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Chambered Guitar - Top Thickness?


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My last chambered/thinline style guitar had a 1/4" top - can I get away with 3/16"? I've got the chance to pick up some nice wood, but i'm worried about going thinner. I'm assuming since it'll be supported over both the center block and the edges, and won't be spanning too much distance, it shouldn't be too much of a problem, but I thought I'd ask before I went ahead.

Thanks!

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Acoustic guitars typically have a 1/8th inch top, so 3/16th will be fine.

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Acoustic guitars typically have a 1/8th inch top, so 3/16th will be fine.

Acoustic soundboards serve a totally different function. The top should be thick enough to limit feedback issues when played at volume. Chris mentioned he was successful at 3/16" using Spruce worked well with his design. The design should be considered when choosing this thickness. Depending on how much material is remaining under the top(supporting it), and type of top wood. You may want a bit more or less thickness.

Peace,Rich

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The top should be thick enough to limit feedback issues when played at volume. Depending on how much material is remaining under the top(supporting it), and type of top wood. You may want a bit more or less thickness.

I've been wondering about this too -- are you talking about the 'musical' feedback or the squealing microphonic type?

What does one need in order to prevent excessive feedback when dealing with a chambered guitar--is it okay, for example, to hollow out the area beneath the pickups, or is that asking for trouble?

I've been looking at (ogling) Scott French's tuning fork innards lately --

features_Body.jpg

He says he uses a 3/4 inch top though --so I imagine that helps. But suppose one were to put a 1/4 inch cap on this kind of innards?

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Yeah, he uses a 3/4" top, but it's also a 3/4" top that's CARVED. So it's only 3/4" in the center. It gets close to nothing near the edges. Well not nothing, but I'm sure it's a transition that's something like 3/4" to 1/4" (or less) so in places it'll be less than 3/4" IE: like above those side chambers.

Chris

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Hopefully one of the fellas that has more experience with these can give you more accurate information(maybe; Scott, Myka, Chris or some of the guys have looked more closely at how to limit the effect).

I've been wondering about this too -- are you talking about the 'musical' feedback or the squealing microphonic type?

I don't know if I understand the question well enough to answer. :D

What does one need in order to prevent excessive feedback when dealing with a chambered guitar--is it okay, for example, to hollow out the area beneath the pickups, or is that asking for trouble?

Limit how much the body is vibrated by the speakers pumping at high volume near you. A hollow body guitar with thin sides/ back and top will be pretty easy to vibrate if you play it in a room at high volume levels. It is not as easy to make a solid body guitar vibrate, thus the effect is minimal. A semi-hollow is somewhere inbetween. It has thicker sides, much more material is used brace the bridge, pickups and so forth limiting the top/back and sides ability to vibrate. When the body vibrates it vibrates the strings(transfers that energy) and it is induced into the pickup. So to control how much energy is introduced by outside forces and still allow some string energy to effect the bodys vibration. Man, I sure hope that makes some sense. I am not very good with words.

This is all about the strings and pickup. An acoustic instrument its about transfering string energy to the body to make the body vibrate and move that air.

Hopefully the other guys will provide a little better explanation, and some more specific examples or rules of thumb they use.

Peace,Rich

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I JUST wrote a HUGE essay on this exact topic in another thread in the hollowbody section, here's the link, go read up, it'll help ya out a bit (I think... if I'm accurately remembering what I wrote... but it WAS awhile ago, but I'm pretty sure it'll give you a good ground base).

That Post

Chris

Oh yeah, and one more thing to add to that linked post. This is MY humble opinion as to how/why the Scott French design works. If you read that other post I talk alot about the connected volumes of air (ie: one big pocket vs. bunches of disconnected mini pockets) that are allowed to be affected by the outside vibrations and cause feedback. My take on why the tuning fork design doesn't feedback despite how much wood is missing and how it's one big connected chamber is simple... his bodies are very thin. Therefore, YES he did take out a big AREA.... but the VOLUME is not as big as one would think because of the fact that the body is thin (ie: on his last GOTM entry it was .5" at the sides... and the carved top (to my knowledge I could be wrong though) was not carved on the underside. Therefore, that hollowed area that you see isn't in fact that deep at all, and therefore the air volume to be affected would probably actually be LESS than a normal 1 3/4" thick semi-hollow with two chambers, one on each side.

Another example of this is the Pagelli Jazzability. Watch the "Gourmet Guitars" video that has his in it and you'll see a METAL player rock out on one of these bad boys right next to a stack without a problem. And those are fully hollow, carved inside and out, back and front (although you CAN get them with a center block). The trick to not feeding back... find a pic of the side profile of the jazzability on Pagelli's site... it's IMMENSELY thin. You'd think from the front it's a fully thick hollowbody... NO I'd estimate it at an inch or less at the sides. IE: Low air volume to be affected by vibrations, therefore, less feedback.

Chris

PS: Scott or Pagelli... feel free to correct me if I'm wrong.

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I JUST wrote a HUGE essay on this exact topic in another thread in the hollowbody section, here's the link, go read up, it'll help ya out a bit (I think... if I'm accurately remembering what I wrote... but it WAS awhile ago, but I'm pretty sure it'll give you a good ground base).

That Post

Chris

Oh yeah, and one more thing to add to that linked post. This is MY humble opinion as to how/why the Scott French design works. If you read that other post I talk alot about the connected volumes of air (ie: one big pocket vs. bunches of disconnected mini pockets) that are allowed to be affected by the outside vibrations and cause feedback. My take on why the tuning fork design doesn't feedback despite how much wood is missing and how it's one big connected chamber is simple... his bodies are very thin. Therefore, YES he did take out a big AREA.... but the VOLUME is not as big as one would think because of the fact that the body is thin (ie: on his last GOTM entry it was .5" at the sides... and the carved top (to my knowledge I could be wrong though) was not carved on the underside. Therefore, that hollowed area that you see isn't in fact that deep at all, and therefore the air volume to be affected would probably actually be LESS than a normal 1 3/4" thick semi-hollow with two chambers, one on each side.

Another example of this is the Pagelli Jazzability. Watch the "Gourmet Guitars" video that has his in it and you'll see a METAL player rock out on one of these bad boys right next to a stack without a problem. And those are fully hollow, carved inside and out, back and front (although you CAN get them with a center block). The trick to not feeding back... find a pic of the side profile of the jazzability on Pagelli's site... it's IMMENSELY thin. You'd think from the front it's a fully thick hollowbody... NO I'd estimate it at an inch or less at the sides. IE: Low air volume to be affected by vibrations, therefore, less feedback.

Chris

PS: Scott or Pagelli... feel free to correct me if I'm wrong.

Chris, do you know what the thickness of the bodies(top/back/sides) you are describing are? Are you saying the air volume of the guitars chamber is the variable to control feedback directly or thickness of sides and volume of air effect the feedback equation? I suppose I should look into this more closely, rather than ask for more explanation.

Peace,Rich

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I DO think that side and back thickness, etc. is a factor. But not as much as many would have you believe. But also, you should take this with a grain of sand because I AM only a novice builder, however because 2 out of 3 of my builds have been some form of hollowbody I've done ALOT of research on the subject. Here's why I think so:

1- Warmoth offers honey-comb hollowing techniques that make you lose weight but have NO more feedback than a solid body.. why? Because they're essentially just drill holes, meaning they are small pockets that aren't connected, and even though there IS an air volume in the guitar, each pocket is so small the effects are negligable.

2- The Gibson ES-335 has a thin top and back (carved inside and out) and two big chambers, but punk rock bands like matchbook romance still use them without great feedback issues. Since they have thin tops you'd expect alot of vibration and feedback, but you don't because the two sections are cut off from each other.

3- When I played with Avril lavigne her guitarist used a Gretch FULL hollowbody (kinda like Ted Nugget) and this was a 7000 person crowd and the sound in that room was HUGE yet he didn't feed back because..... he stuffs. Not his pants, but his hollow guitar to limit the amount of air the can circulate and vibrate in it.

What I'm trying to say is it's a trade off between the two. Picture this, if you have a guitar with thin tops and sides and you have SUPER loud noise playing... in a vacuum... nothing will happen. Sound doesn't travel through a vacuum. HOWEVER the minute you add air the sound travels and vibrates the wood making feedback. Now that we know this we can say "so what makes the wood vibrate????" THE AIR. Therefore, if you have air INSIDE and OUTSIDE your guitar, it's gunna be affected more by the air. Not to mention, air OUTSIDE the guitar carrying sound hits it and goes past it, however the air INSIDE will beat all the sides of the cavity and not be able to leave the guitar as easily. Try this, go home fill your tub with water. Then take a tupper-wear bottom and fill it with half water so it sits about halfway down in the water and is stable. Drop a spoonful of water near the floating tupperwear outside it's walls and watch it rock like a boat. Then when everything is calm again, take a spoonful of water and drop it in the water INSIDE the tubberwear boat and watch it rock, but keep an eye on the ripples, they'll go out from the point of impact, hit the walls, turn around, come back, hit another wall, etc. until they've dissipated. Now add these two affects together... and you have the outside ripples rocking it, and the inside ripples bouncing off the walls SEVERAL times. It's the same with a guitar cavity. The air gets trapped inside and hits the walls of the guitar SEVERAL times. The freer the air is to move around, the more impact the waves will have because more of the outside waves can get in and get trapped. Adding an f-hole makes this worse because it's like opening a door INTO the cavity making the air carrying sound waves easier to get in.

So, it's a combination of things. If you have a big coninuous cavity that allowings for things to reverberate around and shake things up, then it'll have a bg impact... however this CAN be countered with thick walls that are less prone to the attacks of the trapped reverberations. If you want a light guitar... do small chambers, or slightly larger chambers that aren't connected and won't allow reverberation to travel. If you want huge feedback, make a giant single cavity and thin walls. Or balance things, such as a large cavity... but make it a shallow one? Or put thick walls around it and you'll be pretty safe.

Did that make sense??

Chris

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I've been wondering about this too -- are you talking about the 'musical' feedback or the squealing microphonic type?

I don't know if I understand the question well enough to answer. :D

Heh heh...I mean, there's feedback you can play with, make musical noise with....and then there's that microphonic squealing pickup thing, which is anti-musical.

But from what you and Chris are saying, it sounds like the feedback I'd be worried about would be more of the musical variety...which is not so much of a worry (rare that I play at super high volume anyway)

I had a Samick Royale for a while --a semi hollow featuring a 'monoframe' construction --in other words, it was more like a chambered guitar. And I could NOT get that guitar to feedback, unless I jammed the headstock right against the amp.

As for Scott French's guitar --the one in the photos isn't all that thin --the body is 1.5 inches thick, and the cap (before carving) is .75 inches....

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You'll have to ask him about that one... from what I understand that's one of his OLDER builds in that picture, and therefore may be a prototype of the design. Maybe that one IS prone to feedback cause it's thick creating a bigger hollow? I dunno, like I said you'd have to ask him.

Wait a sec, I just re-read what you wrote... the guitar is 1.5" thick, the cap before carving is 3/4" thick. That means the backwood is only 3/4" thick too. Lets assume he's doing the norm and leaving 1/4" thickness on the back. That means that that cavity is only 1/2" deep (cause to my knowledge he doesn't carve out the bottoms of his tops too). 1/2" cavity isn't all that deep when you think about it.... especially compared to when it's reverberating onto a top that's 3/4" thick where the bridge is.

:D

Chris

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Essentially here is MY theory.

Verhoeven's Fundamental Theory of Musical Feedback:

k*(V*aS)/T=F

where k=a universal constant

V= the volume of the chambers in the guitar

aS= the average size of the chambers

T= the average thickness of the top, sides, and back

F= the amount of feedback.

Although it's not an exacting formula, it illustrates quite well the effect each variable has on the feedback. Just put in some random numbers, and then change one and see if the feedback goes up or down!?

Chris

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Acoustic guitars typically have a 1/8th inch top, so 3/16th will be fine.

Acoustic soundboards serve a totally different function. The top should be thick enough to limit feedback issues when played at volume. Chris mentioned he was successful at 3/16" using Spruce worked well with his design. The design should be considered when choosing this thickness. Depending on how much material is remaining under the top(supporting it), and type of top wood. You may want a bit more or less thickness.

Peace,Rich

He implied strength of the top, hence if 1/8th will hold the guitar together, 3/16th will be fine.

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My take on chambered guitars is much like Chris', although I'm certainly not going to bother to even attempt to turn it into something vaguely mathematical. Too many damn variables.

The types of semis I've built (none have F-holes of any type, before anyone asks):

-deep body, chambered, block under the bridge, maple top carved inside and out to about 1/4" thick, maybe a hair less. Bridge pickup suspended in top. Feedback: highly controllable; if you want some, it's there. Don't? Don't let it happen. Bridge pup is slightly microphonic, and needed to replace the springs with surgical tubing because it tended to vibrate too much.

- honeycombed: forstner bit, drill holes, then used those as a guide to drill out even more wood using a 3/4" router bit with the 1/4" shank riding the edges of the 1" drilled holes to hollow the inside out even more. Very light, acts like a solidbody re: feedback

- My 'standard' ish method: several chambers (3 to 4), solid under the bridge, solid center block (long tenon set neck). Top carved in and out to about 1/4" in those areas above the chambers. This gives you a fair amount of that hollowbody character (lots of wood removed), a lightweight guitar, a slightly more feedback prone instrument (in a good, controllable way).

The feedback/sound issue boils down to vibration; remove more wood, have more thinner surfaces that can vibrate, and you have more effect on tone. Use lower-gain single coils (P-90, whatever), and you seem to get even more of an effect.

As for thickness, it depends on the unsupported span and the wood species. 1/4" is safe for even the most chambered of guitars, assuming you're not using spalt maple that's about to fall apart.

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Here's a question: what difference do you think it would make if I glue the cap on the bottom, and not the top --I'd still chamber out the main portion of the body, then cap the back.

I'm guessing that would stop a potential feedback problem --but would I lose that semi-hollow sound?

Assuming all the woods stay the same, methinks the feedback issue (it's NOT A PROBLEM. It's perfectly controllable, even at very high volumes. it's a Good Thing.) is the same. Glued or solid, no difference, assuming thicknesses are the same as well.

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Exactly what mattia says. Having the cap on the top or on the back, assuming you're using the same woods and thicknesses and chambering pattern, the feedback proneness will stay the same.

Chris

OK, that's good to know --it'd been fun to have a feedback-worthy guitar.

What about sound --or is that a non-issue too?

I've got this nice-looking ash, it'd be a shame to hide it on the back of the guitar, that's why.

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Then why not re-saw it into a matching back and top.... and then chamber out the back, glue the top on top and it'd look close to a solid body cause the top and back would match grain almost spot on... but it'd be hollowed.

Chris

Not thick enough for one thing--it's only 30 mm, so with the cap I'll get 35 mm or so (I'm into really thin guitars right now).

For another, I don't have access to the right tool for that...I might be able to scare up some more of the same wood (came from scrap a friend gave to me) and thickness those.

But yeah, your idea would be very cool looking...ah, now see what you've done...you've got me started dreaming about yet another guitar...I'm trying to take a break from this guitar building thing, the wife wants me to start working around the house now...

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I had (yet) another question:

How important is it to have wood below the pickups?

For example, I'm using humbucker-shaped pups --so, I could use Scott French's tuning-fork style innards to provide supports for the pickup wings (the screw/spring assembly), so that the pickup will be suspended over the hollow cavity.

Seems to me that that's the best way to pick up that hollowbody sound.

But will that make the guitar excessively prone to uncontrollable feedback? (I suppose I could add in some kind of foam support beneath the pickups if that becomes a problem).

Apparently, the SF3 suspends the bridge pickup, but houses the neck pickup completely in wood --I'm assuming that's done for neck joint strength though?

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I had (yet) another question:

How important is it to have wood below the pickups?

For example, I'm using humbucker-shaped pups --so, I could use Scott French's tuning-fork style innards to provide supports for the pickup wings (the screw/spring assembly), so that the pickup will be suspended over the hollow cavity.

Seems to me that that's the best way to pick up that hollowbody sound.

But will that make the guitar excessively prone to uncontrollable feedback? (I suppose I could add in some kind of foam support beneath the pickups if that becomes a problem).

Apparently, the SF3 suspends the bridge pickup, but houses the neck pickup completely in wood --I'm assuming that's done for neck joint strength though?

On my first, the bridge pickup was totally suspended, the neck encased in wood (for, as you rightly guessed, purposes of maintaining a solid neck joint). I don't think the air below has anything much to do with the tone, but how freely the wood the pickup is mounted in can vibrate is important. After all, pickups don't pick up wood vibration. The movement can move the pickup relative to the springs (which caused weird sounds at first because the springs were vibrating; damped that using surgical tubing instead), and the body filters string vibration.

You 'pick up' that hollowbody sound regardless, though. I tend to play through the normally mounted and surrounded neck pickup, and it's definitely that rounder hollowbody tone.

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