Most dulcimer makers offer instruments with a choice of plywood or solid wood soundboards. Plywood refers to the method of laminating extra thin sheets of wood and has nothing to do with the quality of the product.
Solid wood soundboards made of sitka spruce, cedar, or mahogany are used on higher priced instruments, which makes sense because solid wood is much more expensive than plywood, and takes more labor to turn it into a soundboard. The jury's still out, however, on whether a solid wood soundboard sounds appreciably better than plywood, especially since the bottom, which really acts as a second soundboard, is usually made of plywood anyway.
Mind you, plywood soundboards are euphemistically called laminatedhardwood veneer boards, a much more elegant description than the material used for tool sheds and doghouses. Let's face it -- if somebody asks you, what your instrument is made of, you wouldn't really want to say, uh...plywood...
Be that as it may, when it comes to modern technology, I have the feeling that instrument makers of old would kill for a sheet of veneer plywood. They would also kill, I'm sure, for central heating and air conditioning, but that's another story.
TO PLY OR NOT TO PLY?
There are two basic designs of dulcimers, both centered around the soundboard. One design favors a closed system, where the body of the dulcimer and the soundboard are glued together -- something like the construction of the harp or the guitar. In the closed design, the soundboard is part of the entire load-bearing structure and acts as a brace.
The second design favors a floating soundboard. In that, the box carries the load of the entire structure, while the soundboard is held in place only by the pressure of the strings on the siderails and bridges. The top and bottom edges of the soundboard are free to vibrate. Also, the soundboard moves independent of the box with temperature and humidity changes and thus has less of a chance to warp.
The floating soundboard gives the dulcimer its characteristic bright sound and volume. On the other hand, instruments with a floating soundboard are usually heavier than those of closed design of the same size.