Sound Transmission Class (or STC) is a test rating of how well a building partition reduces sound. STC is the most common sound reduction measurement which is widely used to rate interior and exterior walls, ceilings, floors, doors, and windows. The STC rating figure roughly reflects the decibel reduction in noise that a barrier can provide. In short, STC gives you an idea of how much sound a structure might stop.
STC is calculated by taking the Transmission Loss (TL) values tested at 16 standard frequencies over the range of 125 Hz to 4000 Hz which are then plotted on a graph. The curve created by data points on this graph are then compared to standard STC reference curves. The rating of the curve from the standard STC reference that matches most closely to the curve from testing is how the STC rating is determined.
STC ratings are the only way to accurately compare noise reduction capabilities between different products.