STC is sound transmission class - the ability of, say, a wall or roof to stop sound from "seeping" through. The number is equivalent to a dB figure. Thus, if there is sound coming from the other side of the room and it is measured to have a 100 dB intensity, and your walls have an STC of 60, the resultant "seepage" would be about 40 dBA which is almost inaudible. Maybe you are confusing STC with NC or Noise Criterion. NC is the measured result on how quiet the room is, with the air con, etc. on. For recording studios, an NC30 is acceptable, but the better ones should have an NC 20 or maybe 25. The lower the STC number, the better the noise or ambient performance - meaning, quieter. A single layer of CHB 6 inches, filled, plastered and all side connections totally caulked and sealed, would give you an STC of 45 to 50. A 6 inch CHB wall with a .90 meter air gap, then another CHB wall 6 inches, will give you a very high STC. But you have to do this on all 4 walls, ceiling and flooring to achieve the very, very high STC rating (would you have the real estate space?). Your problem also is the weight of CHB. If you are on the ground floor, it is fine. But if you are on the second floor, you have to consult your Structural Engineer if putting heavy CHB walls is acceptable to the weight load capacity of the building.