Describing a large acoustic guitar with a bass scale as an "Acoustic Bass" is confusing, because an upright bass fiddle can also be called an Acoustic Bass. "Acoustic Bass Guitar" is a more accurate term for the hybrid beast you describe. But I still don't like it completely, because it lends an air of legitimacy to a hybrid form which is really quite useless. Useless? Well, it's useless outside of playing one in your living room accompanying friends -- and even THEN it's useless if your friends are playing big dreadnaughts with heavy strings and thick picks and if someone else is playing a banjo. It really cannot hold its own when pitted against all those "real" instruments. If you're playing outdoors, on stage or recording, you will then need to install a pickup in it or it won't be heard at all. Then, its kinda dumb to call it an "acoustic bass guitar" isn't it? If unvarnished truth is your goal, it should righfully be called a "hollow-body electric bass guitar." (I can already read the mail: "but acoustic guitars with pickups are STILL acoustic guitars!!" Okay, okay have it your way...)
The other reason that Acoustic Bass Guitar is an inappropriate term is because, for my money anyway, it refers to an instrument OTHER than the only REAL acoustic bass guitar (one that you DON'T need to amplify, one that you can play out on the street or on stage and be heard just fine, like an upright bass fiddle): the
Mexican Guitarron. The Guitarron is not a hybrid at all but a full fledged instrument form which developed in an unbroken line of descent from the sixteenth-century Spanish Bajo de Uña.
So then, why don't you go out and find yourself a Mexican Guitarron and forget about this silly hybrid which is really a non-acoustic acoustic guitar with a bass scale? Well, trying to play a Mexican guitarron is something like trying to play a small bathtub. Physically you have to look like a WWF wrestler to hold the huge thing all night long, and if you're lucky, have a belly big enough to rest it on and hands as big as baseball mitts to play those telephone-cable strings that are usually about half an inch off the fingerboard.
You should have deduced by now that the reason the guitarron works so well IS because it's really big. Period. So I think the concept of a guitar-sized "acoustic bass guitar" is really kind of a hustle.
But, if a Fender Electric Bass is going to look silly in your earthy-crunchy folky string band or a washtub bass in your authentic bluegrass group looks too geeky, I can't think of any solution better than one of those so-called "Acoustic Bass Guitars" I've just disparaged. With the right pickup in it, it sounds pretty good on stage. In fact, it sounds almost (but not quite) as good and is easier to play and to carry than an upright string bass...and costs a fraction of the price of a good one.
Since an "acoustic bass guitar" is not acoustic (you've got to plug it in to hear it), and really not a guitar, and it doesn't really work very effectively as a bass, maybe we should call it a Fred. Or, okay, a "flat-top bass" if you prefer.
So, you want to build one of these Hollow-Body Electric Bass Guitars: then build a jumbo guitar with 5" deep sides, make the top and back a full 1/8 thick, and the sides as thick as you can bend them without breaking them. Dome all the back braces about twice as much as you would ordinarily. On the top, you can leave the x only a bit beefier than the ordinary ones. Increase the upper transversal by 50% in cross section, and make your bridge patch half again as thick and half again as large. This is for a four string. For a five or six string, you're on your own.
Get a 34-inch scale "bass guitar" fingerboard (Martin sells them already slotted and radiused, don't kill yourself trying to make one). So things don't collide disastrously on your actual project, do a full-sized layout drawing on a large piece of paper, like I describe in
my book. In order to have the bridge land about where it does on an ordinary guitar (about at the widest part of the face) the fret that first touches the body will end up an oddball, like the seventeenth or eighteenth (depending on the length of your soundbox). But that doesn't matter.
The first complaint from bass players is that it's going to be neck-heavy. Well, that's no surprise: this thing has a large solid neck is on one side, and a hollow, thin-walled box is on the other. The only way you can get it to balance properly is to get the strap to attach about at the midpoint of the entire length of the instrument. But there is no soundbox there to attach it to. Too bad! Sorrrry.... (The Guitarron balances perfectly because it has a tiny short neck--enough for seven or eight frets-- and a huge soundbox).
SO the only recourse for comfortable playing is to use a strap that attaches at the butt on one end, and behind the nut at the neck end. That's still not good enough, because most good players hate to put their strap there, and it shoves the soundbox over to the right, and you have to be readjusting the instrument constantly. But hey, we're talking hybrid here!
So, that's why when you see death metal and thrash bands displaying their "sensitive" sides on MTV Unplugged (unplugged? HA!), the guy with the nose rings playing the Fred is sitting down! Oh, and his Fred usually has a pretty tight waist, so it won't slide off his knee.
