STC stands for "Sound Transmission Class", a measure of the extent to which sound is prevented from being transferred from one area to another. The higher the STC value, the less sound transferred from one area to another.
The STC scale is a logarithmic progression, meaning that a jump of 3 points in an STC rating equates to a doubling in a door's ability to prevent sound transmission. Acoustical doors are tested as an assembly comprised of the door, frame, hinges, and sound sealing system. See SDI 128 for more information.
The following chart illustrates the sound retardant performance associated with a range of STC values.