Actually he explains principle in other part.
It is physics, and not "sound like plastic" statements.
Well, for cheap & quality nothing beats bonded logic...problem with blankets (well, we all use blankets sometimes) is that they act as a low pass filter.
On the cheap it is better off using moving blankets than acrylic sheets.
It's so funny that someone would think that putting a big acrylic sheet over a window will improve the sound. It will keep some of the noise out, sure, but your studio will sound like plastic. Because it is.
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