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Does anyone know the formula to measure the volume inside an acoustic guitar? I think I have it figured out but I just wanted to pick your brains on it.
I don't know what useful information it would be but it is interesting to think about.
My theory is it would be like finding the volume of a cylinder. 2 pi r h
But since a guitar is not a circle you would have to find the circumference measurement and solve for diameter then you would know the radius.
What's your thoughts?
Hutch


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"After forty-nine years of violin building, I have decided that the search for a varnish is similar to the fox hunt. The fun is in the hunt."
Jack Batts Maker and Repairer of Fine Violins

Do you need to smuggle some stuff?

The curves are so irregular on most models that I think it would be very difficult to get something accurate. You basically need to divide the outline into smaller shapes, find the area of those shapes, add them together, and multiply them by the height. (Then subtract the volume of the braces, if you care?)

Or you could just see how much water it holds.

Alternatively, I recommend a new system be developed for measuring the volume of guitars based on the number of specified units that fit inside said guitar. We could use cats, but they're not standardized. Beer cans, maybe? Like...the volume of this guitar is 33 cans?

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David Morris

Tape up the soundhole and dunk it in a large enough container of water.

Measure the volume of water displaced.

Keep in mind that this will only give you an approximation as the volume of water displaced displaced will include the thickness of the wood dimensions, and the actual internal volume of the instrument is decreased by the volume of the internal structure (kerfs, braces, etc.)

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Old growth, shmold growth!

The volume inside a guitar is directly proportional to the intensity by which it is being played on the outside. Put a microphone and decibel reader inside to measure volume......sorry, couldn't resist.

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Doug

Don't let fear or common sense stop you from trying to build something

Dang it, you beat me to it

You could also fill it with beans. Shouldn't be too terribly time consuming to get them all back out. But unless you need it really accurate, I'd just estimate the soundboard area by dividing it up into a few rectangles/triangles (remember, you only have to do half of it if it's symmetrical), multiply by the average depth (not including the soundboard/back thickness), and subtract some more for the side thickness and braces/linings/blocks.

EDIT: Dang it again, ninja'd by Todd now Although I still say beans are better than rice, being bigger and thus easier to get out (and wash for eating afterward). Actually, you'd probably want to wash the rice before putting it in as well, so it doesn't leave dust in there.

You can measure the area of the back by tracing on graph paper, say one inch squares. Then count up all of the squares that are completely inside the line plus all of the squares that are one half or more complete. Subtract an estimate of the side thickness. Then you need to average the height from top to back and multiply height times area.

It's as easy as taking the integral of the 4th degree polynomial that best approximates the silhouette from neck block to tail block and multipling by twice the average depth. Which is to say, not very easy.

I guessed the number of beans in a jar once...won a fern.

Pour the beans in a jar and post a pic....I'll tell you how many beans you have.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I think the decision comes down to beans or rice.............................!!! But water may be more accurate??

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A person who has never made a mistake has never made anything!!!

Lol....... I guess I set myself up for this one. I haven't laughed so hard in a month.
Well, since my wife is Mexican I better use her black beans. I don't think she'll mind too much

Thanks guys for ideas, but I was looking for something a little more scientific.


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"After forty-nine years of violin building, I have decided that the search for a varnish is similar to the fox hunt. The fun is in the hunt."
Jack Batts Maker and Repairer of Fine Violins

If you just need a rough estimate, Romanillos gives this formula for an approximate top area on his Torres book:

{Length*(Upper_Bout_Width+Waist_Width+Lower_Bout_Width)}/3*0.96

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member of the guild of professional dilettantes

"(Upper_Bout_Width+Waist_Width+Lower_Bout_Width)}/3"

That's just the average width of a guitar and the .96 must be thickness of sides. My guess was similar - take the circumference of the instrument, then the average radius of upper and lower bouts, multiply by the .96 that Romm. uses. Use the volume formula for a cylinder.

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It's this new idea from recent decades that everyone gets a participation award. - MUX

I'd say the easiest and most accurate way to measure the volume inside of a guitar is with a decibel meter. Stick it in there and give it a strum. It'll tell ya immediately what the volume is.

I would suggest you consult with a fellow named Gottfried Wilhelm Leibniz.

Dried peas, rice, polystyrene beads etc. will all work. If you want more scientific and have a bit more time, download Jon Sevy's guitar outline design program and "sketch" up an outline for the guitar you have. (Print a blank outline to get the squares, draw around the guitar you have onto the squares, then match the drawn outline to what you create in the app). The app. will give you the length of the outline (for side panel requirements) and the area inside the outline. Multiply that area by an average body depth and you'll get a close approximation to the volume. Having found the volume, I'm not sure what you'll use it for, but if you're trying to calculate a Helmholtz resonance, you won't get a decent answer from the "simple" formulae because they rely on the vessel being rigid and a bunch of other assumptions that don't hold for guitars.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

True, not very easy to do by hand. But, with a computer this shouldn't be too bad. First you need a digitizing program (not sure if that is the right term). These allow you to take a picture, put in some points, and fit a curve. Then, once you have the equation (yep, should be roughly 4th order, may get some higher order terms with small coefficients that you could ignore), you can use something like MATLAB or MathCAD to integrate it.

But Todd's idea is pretty clever. The shape of beans seems like it might leave a good amount of empty space between each one, but I guess you will get roughly the same amount of empty space in the measuring containers, so it should even out.

I think in all seriousness dumping rice or the like, maybe even sand?, into the instrument then removing it and measuring would be the most accurate. Because even if you find the area of the outline and calculate for depth you are not calculating for the loss of volume to braces, kerfing, and endblocks.

Have any of you actually filled a guitar with rice? I did, once, because someone told me it was a good way to clean out old dust and crud; and it worked, sort of. I think it was an old Guild, anyways it had traditional kerfed linings. I poured some rice in, shook it around well, and got most of it and the debris out, the rest of the rice got stuck in the kerfs! I could get most of it out by some more rigorous shaking, and a pointy stick, but some were really hard to reach, so they stuck until they fell out, one by one, for a long while... Never again. Beans (or water!) sounds like a better idea.

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Rian Gitar og Mandolin

The bit that's always left out is to use arborio...

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I filled two guitars with water. Strangely the one with a dovetail neck joint fell apart but the bolt on was perfect

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Doug

Don't let fear or common sense stop you from trying to build something

Yikes! Fill the guitar???
My relatively unsophisticated Turbocad allows me to trace around the perimeter of any shape & automatically calculate the area. If a guitar has an average inside depth of (whatever), volume est. is possible. (I suppose you could break the areas into multiple sections to get a more accurate estimate)
FWIW I made note of a few I saw come up one time ... never checked them
Box Volumes:
OM (4.09-3.35" sides): 845.124 cubic inches
00 (4.13-3.385" sides): 784.915
L-00 (4.37-3.46" sides): 836.110
Dred (4.875-3.75" sides): 1109.705

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Dave
Milton, ON

My water idea was actually an absurd joke.......

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Old growth, shmold growth!

I like the standardized units idea, like beer cans. Although I can see how using a liquid would be more accurate. Perhaps we could fill guitars with beer and state their volume in pints.

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George :-)

Does this mean domestic guitars should be measured with long grain rice and asian ones with sushi rice (arborio)? What about Mexican and Spanish guitars - Spanish rice?

ScooberJake: In all of what is left of seriousness, the curve fit wouldn't be that hard to manage (famous last words). Wait a minute, I think I like the rice & beans best for the Spanish.

How about placing the guitar into a rectangular box, then fill the area that the guitar isn't in with rice or beans to the level of the top. Calculate the volume of the box, and subtract the volume of rice or beans to get your measurement. You can then subtract for the volume of sides, back, top and braces. Just trying to think outside the box.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker