sitar making
here's an page from a sitar maker that runs through the various parts of the instrument. the thing you're referring to is a tumba (resonator)...there's an outline of the functions it has (or is supposed to have) on the page.
interesting idea for a thread.
thinking about a question that might be relevant to it.
all that's in my head at the moment concerns microtonal tuning systems, whether anyone is using them and what they're doing with them...but this may be more a compositional question than an instrument one (it's both really, but more general)....