STC stands for Sound Transmission Class. It’s a way to rank how much sound a building partition—like a wall or ceiling—blocks sound traveling through the air. STC is used in the United States while it’s cousin, the SRI (Sound Reduction Index), is used elsewhere. You can think of the STC rating as representing approximately how many decibels the partition can drop the noise. So, a higher STC number means more sound blocked. Although this is a rule of thumb, it ...