(1) The piece of wood is held between two screw holes with low amount of pressure (shown clearly in the first picture). I don’t really know if it actually touches the internal body, but shaking the instrument won’t make a noise so I guess that it does. It sits directly near the opening of the key pads, where most of the sound is projected from (I did an experiment on the inner body to determine where most of the sound is projected from).
I had several experiments putting it in different places where it either didn’t fit, or interfered with the key pads opening.
(2) Although I think that THE flaw of Yamaha’s design is its stupid moisture release (inside the box), I actually found a way to let it out fast, keeping it very dry internally (more on the Yamaha P37D review I posted recently). Therefore, I estimate that moisture will probably cause no real damage the wood.
(3) In contrast, the reed chamber is very wet. Putting fabric of any kind in it will probably cause more standing moisture there, which may lead to future problems. Even if that wasn’t a problem, I think that the extremely tight air chamber won’t leave room for any additional materials in it. One wishing to mute the Yamaha may use my technique with a fabric instead of wood (in the relatively dry area). Anyway, I don’t see any reason to reduce the size of the air chamber; I really love the current design, it’s REALLY small! 🙂
As for the vibration, in the first test I made (putting the naked instrument on the table), the table vibrated very well. I guess that it’s basically because the wood in it vibrates (wood is a terrific medium for sound transfer, unlike plastic), and therefore having more moving material generates more momentum.The vibration that I noticed is one that I like, it reminds me of the vibrations of the Pro 36, which has wooden air chamber. Definitely not too much, and gives terrific feedback to the musician.