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Just wondering whether there is any benefit in using reverse kerfing on both back and top. I've seen builders who use them only on the back, or only on the front, and some who use on both.
Your thoughts would be greatly appreciated. Thanks in advance, Mike.

My guess on why they would be used on the back only would be because it’s visible through the sound hole. The suppliers have said it’s added stiffness, maybe somewhere between normal kerfed lining and solid lining? I cannot comment on that claim.

They aren’t more expensive so it would make sense to me to use for both top and back if you were going to use them.


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imho they look different and that's about all. If you glue and compare two sides, one with reverse and one with traditional, you might notice that the reverse side feels stiffer. The thin web of wood that connects the kerfs in the reverse lining 'closed' the kerfs and so they are no longer flexible. The traditional kerfs are still open and so they have flexibility.

Now, once you glue those sides to the top does it really matter? Perhaps to some degree but in my mind it's negligible. Of course I've never tested it.

I find it easier to do a neat job of inletting the brace ends with reverse kerf liners, and that's the main reason I use them. Naturally the real benefit there is with the back braces, since most folks don't look inside with a mirror to see the top inlets.

Reverse kerf liners can add some stiffness to the rim up until the time you glue on the top and back plates. After that the edges are locked in both directions, so there really should be no difference in stiffness. Solid liners, particularly if they're laminated, may add some mass, which will affect the sound to some degree by helping to 'keep the sound in the top'. There are other ways to do that: Trevor Gore builds in fittings on the sides that allow him to bolt weights on inside. With this setup he can fine tune the weight to get the result he wants, rather than being more or less stuck with whatever the side and liner mass turned out to be.

I agree with Alan about the neater appearance of inlet brace ends with reverse kerf linings. I also like not having to deal with removing glue squeeze out in the kerfs after gluing the back on because the web of the reverse kerf lining hides the kerfs where they meet the back.

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Thanks everyone, so the general opinion is not in favour of these linings and thanks for the explanations as to why. It does seem a bit of a fad, so I may well not bother but if I do , it will just be the back lining for the reasons stated above of inletting the brace ends in a neater fashion.
Cheers Mike.

I'm just a bumpkin, but this is why I don't use either laminated solid linings or reverse kerfed linings. While they stiffen the rims before the top/back are glued on, I have not heard solid proof that they make a difference in stiffness for the closed box. I can add mass by making my kerfed linings a bit larger and using a heavier wood (like maple), and the kerfed linings are a lot easier to make and install, so that's how I do linings. No offense to those who like the other alternatives, though.

I use reversed kerf linings precisely BECAUSE they stiffen the rim before the back and soundboard get attached. Everybody's buildup techniques are unique to that particular luthier, but for me, it makes sense.

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Peter Havriluk

I use the reverse kerf linings because they stiffen the sides during construction. That helps me when handling the rim out of the mold. In other words, I use them only for my own convenience.

When I started using them, reverse kerfed linings "double sides" and Somogyi stiff, thin tops were the rage. Like Peter and Bob above and others I used them top and bottom because they greatly increased the stiffness of the rim without resorting to slathering on epoxy and doing the double side thing.

Solid linings are also an option if you want to stiffen up the sides while remaining single sided.

I know a builder who bends his sides at .150" thick.

I used to make my own reverse kerf linings, and preferred them because of increased rigidity, still have some.
But lately I've been using laminated linings with laminated sides) and of course find them much more rigid, to the point I can drive the bus without having them in a mold.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Another thing for the OP to consider is that reverse linings require a bit more work to get right too. You need to either bend them on a hot pipe or the method I like to use is to spritz water on them and clamp them to the OUTSIDE of the sides over night. The next day they will retain their shape.

Otherwise they break easily on the tight bends and God forbid someone look inside your guitar and see that the linings are not one long continuous piece

I have used them for years. Mainly because they look cool and contain glue squeeze out better as already mentioned. As Alan mentioned the brace inlets look better.

I spritz them with water and bend on a pipe, clamp on the inside and they hold shape.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Rather than fight city hall, so to speak, I just make my reverse kerf liners in short pieces. As my violin making teacher pointed out, it's usually OK if it looks as though you meant it.

For those of you who don't mind an extra step here and there, laminating a thin bent liner onto the inside of reverse kerf liners stiffens them up and avoids any chance that they'll break in the kerfs. I've also glued bias selvedge tape onto reverse kerf liners to see if it would make any noticeable difference in the sound. They were really stiff with the tapes on: quite like solid liners in that respect, but there was not enough added mass to be of much help.

I find I can bend solid willow or cedro liners up to about 5mm thick with only minimal crushing failure on the inside of the waist bend (which doesn't hurt anything and can't be seen on the assembled instrument), but with a domed top and back you have to bend them very wide and trim them to fit the dome. I've done this, and also used laminated liners, on the arms of harp guitars to get them to hold their shape as I'm building. Lots of ways of doing things...

I know a builder who bends his sides at .150" thick.

I’ve always wondered what the limits were in that regard but haven’t taken the time to find out. I’m guessing there’s no Venetian curves involved…

Insert 4 pages about their brace ends here.... .....but food for thought that just came in the door.

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This is really interesting food for thought, and I've been playing with it in my mind for the past few days if I'm being honest. Drum shells came to my mind. I grew up in the world of percussion along with the world of guitar and built a few drum kits in my 20's. We often hear people making parallels to drums when we talk about stiff rims and what not... Whether that's a fair comparison or not, I don't know (unlike a guitar, the shell’s actively vibrating when you play a bass drum or tom). Five vs. seven ply drum shells are a real discussion in that world though, and there's a discernable tonal difference between drums made with either. Not only are we talking about something that's already extremely stiff there, but something made out of plywood to boot. I'm not sure how to measure the difference per se, but at least in the world of drums, I can hear it to an extent. Five ply shells tend to be more reverberant. Seven ply shells tend to be more focused.

I build guitars with stiff rims because I like working with the rim better, but there have been guitars that really speak to me made both ways. Some of my most favorite guitars on earth are made with single-ply rims and traditional open liners.

FWIW, the rims I've made with reverse-kerf linings are definitely stiffer than the rims I've made with traditional open kerf linings, and the rims I've made with double sides are almost like drum shells. They still have some flex and give, but not much.

James--

It is definitely something I spend time thinking about for any/all of the ways builders today vary from the way the iconic guitars of the past were built. For different types of linings, as well as different ways of constructing the sides, I come back to the question of whether the end product has attributes that are favorable to the end user, as balanced against the extra work and/or expense of doing it the newer way. I completely understand the benefit of working with stiffer rims during construction. I don't need that myself (I am fine relying on the mold), but others really like it. But if I don't need that, and it doesn't make the end product (the closed box) any stiffer, then I personally feel better off not using the newer ways. YMMV. I'm pretty "big tent" about how these things get made. Lots of different things work and make great guitars.

I'm not convinced that it's settled science that it does not make the rim stiffer after the box is closed. Probably not a way you could "prove" it anyways.

I have only built traditional and lean towards "lighter is better", but I'm intrigued by those emphatic about double sides, engineered sides and reverse kerf as at least a partial means to the same end.

Some that are going for ultra stiff sides (many in the high-end market) are also trying for a "live" back whereas others are doing a heavy solid back. Sounds like they are all trying to isolate the top's energy more. I'd like to hear more thoughts on this.

Any opinions on whether heavy full length side braces would also be a means to this end?

Pat

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Pat

Just thought I'd throw in a picture of my stepped laminated linings into the mix, different ways to do things.
I used quite a dense Poplar on the back, on the top I used 2 poplar but Khaya on the innermost lining.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Pmaj7th wrote:
"Sounds like they are all trying to isolate the top's energy more. I'd like to hear more thoughts on this."

I've done some experiments relating to this; probably not definitive, but certainly suggestive.

First; you have to keep in mind that the only part of the guitar that can produce sound and is driven directly by the strings is the top. The neck can pick up some energy at certain pitches, but it has not nearly the relative importance on an acoustic that it does on a solid body electric. The other parts that can put out useful amounts of sound all get the energy from the top. The sides, being curved, don't move much except for the flat part below the waist. The back points in the wrong direction to produce sound for the audience, it's usually much heavier than the top, and it's often up against your pudgy avoirdupois. From that perspective, then, it makes some sense to keep energy from 'leaking' out of the top to drive the back. It can get more complicated, though.

There are two ways the back can get energy from the top, and they're most effective in the lowest pitch range, where the guitar acts as a 'bass reflex cabinet'. In this analogy the lower bout of the top, working at it's lowest fundamental mode, is the 'speaker', and the sound hole is the 'port'. The strings cause the top to move in and out, which:
a) causes it to radiate sound like a speaker does, and
b) causes the pressure in the box to rise and fall, mosrt effectively at the 'Helmholtz' ait resonant pitch.
As with any bass reflex system the top and the air work together to produce sound. In a speaker cab the speaker resonance and the 'air' resonance are tuned to ber at the same pitch. The twogo through a somewhat complicated dance. Below the resonant pitches the speaker is moving 'in' as the pressure in the box is rising, and there is a lot of flow through the hole. Note that, in this lowest range, air is flowing 'out' of the hole as the top/speaker is moving 'in', so there is some cancellation as air from the hole just slides over into the space the top vacated. As you go up in pitch the air and top motion change phase relative to each other: a pitch is reached where air is flowing 'out' of the hole when the top/speaker ia also moving 'out': the flow and top motion might be a bit less, but now it all adds up to produce more sound out in front.

With a speaker setup the cabinet is more or less 'rigid' so there are no 'wood' resonances to color the sound (ideally), and they use a lot of damping, such as foam lining in the box to spread the resonances out in pitch. The sound level between the peaks and dips doesn't vary much, so to our ears, which are not very sensitive to changes in level, the output seems 'flat'. With only plucked strings to drive things we can't afford a lot of damping: too much wasted energy. OTOH, we're not particularly bothered by changes in level, so long as they're not too extreme, and the added resonances of a thin wood box can make things more interesting.

In measurements I've made, it turns out that having the main back resonance 'tuned' to be fairly close in pitch to the main top 'speaker' resonance can actually enhance the sound output in that pitch range. The top and back respond to the air pressure changes in the box by working as a bellows, to pump more air through the hole. This is the only back resonance, so far as I can see, that doesn't 'steal' more energy from the top than it contributes to the sound output, and it only works if the back resonant pitch is fairly close to that of the top. Other, higher pitched, back resonances to seem to contribute to 'tone color', so they can be good to have so long as they don't steal too much energy from the top. A reasonably heavy back with low damping (think rosewood rather than, say, maple) seems to help with this.

IMO either the 'active back' or 'reflector back' can work, within limits. The only backs I've ever seen that are not 'active' to some extent in the low range are the ones on Ovations; that's the 'extreme reflector', if you will. Most guitars fall somewhere between that and the really 'active' back. Most guitar backs are too stiff to be really 'active', and not heavy/stiff enough to be purely 'reflective'. Often loosening up the back by shaving braces is a better way to enhance the 'bass reflex' action and punch up the bass than shaving top braces, and it's safer too.

Naturally, as is almost always the case with guitars, it's actually more complicated than that, but this should help the conversation along.

"Any opinions on whether heavy full length side braces would also be a means to this end?"

In theory that should help, but note that what counts here is not side stiffness, but mass. In practice, you'd have to add a lot of them to change the side mass enough to really matter. Wood side braces must be inlet into the liners. A brace that ends short of the liner produces a stress riser there, so that's where the side will crack if it does. It's also, of course, the hardest place to do the repair.

That's the way I do it too, Colin. Also, I let the side bracing into the first layer. Got that from Bryan Bear's description. I've been using Mahogany second sides for the strips, thinned to .05"/.06".

I use similar side bracing, when I don't laminate the sides.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Thanks Alan, great insight!


Pat

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Pat