Had an interesting question from a reader and after spending a while replying, thought it would make a good post for others. Norm asked:
I've cut the neck for a 6 string acoustic CBG to learn on. Have a beautiful piece of leopard wood for the fret board. Trying to select the box. Going to go with kind of a plain Jane so I won't feel bad about cutting a sound hole in the middle. Since some of the art work on the CB's I have picked up is so beautiful I can't bring myself to put a big hole through it!!!
My question is this. I'm going with a 25 inch scale, how large of a diameter sound hole should I cut and how far from the bridge do you think I should place it. I'd like to get the biggest sound as possible. Would you suggest reinforcing the inside of the box? I am building with the neck and tailpiece as one unit going all the way through the box.... hmmm that right there would limit placement of a central sound hole. What would you suggest with that in mine? Maybe two smaller sound holes on each side of the neck/tail piece? If so what size diameter or radius?
Answer
There's been a lot of debate on CBG forums about soundhole sizes. I'm not an expert luthier, I reckon you probably know more than me on regular guitar building. I tend to like to keep things fairly quick and simple in the original spirit of CBGs being rough home-made instruments. I do like to know the science etc behind things though but don't actually apply it to my builds. Here's a link to one article http://hal9000.ps.uci.edu/Weber%20H90%20Physics%20Paper.pdf just Google "helmholtz resonator" for more info.
As far as I understand, soundhole size has nothing to do with the scale length but is relative to the volume of air inside the box. I'm sure there are online calculators to give optimum sizes if you can work out the internal volume of your box. Theory suggests that size seems to affect the frequencies more than actual acoustic loudness - larger holes better treble but less bass. Some CBGers don't bother with holes at all and some put one in each corner. It seems that it doesn't make all that much difference what you do. I guess that's becuase the poor acoustic properties of a cigar box can't be altered enough to actually hear a big difference in audio quality. It's more a question of visual aesthetics where you place them.
My personal slant is usually to put one about an inch diameter in one corner. Sometimes two or three smaller ones. (I think the optimum worked out by someone was surprisingly small - only about 3/8 inch for the average size box). I built one with it in the centre but hid the neck by fitting a sink drain cover over it. Very little 'hole area' when you add it together but it sounds great and is one of my favourites. I also fit piezo pickups to all my guitars so if I want some volume and variety of tone I just run it through an amp.
I reinforce the box by simply running a bead of wood glue (PVA) around the inside of the corners. Some people use wooden beadings glued for extra strength. I think soundboard bracing would be overkill.
Theories seem to go right out the window with CBGs. Standard acoustic guitar builders consider the type and thickness of wood for the soundboard, type of internal bracing etc. Doesn't seem to make much difference to a cbg. Example I built one with a very thick lid. I thought it would sound dead but it has the best sustain of them all. In fact it's the thinner box lids that seem to be a little dead, I would have thought the opposite though perhaps they give more volume. A pal built one out of an all-cardboard box and it sounded great.
My advice would be not to get too caught up in all the technicalities or striving for the ulitmate sounding CBG. Just have fun, experiment, try a different approach each time you build one and see what works. You will be surprised what does. Every one I've built has sounded and felt different, some better than others but unless I use the exact same box and neck wood, I doubt I could repeat any one exactly but that's one of the things I like about it all - variety and the element of surprise. Don't get drawn into being too precious about them. It's a guitar made out of a cigar box after all! You're not competing with a Martin or a Gibson. Be guided by your imagination and whims and just enjoy the whole creative aspect.
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