I am designing a home that will have a hybrid system of timber framing and conventional stud walls. The Second floor will be framed with timbers for the floor assembly and have a exposed tongue and groove subfloor that will be the finished ceiling for the first floor and possibly the finished floor on the second floor. My concern is that the sound transmission from the rooms above having hard wood floors will be a problem for the rooms below. It seems that the system I described must be encountrered frequently in timber framing. Is the sound transmission a problem?

Caleb,

Sound transmission can definitely be a problem in the situation you described. Basically, it depends on whether hearing people above you bothers you. There are a number of solutions, both in the layout of the floorplan, as well as in construction details. Some 2x's and an airspace help a lot, or you could put soundboard in the cavities. This is also a convenient place to run pipes and wires, etc.
Good luck with your project

Gabel Holder

Gabel-

Could you give more information on the sound board: thickness? manufacturer? cost? typical installation? thank you.

I agree that sound transmission is a problem but not too bad if the floor is 1.5" thick or more.
I've used 2x stripping and then fiberglass fill(tearing up conventional fiberglass insulation) with good results in this situation before, i would NOT recommend leaving empty spaces as i feel it would be worse than one layer of wood (like beating on a drum).
-Mark in Ontario.