Can somene please explain a mass/stiffness/energy scenario?

Halo: Can someone try to explain what is going on with this scenario: If you have seen my earlier posts on the 'studio-room in boston area' thread, you noticed that I 'sistered' the joists of the floor ABOVE where the studio will be.... I did this because I will be adding mass in those joist bays for the outer-leaf ceiling of the studio, and I wanted to strengthen them to ensure safe support... What I have noticed is this: When I knock on the doubled joists, I hear a subdued thud... When I knock on the joists that have NOT been sistered, I hear a much higher pitch and a 'ring' type reverberation... There is very little or no reverberation at all with the sistered joists.... What's up with this?... What I'm thinking is that the mass of the joists is doubled, and so the pitch should be halved?... But of course this is a simplistic model, since the floors are also stiffened (deflection decreased) although the floor loads are still about the same... But again, if things are stiffened, then that would mean an increase in pitch, no?... Yet, what I'm hearing seems to indicate a lower pitch (maybe the stiffeness increase is not as great as the doubling of mass increase?) So, I'm also thinking that the vibration of the joists is of less energy -- not just a lower pitch... Am I on the right track here? Is this good or bad.... or a mixed bag? If the pitch is lowered, it could be bad because higher pitches are easier to block? Although if the energy of setting it in vibration is lowered, maybe it balances out? Gonna see if I can verify the pitch and damping a little bit more closer; maybe I'm mistaken..... thanx, K