtue of the lever arm of that distance.
but there are also needs for the web to have strength for example for the beam to resist shear (typically vertical) forces and to resist torsion

the majority of the mass of the web is too close to the neutral axis (see below) of the beam to build up any significant forces in-plane.  At the neutral axis itself there is zero stress

so optimal beam design is to optimise the distance between the 2 flanges and their thickness/width so that the stresses therein are kept within material failure stress, and which have as light a weight as possible.

we often see the use of composite beams, which only have one real flange made of concrete connected to a “deep” flange of steel.  The concrete takes the bending (moment) in/by compression which it is very good at.  And just the web is often enough to take the tension stresses induced the other side of the neural axis (the point where on one side it’s in tension and the other in compression)

this then gets more fun when we talk about a “plate” or “diaphragm” where we have the 3rd dimension to consider too and its contribution to resisting loads 

OK, ta. I was just thinking of a ruler bends easily one way but is almost impossible to bend the other way.
Assuming, as you say, a beam where the load is pulled by gravity, as in downward. Like a wooden beam is deeper than it is wide. 
Guess it's not that simple.