that's not only what I was taught, when I was working in some recording studios. The fibres in the insulation split the sound waves as it hits them. Reducing it down with each split.
I'm building a house in Poland, it will be insulated and airtight to passive standards. The design is for 200mm EPS and 250mm of concrete(it's a raft foundation) how did the 250mm of concrete heat up in the end