Sound rating systems
STC - Sound Transmission Class -
STC rates the effectiveness of building components - walls, floors, windows, doors etc. - as barriers to airborne sound. The greater the value of STC, the less sound is transmitted through the building component. This rating system applies only to airborne sound, i.e. the sound from sources like voices, TV sets, stereos etc. It does not apply when the source is some object that strikes a surface in the building or when a vibrating object is in contact with the building.
ASTC - Apparent Sound Transmission Class -
ASTC rates the sound isolation between two rooms in a building. It is measured according to ASTM E336, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings. ASTC includes the effects of sound transmission along all paths between two rooms, not just the path through the common wall or floor. Many building codes incorrectly use STC as a criterion for sound isolation.
IIC - Impact Insulation Class
IIC rates the effectiveness of floors at reducing the transmission of impact sound. The greater the value of IIC, the less sound is transmitted through the specimen.