Q: What are CAC ratings?
A: Ceiling Attenuation Class (CAC) ratings are used to determine the sound transmission loss through an acoustical or dropped ceiling. A CAC rating is determined by ASTM test method E 1414 "Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum" and provides a value that is equivalent to an STC value. When designing partitions, the CAC rating of the ceiling should be close to the STC rating of the wall for optimal performance of the system. Fiber glass batts placed 4 feet from each side of the partition wall on the dropped ceiling can increase the CAC 6 to 15 points depending on the system1.
1 Halliwell. R. E. and Quirt, J. D., Controlling Interoffice Sound Transmission Through a Suspended Ceiling, The Journal of Acoustical Society of America,Vol. 90, Sept. 1991.