Double Top Guitars

[Tim McKnight]

I have build 30+ doubletops, all steel strings, so here is my take, from a builder's perspective:

- Doubletops, regardless of the woods or combinations thereof used, have the widest dynamic range of any top, period. Many players don't need this wide dynamic range so they certainly won't appeal to everyone.

- Doubletops, by design, are inherently stiff. Therefore they are more difficult, but not impossible, to get bass out of.

- Double tops [CAN BE] much lighter than a solid top and therefore takes less energy to set in motion. They [can] have the responsiveness of a very light top like Cedar for instance. Yet they [can be] extremely stiff both along and cross grain which [can] increase the amount of headroom and volume if driven accordingly.

- They are VERY difficult to construct, the correct way. You are working with two thin plates ~.030" thick and trying to glue Nomex to each plate. Keep in mind that the gluing surface of the Nomex is like trying to apply glue the edge of a single sheet of notebook paper. Herein lies the difficulty. If one applies too much glue it defeats the entire purpose of building a super light weight top structure. If one applies too little glue then it will literally fall apart. On top of that imagine trying to install a rosette & purflings in a .030" thick piece of wood.There is just no room for error.

- Doubletops seem to have unlimited headroom without sacrificing tonal distortion.

- If two dissimilar woods are used, which ever wood the bridge is glued to has the most effect on the overall tone of the top. The tone is a slight hybrid but I would say you will hear 85% of the outer top and 15% of the inner top.

Keep in mind that these comments are limited to the guitars we build and do not apply to other builders results.

Here is more information on our double tops if you care to read it?

[rick-slo]

Hi Tim. Thanks for the details. Construction does sound very tricky. Loved playing your roadtrip guitar.

[jrankin]

Originally Posted by Tim McKnight

I have build 30+ doubletops, all steel strings, so here is my take, from a builder's perspective:

- Doubletops, regardless of the woods or combinations thereof used, have the widest dynamic range of any top, period. Many players don't need this wide dynamic range so they certainly won't appeal to everyone.

- Doubletops, by design, are inherently stiff. Therefore they are more difficult, but not impossible, to get bass out of.

- Double tops [CAN BE] much lighter than a solid top and therefore takes less energy to set in motion. They [can] have the responsiveness of a very light top like Cedar for instance. Yet they [can be] extremely stiff both along and cross grain which [can] increase the amount of headroom and volume if driven accordingly.

- They are VERY difficult to construct, the correct way. You are working with two thin plates ~.030" thick and trying to glue Nomex to each plate. Keep in mind that the gluing surface of the Nomex is like trying to apply glue the edge of a single sheet of notebook paper. Herein lies the difficulty. If one applies too much glue it defeats the entire purpose of building a super light weight top structure. If one applies too little glue then it will literally fall apart. On top of that imagine trying to install a rosette & purflings in a .030" thick piece of wood.There is just no room for error.

- Doubletops seem to have unlimited headroom without sacrificing tonal distortion.

- If two dissimilar woods are used, which ever wood the bridge is glued to has the most effect on the overall tone of the top. The tone is a slight hybrid but I would say you will hear 85% of the outer top and 15% of the inner top.

Keep in mind that these comments are limited to the guitars we build and do not apply to other builders results.

Here is more information on our double tops if you care to read it?

Thank you, why Nomex?

[Tim McKnight]

Originally Posted by rick-slo

Hi Tim. Thanks for the details. Construction does sound very tricky. Loved playing your roadtrip guitar.

Thanks Rick. We will have to do it again some day.

It was a VERY expensive learning curve, for me, to learn how to build these things properly. I have lost count how many tops I've had to scrap in the process.

BTW, I have played a couple other DTs that did not use braces and I found them far less interesting than DTs with braces. IMO you need braces, however quite minimal, to distribute vibrations and energy across the face of the top(s).

[Tim McKnight]

Originally Posted by jrankin

Thank you, why Nomex?

The technology was borrowed from aviation. It was originally used in the construction of airplane wings and fuselage parts. Its the material of choice because its super light weight, much lighter than Kevlar. Its basically an Aramid synthetic paper product. We (builders) use a similar but slightly different material than is used in the aviation industry. Nomex, by itself, is quite weak with the strength of a limp rag. However, after its layed up in a three ply sandwich it takes on the form of an I-beam which is a proven structural member used for its strength of design.

[Sage97]

I'm not a luthier but Paul Woolson built me a double topped guitar a few years back. I agree with Tim's comment about dynamic range. My Woolson responds very well to various attacks from light fingerstyle to heavy strumming. You'd have to really drive it to get some distortion.

[ljguitar]

Hi all...

I've played Paul Woolson's double topped guitar on the road trip, and I've played several of Tim McKnight's - the most notable of all of them was the Diamond.

That guitar was simply explosive dynamically. You could go from ppp-to-fff (softest soft to loudest loud) instantly with just a firm pluck of the strings, and then it still seemed to have headroom. It was pleasantly disconcerting to play it at first till I learned to adapt my style to it.

It was really a lot like driving a racing car in town. All that power under the hood to drive city streets.

[alembic1989]

Originally Posted by Tim McKnight

I have build 30+ doubletops, all steel strings, so here is my take, from a builder's perspective:

- Doubletops, regardless of the woods or combinations thereof used, have the widest dynamic range of any top, period. Many players don't need this wide dynamic range so they certainly won't appeal to everyone.

- Doubletops, by design, are inherently stiff. Therefore they are more difficult, but not impossible, to get bass out of.

- Double tops [CAN BE] much lighter than a solid top and therefore takes less energy to set in motion. They [can] have the responsiveness of a very light top like Cedar for instance. Yet they [can be] extremely stiff both along and cross grain which [can] increase the amount of headroom and volume if driven accordingly.

- They are VERY difficult to construct, the correct way. You are working with two thin plates ~.030" thick and trying to glue Nomex to each plate. Keep in mind that the gluing surface of the Nomex is like trying to apply glue the edge of a single sheet of notebook paper. Herein lies the difficulty. If one applies too much glue it defeats the entire purpose of building a super light weight top structure. If one applies too little glue then it will literally fall apart. On top of that imagine trying to install a rosette & purflings in a .030" thick piece of wood.There is just no room for error.

- Doubletops seem to have unlimited headroom without sacrificing tonal distortion.

- If two dissimilar woods are used, which ever wood the bridge is glued to has the most effect on the overall tone of the top. The tone is a slight hybrid but I would say you will hear 85% of the outer top and 15% of the inner top.

Keep in mind that these comments are limited to the guitars we build and do not apply to other builders results.

Here is more information on our double tops if you care to read it?

Thank you so much for your very informative reply. I believe you have answered all my question..I will follow your link..a read some more.

[alembic1989]

I wonder if " double top technology " could be applied to archtop design...its out there now.....so someone should give it a go!

[ljguitar]

Here are some videos of the Diamond double top (double back) guitar Tim McKnight built...








And a friend of mine stopped by and played a piece on the Woolson LG double top/double back that was on the road tour.

This was just a quick grab in the living room for me to practice with (he and I were playing a duet at our guitar society the following week)...maybe with a Kodak Video camera. John is 6'3'' tall, and plays a Jumbo taylor, so this guitar looks so small on his lap - yet is was a powerhouse.

[Jim.S]

Originally Posted by Tim McKnight

- Doubletops, by design, are inherently stiff. Therefore they are more difficult, but not impossible, to get bass out of.

Hi Tim, when you say "inherently stiff" do you just simply mean that their stiffness to weigh ratio is better, rather than that they are stiffer than say the same bit of spruce without the nomex in the guts?

Like, if you take a bit of spruce say 3mm thick with E long of 10Gpa then slice 1.5mm out of the guts and replace it with 1.5mm of nomex (doing a good job of it) shouldn't it just remain at 10Gpa E long and become lighter? Or maybe even drop a tad on the 10Gpa but still gain in stiffness to weight?

Jim

[MJRB]

Originally Posted by Tim McKnight

I have build 30+ doubletops, all steel strings, so here is my take, from a builder's perspective:

- Doubletops, regardless of the woods or combinations thereof used, have the widest dynamic range of any top, period. Many players don't need this wide dynamic range so they certainly won't appeal to everyone.

- Doubletops, by design, are inherently stiff. Therefore they are more difficult, but not impossible, to get bass out of.

- Double tops [CAN BE] much lighter than a solid top and therefore takes less energy to set in motion. They [can] have the responsiveness of a very light top like Cedar for instance. Yet they [can be] extremely stiff both along and cross grain which [can] increase the amount of headroom and volume if driven accordingly.

- They are VERY difficult to construct, the correct way. You are working with two thin plates ~.030" thick and trying to glue Nomex to each plate. Keep in mind that the gluing surface of the Nomex is like trying to apply glue the edge of a single sheet of notebook paper. Herein lies the difficulty. If one applies too much glue it defeats the entire purpose of building a super light weight top structure. If one applies too little glue then it will literally fall apart. On top of that imagine trying to install a rosette & purflings in a .030" thick piece of wood.There is just no room for error.

- Doubletops seem to have unlimited headroom without sacrificing tonal distortion.

- If two dissimilar woods are used, which ever wood the bridge is glued to has the most effect on the overall tone of the top. The tone is a slight hybrid but I would say you will hear 85% of the outer top and 15% of the inner top.

Keep in mind that these comments are limited to the guitars we build and do not apply to other builders results.

Here is more information on our double tops if you care to read it?

Thanks Tim, very interesting and informative.
MJRB

[Tim McKnight]

Originally Posted by Jim.S

Hi Tim, when you say "inherently stiff" do you just simply mean that their stiffness to weigh ratio is better, rather than that they are stiffer than say the same bit of spruce without the nomex in the guts?

Like, if you take a bit of spruce say 3mm thick with E long of 10Gpa then slice 1.5mm out of the guts and replace it with 1.5mm of nomex (doing a good job of it) shouldn't it just remain at 10Gpa E long and become lighter? Or maybe even drop a tad on the 10Gpa but still gain in stiffness to weight?

Jim

Logically it sounds like it should remain the same stiffness but it does not because you are replacing the "guts" with a material that is MUCH stiffer & lighter than the material which was removed. Because the finished sandwich top is lighter and stiffer than a solid top it takes much less brace mass to brace the top with. This also contributes both to the mass and weight reduction of the top "system".

[Jim.S]

Thanks Tim, so the best I can think that through is that once the nomex is glued between the wood it then gains stiffness itself, cause it doesn't look like it has much stiffness on its own. Or are you saying that say 1.5mm of nomex on its own is stiffer than 1.5mm of spruce?

Jim

[Tim McKnight]

Originally Posted by Jim.S

Thanks Tim, so the best I can think that through is that once the nomex is glued between the wood it then gains stiffness itself, cause it doesn't look like it has much stiffness on its own. Or are you saying that say 1.5mm of nomex on its own is stiffer than 1.5mm of spruce?

Jim

Nomex by itself is not stiff at all. The Doubletop gains its stiffness after the Nomex is glued to the inner and outer skins. Nomex is not stiffer than spruce if comparing the two different materials. The spruce is stiffer than the Nomex when they are compared before they are joined.

Imagine if you had two playing cards, you held one between both hands, you are able to flex them easily, right? The playing cards would be an example of the inner and outer spruce skins. Now imagine that you separated the two playing cards by a certain distance, say .060" and glued a perpendicular member (the Nomex) between the two cards. You have created an I-Beam shape with the three separate pieces. The I-Beam shape is what gives the three "joined" pieces it's unique stiffness.

I have glued Nomex to only one piece of spruce and it is extremely weak, floppy and not stiff at all. Its not any stiffer than the single piece of spruce. After the second pieces of spruce is glued to the other side of the Nomex, that is when the sandwich gains is stiffness.

A sheet of Nomex is made up of unique hexagonal shaped cells and you can purchase Nomex with different sized cells, ie 3/16", 1/8", 5/16", 1/4", etc... You can also purchase the sheets in various thickness as well as the thikness of the Nomex itself as well as a plethora of coatings. All of these factors affect overall weight and stiffness of the Nomex itself. As the thickness of the Nomex increases so does the stiffness of the completed sandwich based on the cube root rule. Therefore the overall stiffness of the sandwich is controlled by the thickness of the Nomex, the cell size, the thickness of the Nomex walls, the Nomex coating, the adhesive and the amount of adhesive used as well as the way that one graduates the Nomex before gluing the second piece of spruce to it.

Traditionally we adjust the stiffness of the top plate by graduating it. One is unable to do this on a Double Top because a .030" piece of spruce does not have enough material to graduate. Add to that if one dings the top, during construction there is no way to sand that ding out. Its a mind boggling and tedious process to say the least.