What Is Ceiling Attenuation Class (CAC) and Why It Matters

CAC Defined

Ceiling Attenuation Class measures how well a ceiling tile blocks sound from passing through it. Specifically, it measures the tile's ability to reduce sound transmission between two enclosed rooms that share a common plenum (the space above the suspended ceiling).

Think about a typical office building: you have a conference room next to a private office. The demising wall between them goes up to the ceiling grid but stops there — it doesn't extend to the structural deck above. Sound from the conference room hits the ceiling tile, passes through it into the plenum, travels across the plenum, passes through the tile in the adjacent room, and arrives as audible speech. CAC measures how much the tiles reduce that sound along the way.

How CAC Is Tested

CAC testing follows ASTM E1414 (Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum). The test setup creates two rooms separated by a wall that goes to the grid but not the deck. A sound source plays in one room, and microphones measure the sound level in the receiving room. The difference between the two levels, adjusted for room acoustics, gives the CAC number.

The test includes the ceiling tile, grid system, and plenum depth. It's a system rating, not just a tile rating — changing the grid or plenum depth can change the CAC result.

What the Numbers Mean

CAC < 25: Poor. Speech is clearly audible through the ceiling. Standard economy tiles fall in this range. Not acceptable for any space requiring speech privacy.
CAC 25-29: Fair. Loud speech may be partially intelligible. Marginal for private offices.
CAC 30-34: Good. Normal speech is generally not intelligible but awareness of conversation exists. Acceptable for most offices when combined with sound masking.
CAC 35-39: Very good. The standard for commercial office speech privacy. Loud speech may be faintly audible but not intelligible. Most specification requirements call for CAC 35 minimum.
CAC 40+: Excellent. Even raised voices are effectively blocked. Required for medical exam rooms (HIPAA compliance), executive offices, and HR/legal conference rooms.

CAC vs NRC: Different Jobs

NRC (Noise Reduction Coefficient) measures how much sound a tile absorbs — it controls the noise level within a room. CAC measures how much sound a tile blocks — it controls noise between rooms.

Here's the confusing part: the same tile properties that make NRC high can make CAC low, and vice versa. A highly porous, absorptive tile (high NRC) may also let sound pass through easily (low CAC). Dense, heavy tiles block sound well (high CAC) but may not absorb as much (moderate NRC).

The best tiles for offices balance both — NRC 0.65-0.75 and CAC 35+. Products like Armstrong Ultima (NRC 0.70, CAC 35) and USG Halcyon (NRC 0.75, CAC 38) are designed for this balance. Read our complete NRC and CAC guide for deeper detail.

When CAC Matters Most

Private Offices

Any enclosed office where confidential conversations happen — HR, legal, executive, finance — needs CAC 35+ ceiling tiles. Without high-CAC tiles, walls that stop at the grid are acoustically bypassed through the plenum. It's one of the most common acoustic complaints in office buildings: "I can hear everything from the next room."

Conference Rooms

Conference rooms are louder than individual offices (more people, louder speech, speakerphones) and often sit adjacent to other occupied spaces. CAC 35+ minimum. If the conference room shares a plenum with a quiet workspace like a private office, consider CAC 40+.

Medical Facilities

HIPAA requires "reasonable safeguards" for patient health information. In practice, exam room conversations shouldn't be intelligible from the corridor or waiting room. CAC 35+ is the standard for exam rooms and offices. See our healthcare ceiling guide.

Courtrooms and Government

Jury deliberation rooms, judge's chambers, and sensitive government offices need CAC 40+ to prevent confidential speech from leaking. Read about government ceiling requirements.

When CAC Doesn't Matter

Open-plan offices don't have enclosed rooms, so there's no plenum path between rooms. CAC is irrelevant — focus entirely on NRC. Same for lobbies, corridors, and any open space. Warehouses, gymnasiums, and spaces with no shared plenums can ignore CAC entirely.

Improving CAC Beyond Tile Selection

High-CAC tiles are necessary but sometimes not sufficient. Additional measures for spaces requiring maximum speech privacy:

Extend walls to the deck: The most effective solution. If the wall goes all the way up to the structure, sound can't travel through the plenum. This eliminates the plenum flanking path entirely.
Plenum barriers: A sheet of drywall or mass-loaded vinyl installed above the ceiling grid between rooms. Cheaper than full-height walls but effective.
Sound masking: Electronic systems that add a low-level background sound (like gentle airflow) to raise the ambient noise floor, making speech less intelligible even if some sound gets through. A complement to high-CAC tiles, not a replacement.
Seal penetrations: HVAC ducts, electrical conduits, and plumbing that cross between rooms through the plenum create additional flanking paths. Acoustic wrapping and duct lining help.

How to Specify CAC

In your specification, call out CAC as a performance requirement:

"Acoustical ceiling tiles shall have a minimum Ceiling Attenuation Class (CAC) of 35 when tested per ASTM E1414 in enclosed offices, conference rooms, and exam rooms."

Some specs reference the older test standard (ASTM E413) or the term "AC" (Articulation Class). CAC has replaced AC in current standards, but the numbers are similar. Read our guide to reading spec sheets for more on interpreting acoustic data.