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NRC & CAC Ratings Explained

The two most important numbers on a ceiling tile spec sheet. Here's what they mean.

NRC — Noise Reduction Coefficient

NRC measures how much sound a material absorbs. It's a single number from 0.00 to 1.00 (and sometimes above 1.00 for baffles and clouds).

  • NRC 0.00 = perfectly reflective. Sound bounces right off. (Like a mirror for sound.)
  • NRC 0.50 = absorbs 50% of the sound hitting it.
  • NRC 1.00 = absorbs 100% of the sound.

How NRC Is Tested

NRC is tested per ASTM C423 in a reverberation chamber — a specially designed room with hard, reflective surfaces. Technicians measure the reverberation time of the empty room, then place the test material on the floor and measure again. The difference tells them how much sound the material absorbed.

The test measures absorption at four specific frequencies: 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. NRC is the average of those four measurements, rounded to the nearest 0.05.

These four frequencies cover the speech range — the sounds you need to control in most commercial spaces. NRC doesn't tell you about low-frequency performance (bass), which matters for music rooms and theaters.

NRC Numbers to Know

  • 0.50–0.55: Standard mineral fiber ceiling tiles (Armstrong Cortega, USG Radar). Adequate for general commercial.
  • 0.65–0.75: Better tiles for offices and conference rooms (Armstrong Ultima, Calla). Noticeable improvement over standard tiles.
  • 0.80–0.95: Premium fiberglass tiles and thick wall panels (USG Halcyon, Ecophon Focus). For spaces where acoustics are a priority.
  • 1.00+: Baffles, clouds, and thick absorptive panels with both sides exposed. Theoretically absorbing more than the area they cover because sound hits edges and wraps around.

CAC — Ceiling Attenuation Class

CAC measures how well a ceiling system blocks sound from traveling between adjacent rooms through the shared plenum (the space above the ceiling).

Here's the scenario: you have two offices side by side. The walls between them stop at the ceiling grid — they don't go up to the deck above. Sound from Office A goes up through the ceiling, across the plenum, and back down through the ceiling into Office B. CAC measures how much the ceiling attenuates (reduces) that sound path.

How CAC Is Tested

Tested per ASTM E1414. Two rooms are built with a shared ceiling plenum. Sound is generated in one room, and the level is measured in both rooms. The difference — adjusted for room characteristics — gives the CAC rating.

CAC Numbers to Know

  • 25–30: Low performance. Sound travels easily between rooms. Avoid if privacy matters at all.
  • 33–35: Standard commercial tiles. Adequate for open offices where privacy isn't critical.
  • 35–39: Better. Suitable for standard private offices.
  • 40+: High performance. Recommended for medical offices, law firms, executive suites — anywhere speech privacy is required.

Important: CAC only matters when walls don't extend to the deck. If walls go all the way to the structure above (full-height walls), the ceiling plenum isn't a sound path and CAC is irrelevant. The wall's STC rating becomes what matters.

NRC vs. CAC — Which Matters More?

They solve different problems.

NRC matters when: The room is too loud or echoey. People can't concentrate because of noise. Speech is hard to understand. You need to absorb sound inside the room.

CAC matters when: Sound is leaking between rooms through the ceiling. People in one office can hear conversations from the next office. You need to block sound paths.

Some spaces need both. A medical office exam room needs high NRC (clear speech for the doctor-patient conversation) AND high CAC (HIPAA privacy — don't let the conversation carry to the waiting room). That's why products like Armstrong Total Acoustics exist — NRC 0.70 and CAC 40 in one tile.

Other Ratings You'll See

  • AC (Articulation Class): Measures how well a ceiling absorbs sound that would travel horizontally across an open-plan office. AC 170+ is recommended. Only relevant for open-plan spaces.
  • STC (Sound Transmission Class): Like CAC but for walls and floors. Measures sound blocking through a partition.
  • LR (Light Reflectance): How much light the tile bounces back. Higher LR = brighter room. Not acoustic, but on every spec sheet.

Bottom Line

If you remember two things: NRC = absorption (quieter room), CAC = blocking (privacy between rooms). Pick tiles with the right numbers for what your space needs. If you're unsure, contact us and we'll help you figure it out.