If want Metalica at 120 db and to be quiet outside--full on sound isolation, then separate wall structurs inlcluding Quiet rock (the heavy 160 lb a sheet stuff), floating floor, hung ceiling, windows are possible, but difficult and expensive, double fourier entry studio doors, specialized isolation for all penetrations including electrical and some very inventive HVAC solutions. Build cost would likely be $30k, maybe more depending on location and how complex the HVAC gets. And figure on losing 22" in length and width and close to the same in height to construct what's needed for isolation.
We do recording studies--where it IS Metalica or Ozzy not reproduced--but actually recording and producing music. Sometimes the studio is near a highway--always difficult. Then the same studio is used for a vocalist and it has to be absolutely quiet to hear every nuance--no HVAC noise at all because the ribbon mic will pick it up. The engineering is expensive, the build out is very expensive, but it can be and regularly is done for commercial endevours. It is very rarely done at this level for residential environments, but we have done it for a few clients.
