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| Author: | B. Howard [ Mon Jul 09, 2012 4:25 pm ] |
| Post subject: | Cut-away box? |
So I am planning to do a venetian cut-away on my next build. I want it to maintain all the tonal qualities I can from the non cut-away version. Since the cut out decreases box volume I figure I should do something to address that. I could simply add a little depth to the box, I'm guessing about 1/8" would be about right. But that may maker the guitar a little snug in the case for my liking. What I was wondering is if I could simply change the opening of the soundhole? Making it larger should bring the pitch back to where it should be, am I thinking about this correctly? If so is there anyway to calculate what size the new soundhole should be? As always, thanks in advance. |
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| Author: | Kent Chasson [ Mon Jul 09, 2012 5:08 pm ] |
| Post subject: | Re: Cut-away box? |
B. Howard wrote: ...What I was wondering is if I could simply change the opening of the soundhole? Making it larger should bring the pitch back to where it should be, am I thinking about this correctly? ... If your goal is to keep the cavity resonance the same, reducing the cavity volume by having a cutaway will increase the resonant frequency. Increasing the size of the soundhole will also increase it. If you want to lower the frequency, reduce the size of the soundhole. |
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| Author: | B. Howard [ Wed Jul 11, 2012 3:39 pm ] |
| Post subject: | Re: Cut-away box? |
Thanks Kent. I guess this is something most have not put much thought into. So is there any type of equation that I can use to figure out how much smaller to make the sound hole? Or do I need to do some experiments? |
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| Author: | meddlingfool [ Wed Jul 11, 2012 4:34 pm ] |
| Post subject: | Re: Cut-away box? |
I wouldn't say no one's put much thought into it. I'd suggest rather that most find the difference in the box resonance not to be significant enough to need to design around. When you consider that the box resonance is coupled with the top and back and will vary in relation to the stiffness of the plates, the box resonance ends up being 'somewhere around' as opposed to 'precisely X'... Do you think you'd hear the difference between the box being 92hz vs 93.5hz, or what have you? That being said, I did a cutaway dread with a 3 7/8" sound hole to find out, and I didn't like it. It sounded a bit muted like it couldn't get the air it needed to breathe. was it the sound hole alone or some other factors, who knows? It didn't make me want to run out and do it again, anyway... Not really getting the traffic around here we used to, are we? Cheers! |
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| Author: | Kent Chasson [ Wed Jul 11, 2012 5:29 pm ] |
| Post subject: | Re: Cut-away box? |
Sorry, don't know a formula. BTW, I don't build to a specific cavity resonance. If there is a magic number out there, it would be pretty hard to hit it while building everything between a parlor and a jumbo. And I've never adjusted anything to make make up for the lost cavity volume of a cutaway. Good luck. |
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| Author: | meddlingfool [ Wed Jul 11, 2012 8:05 pm ] |
| Post subject: | Re: Cut-away box? |
A fairly easy and reversible experiment would be to two stick tape a strip of ivoroid in your sound hole. Close it by an eighth at a time and measure the freq... |
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| Author: | Trevor Gore [ Thu Jul 12, 2012 1:26 am ] |
| Post subject: | Re: Cut-away box? |
There is no simple formula that gives accurate results and the the complex formulae are very complex! The simple formulae assume a "rigid vessel", which a guitar isn't, and do not consider the flexibility of the top, back and sides. When you factor in the air, top, back and sides you get a 4 degree of freedom model of the guitar, which is way too much maths for most people. As it turns out, on real guitars, the fully coupled air resonance is not that sensitive to the volume of the box, but it is sensitive to the size of the sound hole and flexibility of the plates. Depending on the overall stiffness of the box, the change in frequency of the fully coupled air resonance per 1mm change in sound hole diameter ranges from ~0.5Hz to ~0.9Hz. Increase the size of the sound hole and the resonance rises in frequency. Full details/analysis/discussion in the usual place. Because everything is coupled, the frequencies of the other resonances shift a bit, too. |
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| Author: | B. Howard [ Thu Jul 12, 2012 3:16 pm ] |
| Post subject: | Re: Cut-away box? |
So I guess the first thing I need to figure out is how much will the resonance shift so I can see if it is enough to worry about changing anything. I plan on calculating the volume that I will be removing from the box and then I will place a filler of the appropriate volume in a few guitars and measure the change in frequency. Suppose I will use a cloth sack of sand as my filler as I figure it will not contribute anything to the resonance. I will place it in the upper bout in the area of the cut away so hopefully it will simulate the change in plate vibrations as well. I let you know what I find out. |
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| Author: | meddlingfool [ Thu Jul 12, 2012 4:36 pm ] |
| Post subject: | Re: Cut-away box? |
Hmmm... Seems like a sack of sand is a whole whole lotta mass. Could see that seriously messing with your readings. Maybe try slipping a piece of cardboard to mimic the shape of a Venetian inside? Carefully taped around all edges to create a seal? |
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| Author: | Tai Fu [ Fri Jul 13, 2012 3:52 am ] |
| Post subject: | Re: Cut-away box? |
It's hard to say... build 2 guitars where everything is the same except for the cutaway (which is going to be hard) and see how that affect things. I can't seem to tell the difference but some can. |
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| Author: | John Mayes [ Fri Jul 13, 2012 7:09 am ] |
| Post subject: | Re: Cut-away box? |
Don't sweat it. Just build it. I've converted non-cutaways into cutaways and couldn't hear any difference. |
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| Author: | Dave Stewart [ Fri Jul 13, 2012 7:27 am ] |
| Post subject: | Re: Cut-away box? |
B. Howard wrote: ....Suppose I will use a cloth sack of sand as my filler as I figure it will not contribute anything to the resonance. ..... I think this would dampen plate vibration & change things for reasons other than volume. I tend to agree with John, although nothing quantifiable to back it up. I'd try a balloon ... would reduce volume, do little to dampen the plates, and with different inflations, you might see a trend you could track on audacity or something. Interesting expiriment. |
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| Author: | jfmckenna [ Fri Jul 13, 2012 8:33 am ] |
| Post subject: | Re: Cut-away box? |
Tai Fu wrote: It's hard to say... build 2 guitars where everything is the same except for the cutaway (which is going to be hard) and see how that affect things. I can't seem to tell the difference but some can. That's exactly what I did years ago. Top, back and side wood all came from the same tree cut side by side. The two guitars sounded slightly different but then no two guitars sound the same. After that experiment I convinced myself that it's too hard to tell if the cut out has any affect on tone and projection. I still have that cut out guitar in a case in my shop. Think it's time I play that one, been years. |
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