MIMF

Digital pianos are soulless and plastic, real pianos are too heavy (and I don't love them). While I love harpsichords, my wife hates them. So I want to build something like this:
https://www.youtube.com/watch?v=k2FGcggM4Ro
That sounds fantastic, like a cross between a harpsichord and a classical guitar or lute. I also have no need for it to be any louder than a classical guitar.

I have the basics of woodworking down (and I intend to make money building things), and the design looks fairly simple.

There are some fundamental things about acoustic instruments I don't fully understand though, so perhaps someone could explain some things or point me towards some reading?

Are the curves of guitars and lutes and such there for ergonomics? Or do they improve the sound, compared to a rectangle-shaped soundboard? Is there a reason for two bouts on many string instruments besides ergonomics? Is a bigger soundboard generally going to be better, and is there a limit to this?

Why is the bridge slanted on the instrument I linked to? Why is it curved on some others? How do you predict a good bridge location?

How do you predict a good soundhole location?

I've heard that the sides and back of guitars don't contribute much or even interfere with the sound. Would it be a good idea then to have a thick, rigid box with a light flexible soundboard glued on top?

Is it better to have the boards on the soundboard be long, or greater number of shorter boards? The pictures in the video seem to show it done the second way, which sounds wrong to me since it would result in more expansion and more glue.

There's also wood choice. I'd be very happy if I could use inexpensive lumber local to the Arkansas. Certainly I won't be satisfied until I try.

It makes sense that the soundboard should be as light as possible. On the other hand, I have a mahogany steel string guitar that sounds wonderful. And while mahogany can apparently refer to a number of different types of wood, none of them appear to be that light. And it's not that much stronger than say, spruce. I imagine that flexiblity might be a factor, but I'm not sure. Spruce is apparently also unremarkable in that regard. I can imagine board width to be important since glue is stiff, so maybe I should just go with something available in large widths?

Eastern red cedar is both extremely light and flexible, so it's tempting in that regard. Hardwoods tend to look better, so I'm tempted to try something like cherry or soft maple, which on paper sound very similar to mahogany.

Lastly, is there any recommendation for strings? I'm thinking of just doing classical guitar strings. That would limit the length of the lowest strings, but I'm not sure that matters since guitars seem to do just fine with that. Or maybe that would end up with too much tension on the soundboard?

Edit: Another obvious choice for strings occurred to me: nylon harp strings.

Too many questions! You need to do a lot more research, so that you have at least a basic understanding of what you are trying to build. Your questions suggest you haven't got there yet. This means that answers won't be understandable to you, or might even mislead you.

Two things to get you started:

The design is more complex than you think. Consider how pressing the keys is converted into a pluck of the strings at a 90 degree angle. And also how to build the keyboard so the keys don't stick and they return to position when you stop pressing them. And so the strings don't ring out when not being played. I only know enough to know that making all this work is fiendishly difficult!

The bridge shape and location is determined mathematically by the desired pitches of each string, the string thicknesses you have available, the tension needed for each string, and where you plan to pluck it for the best sound. From these you can work out a compromise shape and location, as you can't have it perfect without giving each string its own bridge. If I were trying to work these out I'd start by researching harps.

It looks like a fascinating project though, good luck with working out how to achieve it.

Really cool sounding instrument!

You do need to do a lot of research, but to get you started:
1. Curves - much of the outline is dictated by the string length necessary for each note. Likewise, the slanting bridge reduces the steepness of the top right side to get the strings short enough for the treble end.
2. These instruments are basically lutes, without much of a soundboard. The one you posted is mostly open. Just search Clavicytherium on YouTube and you will find both open and closed back designs, and hear which sounds better to you.
3. If you want a fully topped instrument, copy the soundhole placement of existing instruments rather than trying to design your own.
4. Materials - this guy built one of "discarded wood" https://www.youtube.com/watch?v=J65raxcq6ys I am sure you can make do with local materials. At one point he pulls out a key assembly, so you can get a good look at how it is built.
5. Plans - a person who built #4 wrote: "I got rough dimensions off the Royal College of Music website for keyboards and fudged here and there based on required string speaking length dimensions." You should have a look at their web site.
6. Strings - #4 above used fishing line, but the sound suffers because of it. Harp or classical strings would probably be better. Steven Sorli, who built the instrument you linked to, has some information on his web site about strings, and seems to like the fluorocarbon strings. http://www.lautenwerk.com/lute_and_guit ... r_sale.htm . You might even write to Steven and see if he can offer any advice or source of plans.
5. Construction - this guy has a neat time-lapse construction diary of one https://www.youtube.com/watch?v=B6KE08HBT68

If you do get around to building this thing, by all means please post your progress here!

Thanks for the responses!

I have been doing research, but I've been getting stuck trying to find resources answering the sorts of questions I have. Are there any good books or such on the subject?

Concerning the jack action, I was thinking that the jacks could turn radially, as long as the plectrum was angled right as it crossed paths with the string. passed the string at the right angle. I think that's what's going on in the one made of discarded wood. Apparently many people have been critical of clavicytherium action though, except for one that used a bell crank, which definitely sounds tricky to get working smoothly. So perhaps there's a good reason not to use the simple solution.

It looks like the harp strings run down the center of the soundboard, which is also what I'd expect to be an ideal placement for a bridge since that'd be where the wood can flex the most. I'm thinking that the bridge slant (or downward slant on a harp) is more about keep the jacks (or fingers) in a straight line. And that in the case of the one I linked to, the sound might be compromised by the bridge running to both corners like that?

I was also thinking that these are basically console lutes. The sound on the one I linked to definitely falls short of a lute though, which is why I'd like to understand how to design a better one rather than just copy it.

Just an aside. The instrument in the photo is not actually a clavicytherium. It's a harp-piano. Clavicytheriums don't have an open harp-like frame, but are upright harpsichords, complete with a blind back plate as minimum. Apart from the earliest one known, also a complete soundboard.

Matthew , Did you fine plans for the Clavicytherium ?
Thanks, Larry