Facility managers often receive noise complaints. A department head might report that confidential conversations in their office are audible in the hallway. The most common reaction is to purchase acoustic panels or foam to put on the walls.
These materials typically improve the room’s acoustics but rarely address privacy. The conversations remain just as audible outside the room as they were before.
This occurs due to a fundamental misunderstanding between two acoustic ratings: NRC and STC. One rating deals with how a room sounds. The other deals with how a room contains sound.
This article explains the difference between these metrics and illustrates how confusing them can lead to failed projects and wasted budgets.
The Difference Between Absorption and Blocking
Acoustic materials perform two distinct functions. You must choose the material based on the specific problem you need to solve.
NRC (Noise Reduction Coefficient): A measure of sound absorption. Materials with a high NRC rating act like a sponge. They soak up sound waves that bounce around a room. This reduces echo and makes speech clearer for the people inside that space. Examples of high NRC materials include:
- Carpet
- Fabric wall panels
- Acoustic foam
- Standard fiberglass ceiling tiles
STC (Sound Transmission Class): A measure of sound blocking. Materials with a high STC rating act like a barrier. They stop sound from passing through a wall, ceiling, or floor into the next room. This provides privacy. Examples of high STC materials include:
- Concrete
- Drywall
- Mass-loaded vinyl
- SpeechGuard Shield Guard Tile Backers
A Common Scenario: The HR Office
Consider a standard corporate office environment. The HR Director reports that employees in adjacent cubicles can hear sensitive termination meetings.
The facilities team attempts a fix. They install expensive, fabric-wrapped acoustic panels on every available wall surface in the HR office.
The Result: the problem actually worsens.
Wall panels alone can’t effectively control noise transmission. High-NRC panels remove the background echo in the HR office. This makes the Director’s voice crisp, clear, and loud. It adds no mass to the walls or ceiling. The sound waves simply travel through the existing weak points in the drop ceiling and into the open office.
Because the room is now “dead” acoustically, the voice projects even more effectively into the plenum and out to the staff.
My Old Office
I used to work in a standard corporate office building shared by multiple businesses. It had plenty of privacy between rooms: acoustical wall panels, ample space between offices. Unfortunately, the plenum was completely clear for sound transmission. You could hear conversations in adjacent rooms, and in some cases, you got crystal clear audio from across the floor.
This meant that anyone could listen in on client meetings. We could hear conversations between our boss and management, and this led to a lot of speculation without context.
It’s well established that overheard and misheard conversations can lead to confidentiality breaches, morale issues, and office tension. Mass layoffs have been signaled in advance by employees overhearing HR or management conversations, leading to unnecessary suffering and liability.
Complete speech privacy isn’t just a courtesy; it’s an investment.
The Ceiling is Usually the Weak Link
In most commercial buildings, the walls are not the main problem. The problem is the drop ceiling.
Standard acoustic ceiling tiles are designed for absorption (NRC). Manufacturers make them porous and lightweight so they don’t sound like gymnasiums. This improves comfort but reduces sound blocking performance (STC).
Noise travels through the porous tile, into the plenum, and down into the next room. It also leaks through light fixtures and return vents.
The Correct Solution
To help other companies, we had to stop thinking about absorption and start thinking about blocking. We needed to add mass to the ceiling assembly to prevent noise transmission.
The solution involved two steps:
- Adding Mass to Tiles: We designed SpeechGuard Shield Guard Tile Backers to be installed over existing ceiling tiles. This added the necessary mass to block sound without changing the room’s appearance.
- Sealing Fixtures: We installed Light Hoods over the recessed lighting. This prevents noise transmission through the open holes in the ceiling grid.
Once our high-STC & 1.0 NRC products were in place, the sound path was broken. The conversations stayed inside the office.
The Reality of Ceiling Acoustics: Why Standard CAC Ratings Fall Short
The primary adversary of speech privacy in office environments is sound transmission through the drop ceiling into the plenum, where it reflects off the deck and into neighboring areas. While architects often rely on manufacturer-provided Ceiling Attenuation Class (CAC) ratings, these figures can be misleading; they typically test tiles in isolation, failing to account for the acoustic “leaks” created by necessary lighting and HVAC components.
Field testing using the ASTM Noise Isolation Class (NIC) standard, such as studies conducted at Ortho Biotech, reveals that standard assemblies inevitably fall short when these penetrations are ignored. To bridge the gap between theoretical ratings and real-world performance, integrating targeted barriers like tile backers and light hoods is essential, offering a proven solution to the acoustic weaknesses inherent in modern ceiling design.
Key Takeaways for Facility Managers
- NRC is for Echo: If a room is too loud or clattery, use absorption.
- STC is for Privacy: If sound is leaking out of a room, use blocking.
- Do Not Mix Them Up: Adding absorption to a leaking room will not stop the leak.
- Check the Ceiling: Standard ceiling tiles are not sound blockers.
- Add Mass: Privacy requires heavy, dense materials to block sound.
When you receive a noise complaint, ask yourself if the problem is inside the room or between rooms. The answer will tell you which rating matters.