CAC Rating 101: Understanding Ceiling Attenuation Class

What Is CAC Rating?

• CAC Defined: Ceiling Attenuation Class measures how well a ceiling blocks sound
• Higher = Better: A CAC 35 tile performs better than a CAC 25 tile
• Used In: Offices, healthcare, classrooms — anywhere with plenum noise concerns

CAC, or Ceiling Attenuation Class, is a lab-tested rating that describes how much airborne sound a ceiling system blocks between adjacent rooms sharing the same plenum. Unlike NRC, which measures absorption, CAC quantifies the blocking of sound through ceiling tiles and plenum paths. A higher CAC means better speech privacy in open-plan offices, exam rooms, conference rooms, and private offices.

How Is CAC Measured?

• ASTM Standard: Measured per ASTM E1414 in a controlled lab environment
• Sound Source: Uses a speaker above one ceiling and a mic below another
• Final Rating: Converts sound reduction into a single-number value

Testing for CAC involves simulating a plenum space between rooms and measuring how much sound from one side transmits to the other. Unlike STC, which applies to walls and partitions, CAC is specific to ceilings and the airspaces above them. Ratings typically range from CAC 20 to CAC 45, depending on the construction and material density.

Recommended CAC Ratings by Space Type

Space Type	Recommended CAC
Private Offices	CAC 35+
Healthcare Exam Rooms	CAC 40+
Classrooms	CAC 35–40
Open Offices	CAC 30–35 (with masking)
Conference Rooms	CAC 35–45

Rooms requiring privacy, like doctor’s offices or HR meeting rooms, benefit from ceiling systems with a CAC of 40 or higher. For open offices, a lower CAC may be acceptable if sound masking is in use. Acoustical ceiling tiles with gypsum backing or dense mineral fiber construction often reach CAC 35+, while lightweight ACT tiles tend to perform poorly in blocking sound.

CAC vs STC vs NRC: What’s the Difference?

• STC: Blocks sound through walls, partitions, doors, and glazing
• NRC: Absorbs sound inside a room to reduce echo
• CAC: Blocks sound through ceilings and shared plenum spaces

Each metric serves a different purpose. STC and CAC are both about blocking, but CAC is specific to horizontal transmission through ceilings. NRC, on the other hand, helps reduce reverberation but doesn’t stop sound from traveling to other rooms. For high-performance acoustic design, especially in healthcare or education, a balance of all three metrics is essential.

Improving CAC in Ceiling Systems

• Use High-CAC Tiles: Look for CAC ≥ 35 mineral fiber or composite panels
• Seal Gaps: Ensure tiles fit snugly and there are no leaks at light fixtures or air returns
• Backloading & Barriers: Add gypsum board or MLV above ceiling tiles if needed

Most standard ACT (acoustical ceiling tile) systems fall below CAC 30 unless specially engineered. By upgrading to CAC-rated tiles, sealing penetrations, and using backer boards, you can dramatically improve speech privacy. Consider using our Ceiling Panel Coverage Estimator to determine how much treatment is needed for your layout.

Conclusion: CAC Ratings & Ceiling Privacy Go Hand-in-Hand

While often overlooked, CAC is a critical part of speech privacy and code compliance, especially in buildings where plenum transmission is a concern. Whether you’re designing for HIPAA compliance in a medical office or trying to improve productivity in a corporate setting, specifying high-CAC ceiling tiles can make a measurable difference.

To take your designs further, explore our complete set of ceiling acoustic tools and materials, or contact our team for help selecting the right ceiling system for your project.

FAQs: Ceiling Attenuation Class & Acoustic Ceilings