CAC vs STC

Below is a table showing the difference between the CAC test and the STC test. As shown, the CAC test is specifically designed for ceiling materials. The physical set-up of the test is designed to standardize and approximate the situation in most commercial building applications. “Cross-talk” is a common problem in most commercial buildings because typical interior walls do not extend to the roof-deck. Noise is allowed to flank over interior walls and be reflected down into adjacent rooms. Ceiling tiles with high “CAC” numbers are designed to stop noise from doing this. The problem with most acoustical ceiling tiles is that they try to be two things. The primary function of an acoustical ceiling tile from an acoustic point of view is to provide absorption in a space, thereby cutting down on echo and reverberation. A secondary function would be to stop noise from going from one room to another. These are two distinctly different functions. Acoustically absorptive materials are typically relatively light and porous. Acoustic barriers are typically hard and reflective, the exact opposite of a good absorber. Any homogeneous material trying to function as both is invariably going to be a compromise.

To achieve both good absorption numbers and high CAC numbers, a composite is required. An absorptive material with a backing of material designed to be an acoustic barrier. Our typical AC-100B is a ceiling tile designed to do both. Either a 1” thick (.85 NRC) or 2” thick (1.15 NRC) layer of 6 lb./sq. ft. fiberglass laminated to a 1/8” thick loaded vinyl noise barrier with an STC rating of 26 by itself. Coupled with a 1” layer of fiberglass it achieves an STC 29, or a 2” layer STC 32. STC rating being a much more direct method of measuring a material’s ability to stop noise in a direct path. The CAC test is specific to room-to-room transmission in a commercial setting. The AC-100B ceiling tile is used as much for that as it is to solve an HVAC noise problem, where noisy ductwork, VAV boxes, or other types of air-handling units might be interfering in an office setting. In those cases, a material’s ability to stop noise in a straight line is most important. A material that is easy to compare to ceiling tiles is gypsum board. Often used in metal ceiling applications to provide a higher STC, we know that a ½” layer of drywall achieves an STC rating of 26, while weighing in at approximately 1.8 lbs./sq. ft. Even ceiling tiles with a high CAC number typically weigh significantly less than that as shown below. 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 we know. The typical High NRC/High CAC panel weighs only 1.25 lbs./sq.ft., translating to a much lower STC rating than the drywall, another rigid, homogenous material. While we can’t assign a specific CAC number to our AC-100B, we can see that it will be significantly higher than the 40 exhibited by the Typical High NRC/High CAC Tile, while offering an NRC value of either .85 or 1.15.