Both will block high frequencies. MDF is a much better sound blocker than plexiglass, higher density.
MDF is much heavier and blocks IR signals that the plexiglass passes.
MDF has a relatively high STC (sound transmission class) which is a measure of its ability to block sound, but a low NRC (Noise Reduction Coefficient) which represents its ability to absorb sound. This means that a sheet of MDF or plexiglass under your projector will reflect the sound back at the ceiling and projector.
Adding a layer of 1" of fiberglass duct liner (like Johns-Manville Linacoustic) which has an NRC of 0.70, (on average it will absorb 70% of the sound that hits it) will do a better job than either material alone. Fiberglass ceiling tiles, especially doubled up, with the fiberglass side exposed to the sound can substitute for the duct liner.
A forum community dedicated to home theater owners and enthusiasts. Come join the discussion about home audio/video, TVs, projectors, screens, receivers, speakers, projects, DIY’s, product reviews, accessories, classifieds, and more!