Thoughts about non uniform thicknessing of the top

[Henning]

Hello, I just wonder: if you don't make the top evenly thick, how will that affect the tone?
For instance, suppose that the area around the cross in the X-brace is ~0,5 mm thicker then the rest of the top. I guess that part of the guitar top doesn't contribute that much to the sound production, or does it?
Just my thoughts. Please
Regards

[BlueBowman]

My understanding is that the entire top functions as one unit. On steel string guitars, some makers thin the edges of the lower bout (Bourgeois, I think), but I've also read Allen Carruth and Ervin Somogyi saying that it makes more sense to thin the interior part of the lower bout, and there's one maker who does that (though I can't remember his name).

Here's a great video on thinning a soundboard for a classical guitar. I would love to find a similar video explaining in depth how to thin a steel string top.

https://www.youtube.com/watch?v=woQ3O9sKvBk

[Mandobart]

With archtop instruments the top (and back) are graduated to optimize structural strength and tone.

[Alan Carruth]

I used to string guitars up 'in the white' so that I could work on the top thickness to get the sound the way I wanted. I spent a lot of time thinning them out below the bridge to get the bass to come up, so I started to graduate the thickness at the start, when it's easier.

A simplified view of the function of the top is that the upper end is all 'structural', and the lower end 'acoustic'. The neck puts a lot of load on the top, the there's usually a lot of bracing there, so it's not really very free to vibrate much above the waist. The load on the lower edge is almost all straight tension, with some upward vector due to bridge torque, and those forces are spread out, so the wood doesn't have to be all that thick over the lower end block.

In many ways, it seems to me that the critical area of the top is the spot between the front of the bridge and the bottom of the sound hole. The top can belly up some behind the bridge without much detriment, but load between the bridge and the hole is compression along the top, and torque pushing it inward, so it wants to buckle in that spot. Once it does it seems to me that you lose the sound. I don't think it's any accident that the two most successful and widely copied brace patterns; 'X' bracing on steel strings and 'fan' bracing on Classicals, both concentrate wood there.

So when I decide on the thickness the top 'should' be it's the thickness of that spot between the bridge and the hole. On a steel string I make it .010" or so thicker at the upper edge, and .010" thinner at the lower edge, and a straight taper in between. This helps to bring up the bass, IMO. On Classicals, where the problem is getting good trebles, I make it to the target thickness all along the center line, and taper it thinner in the 'wings'. It seems to work for me.

Arch tops get a lot of their stiffness from the arch shape, and different folks use different shapes. Some of them, such as the so-called 'high' arch that Orville Gibson used seem to work better when they're thicker in the center and thin at the edges. An arch some makers now use, based on the late work of Niccolo Amati and Antonio Stradivari, (the 'curtate cycloid') seems to work well with a uniform top thickness, or even a bit thinner in the very center. I've made them both ways, and they work, although the sound tends to be different.

However you do the thickness you have to get the bracing 'right' for the top. Although 'good' bracing won't rescue a bad top, bad bracing can ruin a good one. " A cup of soup in the garbage is garbage; a cup of garbage in the soup is... garbage"

[nickv6]

Thank you for that interesting and articulate reply

[redir]

If you draw a line across the bridge to each side of the guitar I start there and taper the perimeter in about 2in all around the lower bout thinning more as I approach the tail block and tapping the top where the bridge is listening for it to loosen up. I've never put numbers to it but I'd say it's maybe 20% thinner on average.

[nikpearson]

For classical guitars I start at 2.3 x 2.5 mm at the thickest point and graduate towards the rims, 1.8 - 2.0 mm. The upper bout is left at full thickness for structural reasons; I also employ a significant extension under the fingerboard as part of my Spanish heel construction so this section will already be relatively stiff.

My standard fan brace is very light, similar to many Hauser and Torres guitars. With more substantial bracing I’d go with a thinner top.

I may be wrong but my experience suggests that the sound quality is mainly influenced by the lower bout. A bridge patch is also used for reinforcement in this area.

This is one of the luthier I’ve jobs I enjoy the most. Working with 102 & 101 block planes to graduate the top prior to bracing.

My experience with steel-string guitars is limited, but again I use a similar graduation but with a little more thickness, probably 0.2 - 0.3 mm.

A few years ago Taylor introduced a relief route near to the edge of the soundboard; a production version of graduating. In general these Taylor’s sound more responsive to me.

[Alan Carruth]

Which just goes to show that there are lots of ways to make a good guitar.

[runamuck]

I've played several of Bruce Sexauer"s guitars and they are some of the best I've ever heard. I believe he makes his tops a uniform thickness.

[koolimy]

Originally Posted by BlueBowman

My understanding is that the entire top functions as one unit. On steel string guitars, some makers thin the edges of the lower bout (Bourgeois, I think), but I've also read Allen Carruth and Ervin Somogyi saying that it makes more sense to thin the interior part of the lower bout, and there's one maker who does that (though I can't remember his name).

Here's a great video on thinning a soundboard for a classical guitar. I would love to find a similar video explaining in depth how to thin a steel string top.

https://www.youtube.com/watch?v=woQ3O9sKvBk

This is an amazing video! Thanks!

[Fathand]

Originally Posted by BlueBowman

I would love to find a similar video explaining in depth how to thin a steel string top.

https://www.youtube.com/watch?v=woQ3O9sKvBk

Try this one?
https://youtu.be/JHs7s1_pbAg?si=xV-9eRWgETZiajPe

[Henning]

Originally Posted by Alan Carruth

So when I decide on the thickness the top 'should' be it's the thickness of that spot between the bridge and the hole. On a steel string I make it .010" or so thicker at the upper edge, and .010" thinner at the lower edge, and a straight taper in between. This helps to bring up the bass, IMO. On Classicals, where the problem is getting good trebles, I make it to the target thickness all along the center line, and taper it thinner in the 'wings'. It seems to work for me.

The 'wings' of a classical guitar, do you please mean the area of the lower bout where it is the widest, or something else?
( I'd rather be reasonably certain before I bring out the planer or sandpaper. )
Do you see any risk for any side effects, wished or unwished for?
Like for instance a shorter sustain, please?

[Alan Carruth]

Yes, the 'wings' of the top are the area between the ends of the bridge and edge of the lower bout. Think of a really shallow roof, with the center line of the top being the peak and tapering down on either side.

"Do you see any risk for any side effects, wished or unwished for?
Like for instance a shorter sustain, please?"

If anything, as compared with a top of uniform thickness, this sort of taper might be expected to produce longer sustain, less power, and less bass, but for any reasonable taper such effects should not be problematic. The top, and, indeed, the whole guitar, is a system, with each part affecting the tone in a number of ways. There is more than one way to get the tone and balance you want, and each maker comes up with a way that works for them.

[BlueBowman]

Originally Posted by Fathand

Try this one?
https://youtu.be/JHs7s1_pbAg?si=xV-9eRWgETZiajPe

Thank for the link, FH. While that's a good video of showing how some luthiers go for the "sheet metal" thickness, I should've been more specific in asking for a video showing variable thickness in a top (on a steel string). I've still not seen one of those. But, again, thanks for the link. I had not seen that O'Brien vid.

[Henning]

Thank you for the confirmation and added information here.
Kind regards