How to Design a Soundproof room

Seal up all the cracks and holes that can allow air transfer in the space. This might include small holes, gaps in between pipes and air ducts, electrical boxes, or gaps under doors and windows. Latex caulk is a good filler, as well as drywall compound. Heavy duty metal and rubber door sweeps, and thick heavy sound resistant doors and windows.
Add a second layer of drywall attached only with sound absorber adhesive.
Add a second layer of “floated” floor to insulate from sound from below.
Cover up to 80% of the walls with an absorber type material like rockwool (mineral wool)
Cover as close to 100% of the ceiling and corners as possible with rockwool (Roxul) acoustic panels.
Add diffusion and adjust acoustics.

1. Sealing up sources of air transfer.

Sound will leak out anywhere where air can move. Use Caulk or high density expanding foam so seal up any potential sources of air transfer. Look for holes around electrical boxes, pipes, wires, or gaps around windows and doors that can leak out sound.

2. A second layer of decoupled drywall or isolated studs floated against the existing wall. Any extra layers of material of varying densities and vibration dampening factors can increase the level of isolation.

3.Floated Floor

The same principle is applied to the floor. With a second layer of vibration transfer control such as a rubber isolaters and studs, or sound absorption

A decoupled layer of flooring on top of the existing floor serves to muffle any sounds bleeding through. Don’t expect that just because something sounds like a good idea that it will work. A layer of rubber may in fact transmit sound quite well, and a squishy layer of gummy bears would be a perfect insulator. Who knows.

4. Cover up to 80% of the walls with an absorber type material like rockwool (mineral wool)

Covering the walls with anything else will not absorb lower frequencies. Check the aborbtion coefficients over different frequencies, if you add a lot of foam or soft fabric, it may dampen high frequencies, but low frequencies will not be absorbed and a business or hollowness to the sound of the room.