It's the weight of the combined materials that does the fundamental isolating trick and these two combined materials don't have two resonances but a new single resonance frequency.
So a 12.5 mm gypsum board combined with a 15 or 18 mm MDF board doubles the weigth of one of the panels alone.
MDF will be slightly stiffer than gypsum, but I don't think you're going to notice this.
Mass is starting to isolate somewhat above it's materials resonance frequency. Under that frequency you can even get amplification because of resonance.
The Green Glue between the layers of MDF, gypsum of the combination works as a constrayned layer damper; a material like glass, MDF etc has a frequency region where bending waves are moving along the surface of the material. These waves don't have, as normal soundwaves in air, a fixed velocity. When the velocity of the wave in the material equals that of the speed of sound in air, you get resonance. This so called coincidence frequency generates a dip in the sound isolation.
GG damps this effect (as does PVB foil between window planes, etc.). GG does this very effective.
So mass is isolating from the resonance frequency up, and a CLD (impedance jump) does this for the higher coincidence dip.
Using for example MDF, a layer of softboard, and MDF to improve isolation is a bit of urban myth and will not yield substantial better isolation than the same weight of MDF or chipboard etc.
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