How Can I Improve The Sound Transmission Class Of A Wall?

That depends....on many different things.  Some are obvious, some are not. The Sound Transmission Class (STC) is a rating of airborne sound insulation.  The STC is good at rating sound isolation when the sound is a person speaking, but has limitations when used for sound with lots of low frequency energy, like music.  It is important to understand that the STC of a wall depends on the entire wall assembly....how many layers of drywall?...how thick are the studs?...is there insulation in the wall cavity?

However, there are less obvious things that affect a wall's STC.  Here are a few things you may not have realized.

• using light gauge steel studs can give you a much higher STC than using wood studs or heavy gauge metal studs.

• studs spaced 24" on center will perform better (with a higher STC) than studs 16" on center.

• with a double stud wall (two separate rows of studs), placing two layers of gypsum board on each side of the wall (four layers total) is better than placing one layer on each side of each row of studs (same four layers total). Usually the STC will be higher, and low frequency sound isolation will be better.

The STC of individual components of a wall cannot be added together.  The rating of the assembly depends on how the components work together.  When evaluating the STC  of an assembly, remember to consider the entire assembly, not just individual components.