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.
How STC is measured.
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.
Other ratings for airborne sound
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.
How IIC is measured
Impact insulation class is strongly influenced by the finish layers on the floor - carpet, vinyl, parquet, etc. This especially so when there is a layer of concrete or gypsum concrete on the subfloor.
Impact insulation can also be measured in buildings. The test procedure is essentially the same but the rating is called Field Impact Insulation - FIIC
Sound insulation ratings similar to those obtained in a laboratory are only achieved in buildings when careful consideration is given to the design of the junctions between walls and floors so flanking transmission is minimised.