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Semi-Hollow Chambering
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10123&t=40734		 Page 1 of 1
Author:  absrec [ Mon Jun 24, 2013 11:24 am ]
Post subject:  Semi-Hollow Chambering
I'm about to build a "stage acoustic" and I am thinking about chambering it. Thinking it will make it light weight in addition to making it a little more like a real acoustic. Is there anything to consider from a standpoint of resonance? Are there any guidelines I should follow so I don't end up with some weird sounding guitar or possibly one that is louder in one frequency range than another?

I've seen the chambering on Les Paul and Telecaster bodies and it just looks like they hogged out everything that wasn't necessary to the structure of the guitar. Didn't look to me like there was any magic but I figured I'd ask anyway.
Author:  Rodger Knox [ Mon Jun 24, 2013 12:42 pm ]
Post subject:  Re: Semi-Hollow Chambering
That depends mostly on the thickness of the top and the material it's made of. If your top is 1/4" thick or more, there will be very little acoustic contribution. Chambering will make some difference in the sound, but probably not enough difference to actually hear.
You usually don't have to worry about some notes being louder, but you can run into some frequencies being more prone to harmonic feedback. That's when a natural resonant frequency of the guitar is the same as the frequency being output by the amp. With an acoustic guitar it would be a "wolf" note, but there's not much power in a string driving the top when you compare that to an amp driving the entire guitar. Adding (or subtracting) weight from the headstock is probably the most effective way to alter the resonant frequencies of ellectric guitars.
Author:  absrec [ Mon Jun 24, 2013 2:09 pm ]
Post subject:  Re: Semi-Hollow Chambering
Ok. I am going to have a solid block running up the middle. My top will probably be 1/4" of flame/curly maple. If I'm understanding you correctly, this setup should be fine. I just want it to feel like when you pick up a Thinline Tele, Silvertone or a Danelectro. When you knock the top, it gives off a hollow "thunk". Can't explain why. Maybe for thumb knocking rythym in between chords on solo gigs.
Author:  absrec [ Mon Jun 24, 2013 3:16 pm ]
Post subject:  Re: Semi-Hollow Chambering
Rodger Knox wrote:
Adding (or subtracting) weight from the headstock is probably the most effective way to alter the resonant frequencies of ellectric guitars.

I've never heard that before. Can you explain this a little bit more?
Author:  Rodger Knox [ Mon Jun 24, 2013 4:02 pm ]
Post subject:  Re: Semi-Hollow Chambering
This is an extreame oversimplification, but the neck is a major contributor to the most important resonant modes in an electric guitar.The neck behaves sort of like a tuning fork, and adding or subtracting weight on the headstock is the most effective location for changing the natural frequency.

In acoustic guitars, there are more contributing factors and more resonant modes, and the "neck modes" have very little effect. The resonances are where the frequency response comes from, and the relationships between the various modes are very important to how the guitar sounds.

With a block under the bridge, you're building a solid body. Since mass and stiffness are factors in determining the resonant frequencies, wood selection and chambering will effect them. Usually guitars are heavy and stiff enough so that most of the resonant frequencies are outside the frequency range produced by a guitar, which is what you want in an electric.
Author:  dzsmith [ Mon Jun 24, 2013 8:12 pm ]
Post subject:  Re: Semi-Hollow Chambering
Very interesting about the headstock resonance Rodger.
I'm working on a Tele I plan to chamber with an "F" hole on top. I don't expect any tonal difference with the chamber, but I want to reduce weight and figured the "F" would look cool.
I just did a set up on an '85 Ibanez Alan Holdsworth guitar. Funny, it is chambered under the plastic pick guard.
I hit the pick guard (with a pick) while playing and it made the worst sounding loud "thonks" I've ever heard.
Author:  Rodger Knox [ Tue Jun 25, 2013 12:12 pm ]
Post subject:  Re: Semi-Hollow Chambering
You mean something like this?
Attachment:
IMGP2179.JPG

The 3 piece top on that is 1/8" thick maple, completely hollow except under the bridge for the stringthrus. Here a shot before the back went on. For most practical purposes, it sounds like a solid body.
Attachment:
IMGP2043.JPG
Author:  dzsmith [ Tue Jun 25, 2013 5:57 pm ]
Post subject:  Re: Semi-Hollow Chambering
Yep Rodger, something very similar.
Unfortunately, I don't have a cool Dead towel to lay it on.
I'm doing a three-piece body (Sycamore/Pecan/Sycamore) but I'll just hollow it out under the "F" hole and leave the rest solid. I know the first time I pick it the pick will fly into the F hole but I have tweezers ready.
Does your design sound like a solid body when not amplified?
Nice Axe!
DZ
Author:  absrec [ Tue Jun 25, 2013 7:16 pm ]
Post subject:  Re: Semi-Hollow Chambering
Ok, cool. That's good new as the plan has changed slightly. More specifically - wood choice. I'm doing a Honduran rosewood core (1.5"-1.75") with top and back laminates of either Spruce or Douglas Fir.9 I would like to put a sound hole (purely for aesthetics) where it would be on your average acoustic. The point is for it to look like an acoustic but feel like a solid body. After picking up the glued up blank, I know that a good bit of it will need to be hogged out for this thing to even have half a prayer of not killing my back. It's stupid heavy!

Anyway, does the sound hole idea sound like a bad one? The only reason I ask is because that's the loudest part of the guitar. Judging from the pics of the Thinline tele, I'm thinking it should be fine. But again, before I make a potentially expensive mistake, just figured I'd ask.

I love this forum.
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