STC, or sound transmission class, refers to a system of measurement of an assembly's (wall or floor/ceiling, for example) ability to reduce sound transmission, i.e. to make it quieter. Sound is measured in decibels (DB). A wall with an STC rating of 50 (the minimum required between live/work units) would reduce a 70 DB noise source to 20 db, i.e. by 50 db.