There is often much confusion over the NRC, STC, and CAC of certain products and assemblies as they relate to acoustical properties. NRC stands for Noise Reduction Coefficient and is used to measure the amount of absorption inside a space. STC stands for Sound Transmission Class and is used to measure the amount of sound that is passing between spaces through walls. CAC stands for Ceiling Attenuation Class and is used to measure the amount of sound that is passing through ceilings.
Most acoustical products have a published NRC with the product data. The measurement ranges from 0.00 to 1.00 and measures the efficiency of the material as a sound energy absorber. NRC is derived from an average of absorption coefficients at frequencies on the 1/3 octave band. NRC =( a125Hz + a250Hz + a500Hz + a1000Hz + a2000Hz + a4000Hz)/6. An absorption coefficient is the amount of sound energy that is absorbed at each specific frequency.
Absorption coefficients also range from 0.00 to 1.00. NRC is a relevant measurement when you want to reduce the reverberation within a space. While absorbing sound can help with sound passing between spaces as it reduces the amount of energy at the source, it is not the most relevant measurement as it relates to sound transmission.
Sound Transmission Class is used to measure how much sound passes through a wall assembly. If you are in an office or quiet space that is adjacent to a noisy space, you can often hear that sound passing through the wall. The remedy for this is to construct a partition with a higher STC. STC is calculated in a similar fashion to NRC.
Transmission Loss is the measure (in decibels) of the sound energy that is lost when passing between an assembly at a specific frequency. STC is a formulated rating system measuring the amount of transmission loss of the sound energy lost through a “construction” at the 1/3 band octave frequencies. STC is not associated with specific products as much as it is with the entire assembly. There are other factors that can influence sound passing from one space to another. If there are penetrations in the wall assembly, the sound can “leak” from the source into adjacent spaces. The sound can also “flank” the partition by going above or around and getting into adjacent spaces. This is often the case when the wall assemblies do not extend to the deck. No matter how acoustically sound the wall partition is, the sound will find a way through if it is not adequately sealed. In these situations, it is important to pay close attention to the Ceiling Attenuation Class. CAC is the same measurement as STC, only it is the measurement of sound passing through a ceiling assembly.
To conclude, NRC, STC, and CAC are all important measurements. They each have a role in acoustical design. It is critical that these be taken into account during the design phase. It is much easier and cost effective to account for these needs during the initial design and construction. Acoustical remediation is often times more expensive and less aesthetically pleasing.