Some more on vibration isolators, a floating floor and other stuff to ponder if one wants to DIY an anechoic room. A Master Degree Project: "Design of a Fully Anechoic Chamber". Quite easy and plain language, to get a grip of what matters.
http://www.diva-portal.org/smash/get...FULLTEXT01.pdf
(I'll pass though. My wallet is too thin for what is needed; 18 metric tons of insulation and 368 metric tons of concrete.)