CAC is a laboratory derived rating for sound attenuation of a suspended ceiling system over two adjacent rooms sharing a common plenum. It is used specifically to rate acoustical tile with respect to Sound Transmission Loss between spaces. This rating was instigated to reduce the confusion that resulted from using STC to characterize this type of Sound Transmission Loss. However, only the name is different. All other aspects of this metric remain the same as when the STC rating was used for the same purpose.