Gypsum, which consists of hydrated calcium sulfate, is what makes up drywall. To soundproof drywall, layer plastic polymers, glue, metal or ceramic materials in with the gypsum to stiffen the drywall, lowering the vibrational frequency of the wall and decreasing outside sound.
Most drywall is solid, but it is easily penetrable and susceptible to vibrations. Stiffening gypsum with other materials makes the wall more rigid and constrained, which thereby dampens the sound and reduces the outdoor volume. Sound transmission class (STC) rating is the official measurement of soundproofing. The rating measures how much sound passes through the drywall of a wall. To pass as "soundproof," drywall must have a rating of at least 40. At levels 50 and 55, two people could not hear the other speak on either side of the wall.
Moreover, rather than installing drywall with additional layers, soundproofed drywall sells at a higher price per sheet. It cuts in the same way as standard drywall and comes in 4 feet by 8 feet sheets. The standard sized sheets come thicker and denser, which result in a more difficult installation because the electrical outlets are more strenuous to maneuver around and the attached studs create less space.