When architects, facility managers, or homeowners look into acoustic treatment, two terms come up again and again: NRC and STC. Both describe how a material or assembly behaves acoustically, but they measure completely different things. Confusing the two can lead to wasted budget, disappointing results, or a boardroom that still echoes despite expensive wall panels. This guide breaks down what each rating means, how it is measured, and which one actually solves the problem you are facing.
What Is NRC (Noise Reduction Coefficient)
NRC measures how much sound a material absorbs rather than reflects. It is expressed as a single number between 0.00 and 1.00. A material with an NRC of 0.00 reflects almost all sound that hits it, like a bare concrete wall. A material with an NRC close to 1.00 absorbs nearly all the sound, like a thick fabric-wrapped acoustic panel. NRC is the rating to focus on when the problem is reverberation and echo inside a room, not sound leaking to the room next door.
How NRC Is Measured
NRC is calculated by testing a material’s sound absorption coefficient at four frequencies, typically 250, 500, 1000, and 2000 Hz, then averaging the results and rounding to the nearest 0.05. This standardized process, defined under ASTM C423, allows different manufacturers’ products to be compared fairly. A panel tested in a lab with an NRC of 0.85 will perform similarly across different rooms, assuming similar installation conditions.
Typical NRC Ratings for Common Materials
Painted drywall sits around 0.05, offering almost no absorption. Carpet over padding reaches roughly 0.30 to 0.40. Fabric-wrapped fiberglass acoustic panels, the kind commonly installed in offices, studios, and auditoriums, typically fall between 0.70 and 1.00 depending on thickness and mounting method. Perforated wood panels with an absorptive backing usually land between 0.55 and 0.85. Knowing these baseline figures helps when specifying materials for a renovation or new build.
What Is STC (Sound Transmission Class)?
STC measures how well a wall, floor, ceiling, door, or window assembly blocks airborne sound from passing through it. Unlike NRC, which is about a single material’s surface, STC applies to a complete assembly, such as a stud wall with insulation and two layers of drywall. It is expressed as a single integer, and higher numbers mean better sound blocking. A standard interior partition wall often rates around STC 30, meaning normal speech is audible but not clearly understood on the other side. A wall built for real privacy typically needs STC 50 or higher.
How STC Differs From NRC
The core difference is direction and purpose. NRC deals with sound staying inside a room and bouncing around, affecting speech clarity and comfort within that same space. STC deals with sound trying to leave a room and enter another. A recording studio needs both, high NRC treatment inside to control reflections, and a high STC wall assembly to stop sound from escaping to the hallway or neighboring unit.
STC Ratings for Common Wall Assemblies
A single layer of drywall on wood studs with no insulation rates around STC 33. Adding insulation and a resilient channel can push that to STC 45 or higher. Double stud walls with staggered framing and multiple insulated cavities can reach STC 55 to 65, which is close to what is needed for a professional recording space or a boardroom handling confidential discussions.
NRC vs STC: Which One Do You Need?
Most real projects need a combination of both. A restaurant with a loud, echoing dining room needs high NRC treatment on ceilings and walls to reduce reverberation and make conversation comfortable. A shared office wall between two tenants needs a high STC assembly so confidential conversations in one unit stay private from the other. Getting this distinction right at the design stage avoids costly rework later, such as discovering after installation that acoustic panels absorbed the echo but did nothing to stop sound from traveling into the next room.
Choosing the Right Acoustic Solution With Akinco
Akinco works with architects, contractors, and facility owners across the region to specify the correct acoustic materials for each application, from absorptive wall panels and ceiling baffles to full partition assemblies rated for speech privacy. Every project starts with understanding the actual acoustic problem, whether it is echo, noise transfer, or both, before recommending a solution rather than defaulting to a single product line.
Frequently Asked Questions
Can a material have both a high NRC and a high STC rating?
Generally no, not in the same single material. High NRC materials tend to be lightweight and porous to absorb sound, while high STC assemblies rely on mass and airtight construction to block sound. This is why acoustic design usually layers different materials for different jobs rather than relying on one product to do everything.
What NRC rating is considered good for an office?
For open plan offices and meeting rooms, an NRC of 0.70 or higher on ceiling tiles and wall panels is generally considered effective at controlling reverberation and improving speech intelligibility.
Does a higher STC always mean better soundproofing?
In most practical situations, yes, but STC ratings are based on lab testing at standard frequencies. Low-frequency sound, such as bass from music or heavy machinery, can still transfer through a high STC wall more than the rating suggests, so low-frequency, heavy applications sometimes need additional mass or decoupling.
