The direct answer
NRC describes how much sound a material absorbs inside the room. CAC describes how well a ceiling system blocks airborne sound travelling up through a shared plenum into the next room. STC describes how well a wall or partition blocks airborne sound between rooms. Absorption controls echo and reverberation; CAC and STC control isolation. One does not substitute for the other.
Absorption versus isolation
Noise Reduction Coefficient (NRC) is a single-number average of how much sound energy a material absorbs, tested in a reverberation room per ASTM C423. A higher NRC means the surface reflects less sound back into the space, which shortens reverberation and helps speech clarity. It says nothing about what passes through the panel to the room next door.
Ceiling Attenuation Class (CAC) rates a ceiling system as a barrier to airborne sound travelling up through the panel, across a common plenum and down into an adjacent closed room. Sound Transmission Class (STC) is the comparable barrier rating for a wall or partition. Armstrong describes STC as the wall equivalent of CAC.
Why the distinction matters on drawings
A common open-office ceiling is chosen for high absorption, and that is often the right call for reverberation. The same panel may perform poorly as a barrier when partitions stop at the ceiling line. Manufacturer guidance is explicit that a high-NRC ceiling absorbs sound inside the space but cannot block sound leaving or entering it.
Where privacy matters, the ceiling, the partition and the background sound level work together. If a wall stops at the grid, the ceiling CAC becomes part of the privacy path. If a wall runs to deck, the plenum path changes. Penetrations, return-air openings and light fixtures can also weaken a barrier, so the drawings should show how the assembly is meant to work as a whole.
Performance targets for a specific room come from the design team, an acoustical consultant where engaged, and any applicable codes or owner standards. Manufacturer published values apply to tested assemblies; field results depend on installation and adjacent construction.
Decision points before specifying
- Is the goal reverberation control, privacy between rooms, or both?
- Do partitions stop at the ceiling or extend to the deck?
- Are both NRC and CAC listed for each panel type where privacy is a concern?
- Is the specified product rating tied to a tested assembly that matches the drawings?
- Have return-air paths, fixtures and penetrations been reviewed for the barrier path?
- Who is responsible for confirming room-level acoustic targets?
Common coordination mistakes
- Specifying NRC alone for offices or exam rooms where speech privacy is expected.
- Treating CAC and STC as interchangeable numbers rather than ratings for different elements.
- Assuming a product rating guarantees a field result regardless of partitions and penetrations.
Apply this to your project
These city guides share a related planning emphasis. They are topic connections, not claims about local conditions, offices or completed projects.
- Loveland, ColoradoThe Loveland guide stresses settling acoustic intent before installation, which starts with knowing whether a room needs absorption, isolation or both.
- Salt Lake City, UtahA Salt Lake City basement, gathering room or reused interior needs a clear distinction between echo within the room and sound passing to adjacent spaces.
- Fort Collins, ColoradoIts planning emphasis on how people use a space maps directly to choosing between reverberation control and privacy.
- Denver Tech Center, ColoradoWorkplace planning there pairs open areas, focus rooms and conference spaces, each asking for a different rating mix.
- Cheyenne, WyomingThe Cheyenne guide centers on defining acoustic goals clearly, which is easier once the three ratings are separated.
- Laramie, WyomingPlanning around how people gather and talk raises the same absorption-versus-privacy question.
Sources
- Acoustical Design & Solutions GuideArmstrong World Industries
- Guide specification: On-Center Linear Lighting Acoustical Ceiling Integration (Section 09 50 00)Armstrong World Industries
Manufacturer documents describe their own products and tested assemblies. Confirm requirements for your project with the approved documents, the manufacturer and the responsible design professionals.