Ceiling acoustical performance can be described by at least four different standards, CAC, NRC, STC and AC.

CAC (Ceiling Attenuation Class) is a measure of reduction in sound transmission via plenum path between two adjacent rooms, determined in accordance with ASTM e 1414, and plotting decibel reduction (transmission loss) obtained at 16 frequencies against a standard reference curve, in accordance with ASTM e 413.

NRC (Noise Reduction Coefficient) is a measure of sound absorbed by a material. The single number designation represents the average of the sound absorption coefficients of a material at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz rounded to the nearest 0.05 when tested in accordance with ASTM C 423. A minimum NRC is considered to be 0.65.

AC (Articulation Class) rates a ceiling's suitability for achieving normal speech privacy in open office spaces as a function of noise absorption and reflection into adjacent cubicles. AC is the primary measure of acoustic performance in open plan offices and is covered in ASTM E1110 and E1111. Ceilings with an AC above 170 are preferable.

The Sound Transmission Class (STC) is a single-number rating of a material’s or assembly’s sound barrier effect. Higher STC values are more efficient for reducing sound transmission. For example, loud speech can be understood fairly well through an STC 30 wall or ceiling, but should not be audible through an STC 60 wall. The rating assesses the airborne sound transmission performance at a range of frequencies from 125 Hertz to 4000 Hertz. This range is consistent with the frequency range of speech. The STC rating does not assess low frequency sound transfer. Special consideration must be given to spaces where the noise transfer concern is other than from speech, such as mechanical equipment or music.

Even with a high STC rating, any penetration, air-gap, or “flanking” path can seriously degrade the isolation quality of a wall. Flanking paths are the means for sound to transfer from one space to another other than through the wall. Sound can flank over, under, or around a wall. Sound can also travel through common ductwork, plumbing or corridors.

While there is no direct conversion figure between CAC and STC, we can draw some conclusions using the weight per square foot and the STC and CAC ratings: