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Hi everyone,
I've built quite a few electric guitars in my time, some semi-acoustics and two acoustics. My experience with acoustics is quite limited, but I would like to start building an Arch-top.
Reading about all types of acoustic guitars, it seems that the main sound generating part of an acoustic is the top and back with very little being contributed by the sides. In fact, many builders make their sides extra thick and heavy so as to reduce sound absorption. And some builders leave their backs fairly thick for better projection. The problem with leaving the back thick though, is that it resonates less and therefore contributes less to the tonal qualities of the guitar.
My question is, why is it that spruce is used for the top but never used for the back? If I wanted a guitar that had increased tonal nuance, wouldn't I build a guitar with as much resonance in the back as I could get? Is it just tradition that dictates these material preferences or something more? For example, I have heard of people using catalpa wood for guitar backs, but I have also heard of people using is for the tops. This is a tone wood that is light in weight and strong, often with a tight grain in later yearly growth. As another example, many semi acoustic and acoustic archtop guitars use plywood maple for both top and back and although most of the tone is achieved from the pickups, it makes me wonder why acoustics adhere so rigidly to the spruce top and maple back tradition. Your experience in building archtops with different tonewoods is welcome.

Every so often, people build with soft woods for the backs and sides. The main objection to me is that the wood is soft, so it is not durable and not resistant to buttons, suspenders, buckles, etc. I am sure that the experienced builders can make the tone good using hardwoods for the back and sides, or I think there would be a lot more all softwood guitars. You also need to have enough strength to resist the tension of the strings.

Aside from the objection mentioned above, from a more general guitar making experience, you want the top and back to have different "tap tones" which may be easier to achieve using dissimilar woods. There is also the beauty of hardwoods which used in less critical sound producing areas attracts people to the instrument. In the past some instruments were built with tops and backs and sides of spruce. Usually the back and sides were veneered over with a hardwood. Some instruments are made with hardwood all the way around, so different combinations can be made to work.
The use of plywood was primarily because it can be molded into shape and is less labor intensive than carving plates. A secondary advantage is that it -is- less resonant, which for an amplified instrument makes it less likely to "feedback" to the amp. Some people use "feedback" for creative effect, but when it is unintended it is not desirable.

The top and back do different jobs. Even if the back were made as light and active as the top it would not contribute as much to the sound. It's facing the wrong way, for one thing, and it's usually up against somebody's pudgy avoirdupois, which doesn't help much.

All of the energy to produce sound comes from the strings, and the top is the only part of the guitar that is reasonably effective as a sound producer that's being driven directly by the strings. Any sound energy that makes it to the back has to cone from the top, either through air pressure changes in the box or through the sides. Since the back cant' be as effective at getting sound out to the room, any energy it gets tends to be wasted. If you look at a sound spectrum of a guitar, strong back resonant modes usually show up as 'dips' in the power output.

The one real exception to that is when the 'main top' and 'main back' resonant modes couple in the 'bass reflex' action in the low range. Here a fairly heavy back with low damping can help. It can act like a 'flywheel, storing energy without having to get to a high amplitude. This can help to pump air through the sound hole at the so-called 'main air' and 'main top' pitches. An active back in this range can enhance the output of the guitar, where at higher frequencies it tends to subtract from the overall output.

Even though they cost power, the higher pitched resonances of the back do seem to help in producing 'tone color'. The trick there is to have fairly strong resonances but limit the amount of power they waste. A heavy back seems to help with that: even if the material has high intrinsic damping it's only losing power if it's moving, so a heavy, stiff back tends to conserve energy. Also, if the back has low damping it's resonant peaks will be narrow in frequency, which helps confine the losses.

It's interesting that the resonant modes of a carved, arched back tend to have lower losses than those of a braced flat back made of the same material. A carved maple back can have losses as low as a braced flat one of Brazilian rosewood.

Long story short; the old boys knew what they were doing; it's hard to improve on traditional designs in any big way. "If it were easy, somebody would have already done it".

Thanks for your replies. I have another question along similar lines, regarding the neck. All the guitars I've built, either have maple or mahogany necks. I used these woods because everyone else does. But I know some people have used Spanish cedar which is a much lighter wood. My question is, is it more desirable to use a denser tone wood for a neck or a lighter one. I understand each wood has a different tone and strength, but is there an advantage to using a denser wood over a less dense one, with regards to sustain or are there light woods that have great tone and sustain too. For example, I know that spruce would be too soft for a neck, but would the tone of it be pleasant? A little insight would be very welcome.

I'm of the opinion that the neck has very little to do with the tone of an acoustic guitar. Not 'nothing', mind you; there is a specific 'neck' resonance that can alter the timbre noticeably in the bass when the pitch of that mode lines up with the 'main air' resonance, but you're unlikely to get that on an archtop anyway.

The reason neck wood is such a topic for solid body makers is simple; it's the most flexible part of the instrument. Resonant modes involving neck bending can cause 'dead' notes, particularly on basses where the bridge is so far down on the top. On many solid bodies that I've checked out the bridge is near the 'nodes' of the lowest two or three resonant modes of the instrument, where the forces from the string can't drive body/neck vibrations anyway. These are generally the only odes that are low enough in pitch and active enough that thy could cause problems. The only other way they could be issues is if the note was being fretted at the most active 'antinode' point of a resonance that has that exact pitch. This in unlikely in general, although it can happen. As I say, solid body basses are another animal. The bridge in close to the lower end of the instrument which is an antinode for every resonant mode, so it's bound to be moving at some pitches. Your only hope then is to either make the the hing so heavy if can't move enough to be a problem, or to 'tune' those modes so that they fall between played pitches.

On an acoustic guitar the top is simply so much easier to move it pretty well preempts and 'neck' mode problems, relegating them to the status of minor issues, if they're noticeable at all. The exception, the couple between the lowest neck-body mode and the 'main air' resonance, actually tends to enhance the toner in the low range. This is because the 'neck' resonance can work with the top to 'pump' air through the sound hole at that pitch. Normally, on most guitars, this mode is down around C below the low E pitch, so it's not in the running, but if the neck is stiff, and the head light, it can actually be up in a useful range. You will see this sometimes on Classical guitars with 12-fret necks made of cedro, and i'ts more common on Flamencos, which often use light weight peg tuners. It can contribute to a 'dark' timbre that is often prized, so it's worth getting, but there are so many things that have to be 'right' that it's hard to get in any sort of controlled way. Note that what seems to count are the stiffness of the neck, and the mass of the head; you need to keep neck stiffness up and head mass down. You can see why the usual 'New Yorker' style arch top is unlikely to get there.