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Published 2026-02-06 · 9 min read

Understanding NRC and CAC Ratings

The two most important numbers in ceiling acoustics — and what they actually mean for your space.

If you've ever looked at a ceiling tile spec sheet, you've seen NRC and CAC ratings. These two numbers are the primary metrics for acoustical performance in ceiling systems, but they measure fundamentally different things. Understanding the difference — and knowing what numbers to target — is essential for selecting the right ceiling for your space.

NRC: Noise Reduction Coefficient

What It Measures

NRC measures how much sound a material absorbs. Specifically, it's the average of how much sound the material absorbs at four frequencies: 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. These frequencies roughly correspond to the range of human speech.

The Scale

  • NRC 0.00: Perfect reflection — all sound bounces back (like a polished marble floor)
  • NRC 0.50: Absorbs half the sound energy hitting it
  • NRC 1.00: Absorbs all sound energy (like a wide-open window)

NRC values above 1.00 are possible due to the averaging method and diffraction effects at panel edges. Values are reported in increments of 0.05.

How It's Tested

NRC is tested per ASTM C423 in a reverberation chamber — a large, hard-surfaced room with known acoustic properties. Test specimens (typically 72 SF of material) are placed in the chamber, and the change in reverberation time is measured to calculate sound absorption coefficients at each frequency.

What NRC Means in Practice

Higher NRC = less echo and reverberation in a room. When sound hits a high-NRC ceiling, most of the energy is absorbed rather than reflected back. This means:

  • Lower ambient noise levels
  • Reduced echo and reverberation
  • Improved speech clarity (you can hear people talking more clearly)
  • Less noise buildup from multiple conversations (the "cocktail party effect")

Target NRC Values

  • 0.50-0.55: Economy tiles (Cortega, Radar). Adequate for basic commercial spaces, storage, back-of-house.
  • 0.55-0.70: Standard commercial range. Suitable for most offices, retail, education.
  • 0.70-0.85: High-performance. Recommended for open-plan offices, conference rooms, healthcare, classrooms with specific acoustic standards.
  • 0.85-0.95+: Ultra-high absorption. Performance venues, recording studios, noise-critical environments.

CAC: Ceiling Attenuation Class

What It Measures

CAC measures how well a ceiling system blocks sound from traveling between adjacent rooms through the shared plenum space above the ceiling. This is fundamentally different from NRC — CAC measures sound BLOCKING, while NRC measures sound ABSORBING.

Why CAC Matters

In most commercial buildings, walls between offices stop at the ceiling line — they don't extend up through the plenum to the structure above. Sound from one office rises, passes over the wall through the open plenum, and drops down into the adjacent office. The ceiling system is the primary barrier to this flanking sound path.

The Scale

  • CAC < 25: Minimal sound blocking — conversations are easily overheard
  • CAC 25-34: Low to moderate — normal speech is audible but not clearly intelligible
  • CAC 35-39: Standard commercial — normal speech is generally not intelligible. Minimum recommendation for most offices.
  • CAC 40+: High performance — good speech privacy. Recommended for confidential spaces (HR, legal, medical).

How It's Tested

CAC is tested per ASTM E1414. A test setup simulates two rooms separated by a wall that stops at the ceiling, with continuous ceiling and open plenum above. Sound is generated in one room, and the sound level is measured in the other. The ceiling's ability to block sound transmission through the plenum is calculated as the CAC rating.

NRC vs CAC: The Key Distinction

Think of it this way:

  • NRC = absorption within a room. How quiet does the room feel? How much echo is there? NRC affects the acoustic quality of the space itself.
  • CAC = blocking between rooms. Can people in the next room hear you? CAC affects privacy and noise isolation.

These are independent properties. A tile can have high NRC and low CAC (great absorption but poor blocking), or moderate NRC and high CAC (decent absorption with good blocking). The ideal tile depends on your specific needs.

The Common Misconception

Many people assume that a tile with high NRC will also block sound between rooms. It won't — or at least, not as well as you'd think. A fiberglass tile with NRC 0.95 is extremely porous (that's why it absorbs so well), which means sound passes through it relatively easily. Its CAC may be only 20-25.

Conversely, a dense mineral fiber tile with NRC 0.55 may have a CAC of 35+ because the denser material blocks sound transmission more effectively.

Total Acoustics: The Best of Both

Products like Armstrong's Total Acoustics line are specifically engineered to deliver both high NRC AND high CAC. The Total Acoustics 1774, for example, achieves NRC ≥0.70 and CAC ≥40 — combining good absorption with excellent sound blocking. These products use layered construction that absorbs sound within the room while blocking transmission between rooms.

Other Acoustic Ratings You'll See

AC (Articulation Class)

Measures a ceiling's ability to reduce sound reflections that travel across an open-plan office. AC 170+ is recommended for open offices. Higher AC means less distraction from conversations across the room.

STC (Sound Transmission Class)

Similar to CAC but measures the sound blocking of the complete wall assembly, not just the ceiling. Used primarily for wall and floor ratings. STC 50+ is the California standard between dwelling units in multi-family construction.

SAA (Sound Absorption Average)

A newer metric that averages absorption coefficients across twelve frequencies (200-2500 Hz) instead of four. SAA is generally within 0.01-0.03 of NRC for most products. Increasingly used in specifications but NRC remains the more widely recognized metric.

Practical Recommendations by Space Type

  • Open-plan office: NRC ≥0.70, AC ≥170. CAC less critical since no walls to block between.
  • Private office: NRC ≥0.55, CAC ≥35 (≥40 for confidential spaces).
  • Conference room: NRC ≥0.70, CAC ≥40.
  • Classroom: NRC ≥0.70, CAC ≥35 (per ANSI S12.60 recommendations).
  • Healthcare exam room: NRC ≥0.70, CAC ≥40 (HIPAA speech privacy).
  • Restaurant: NRC ≥0.70+. CAC usually not relevant (open plan).
  • Corridor: NRC ≥0.55, CAC ≥35.

Use our NRC Rating Comparison Tool to compare specific products, or our Sound Absorption Calculator to estimate how different NRC values affect your room's reverberation time.

Need help selecting the right acoustic performance for your project? Elite Acoustics Inc provides expert guidance and professional installation throughout Sacramento and Northern California. Contact us for a free consultation.