STC (sound transmission class) is an overal average at all frequencies:
Concrete wall (290 mm): PT of 33 at 125 hz, 41 at 250 hz and an overall STC of 50. It's pretty good, but makes up its STC by being good on high frequencies, not really at low frequencies.
Drywall (2 x 12 mm): PT of 19 at 125 hz, 26 at 250 hz and overall STC of 31.
And now the best buys:
Concrete (90 mm) + AIR (yes, air) (25 mm) + Fiberglass/Glass wool (65 mm) + Concrete (90 mm) + Drywall (16 mm): 49 at 125 hz, 54 at 250, and an overall STC of 62.
Drywall (2 x 12 mm) + 40 mm of air + 50 mm of fiberglass + 2x12mm of Drywall: 34 at 125 hz, 47 at 250 hz and overall STC of 55. Less functional than the concrete combo, but as you may see, a lot, lot cheaper.
If you can build the walls, ideally, you'd need concrete + air + insolation + concrete + drywall. And that only covers what goes in and out of the room.