Regarding power levels, I would assume that the greater the sound, the greater the power, but I would also consider that the vessel containing the substance would be under stress. Perhaps not of equal intensity, but still increasing with the power level. As a result, truly loud sounds would potentially destroy the vessel, removing the 'filter' effect. Go to Comment
This is a good, simple straightforward item one could use to add spice to a wizard. I would have the 'destruction' or other baleful effect kick in only after a determined effort to open the book in spite of strong resistance.
I can forgive cliche - I tend in that direction from time to time, but the lack of attention paid to spelling and grammar (Again, I am no saint on that front) is somewhat less forgivable. Go to Comment