I am currently making two types of mandolins. My centered soundhole instruments have a single transverse brace, as did the pre-Loar Gibson A’s and F’s. My offset soundhole instruments have parallel tone bars, which were developed by Lloyd Loar in the early 1920’s.
When I rough-carve my instruments, I carefully select and consider each piece of tonewood. The design of each soundboard is adjusted based on my best guesses about the properties of that particular piece. I back up those guesses with hard measurements like thickness, weight, tap tone, and wood hardness. But it’s mostly just a guess until the plate is rough carved.
I use Sitka spruce primarily, and I think it is an amazing tonewood for mandolins. I also use redwood and yellow cedar, depending on what I’m trying to achieve with the instrument.
Centered oval-hole instruments have a very different soundboard design. A thick core section runs from the neck to the tailpiece, and this center area ranges from .200″ to .300″ thick before fine-tuning. The side areas do need to be more flexible than the center, and those range from .120″ to .170″, depending on the tonewood. This is what it looks like:
The more modern tone of parallel-braced, ff-hole instruments is largely preferred by players. This design is very close to “Loar spec”. I don’t call it “Loar spec”, though, because carving mandolin tops that sound good requires a lot more than mere specifications!
I don’t generally use traditional ff-holes, but I try to use sound ports that function similarly. Tops made in this style are typically between .100″ and .120″ at the recurve, and up to .200″ in the crown.