Ceiling Attenuation Class (CAC) in Acoustic Ceiling Tiles

In the acoustic specification of commercial interiors, there is often a misunderstanding between "stopping an echo" and "blocking a conversation." While the Noise Reduction Coefficient (NRC) handles the former, the Ceiling Attenuation Class (CAC) is the critical metric for the latter.

If you have ever been in a private office and could clearly hear the phone call happening in the room next door, you have experienced a CAC failure.

1. The Engineering Definition of CAC

CAC is a single-number rating that represents a ceiling system's ability to act as a barrier to airborne sound transmission through a shared plenum. In many modern buildings, the interior walls stop at the height of the ceiling grid rather than extending to the structural slab above. This creates a "common attic" or plenum where sound can easily travel over the wall and back down into the adjacent room. CAC measures how much of that sound is blocked by the ceiling tiles.

2. How the Rating Works

CAC is measured in decibels (dB). A higher CAC value indicates a better ability to block sound transmission:

• CAC < 25: Low performance. Standard conversation can be easily understood in the next room.

• CAC 30–34: Moderate performance. Suitable for general office areas where total privacy is not a priority.

• CAC 35–40+: High performance. This is the "Privacy Threshold." At this level, loud speech is reduced to an unintelligible murmur.

3. Why Mineral Wool is the Preferred Material for CAC

Sound blocking requires density and mass. This is where material composition becomes the deciding factor:

• Fiberglass Tiles: These are lightweight and porous. While they are great at absorbing echoes (High NRC), they are essentially transparent to sound transmission (Low CAC).

• Mineral Wool (Mineral Fiber) Boards: These are manufactured by spinning molten rock and slag into high-density boards. This mass makes it much harder for sound waves to pass through, allowing mineral wool tiles to naturally achieve CAC ratings of 35 to 40.

4. Application Guide: When to Spec High CAC

5. Procurement Advice: The "High-High" Ceiling

The most common mistake in procurement is choosing between NRC and CAC. For a truly professional environment, you should seek "High-High" tiles—those that offer both high absorption (NRC 0.70+) and high attenuation (CAC 35+). Dense mineral wool is uniquely capable of providing both, eliminating the need for expensive secondary sound blankets or plenum barriers.