It sound like the tutorial is saying (a video link with a time stamp)
soundproofing as i say it's all about density. it's also about transmission of sound through solid surfaces. so any two surfaces that touch each other sound will very easily transfer between one object that's making a racket through another object that can almost amplify the sound. so floors are a big problem with drum kits, because the floor acts like one giant amplifier ...
Is this true, the floor acts like one giant amplifier? If yes, which acoustics rule is working?