NRC, STC, STI and Beyond: A Practical Guide to Architectural Acoustic Ratings

If you have ever been handed a material test report filled with acronyms like NRC 0.80, STC 52, STI 0.65, and fire classification A2-s1,d0, and been expected to make a specification decision on the spot, you are not alone. For architects, specifiers, and procurement professionals without a dedicated acoustics background, decoding these ratings can feel like learning a foreign language — one where the stakes include compliance, occupant comfort, and project budget.

This article is a plain-language guide to the five most important acoustic metrics you will encounter when specifying sound-absorbing and sound-isolating materials. Each section explains what the metric measures, how it is derived, what range is considered good, and how it should inform real-world project decisions. We will anchor each explanation with real test data from certified laboratories so that the numbers are not just abstractions — they are benchmarks you can use.

NRC: Noise Reduction Coefficient — Your Material's Overall Grade

NRC is the most widely cited metric for sound-absorbing materials, and for good reason. It answers a single, practical question: how much sound does this material absorb, on average, across the frequencies most relevant to human speech and everyday noise?

The NRC scale runs from 0 (perfect reflection — think polished marble) to 1.0 (perfect absorption — think an open window). The value is calculated by taking the arithmetic mean of the material's sound absorption coefficients at four standard octave bands — 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz — and rounding to the nearest 0.05. These four frequencies span the core range of human speech and the most common building noise sources, which is what makes NRC a useful single-number rating despite its simplicity.

NRC reference ranges:

The Silence Tech acoustic nonwoven, for example, achieves an NRC of 0.80 under standard laboratory mounting conditions (50 mm air cavity), as certified by Intertek. Considering that the material is just 0.27 mm thick, this is a striking result — conventional fiberglass typically requires fifty times that thickness to deliver comparable absorption.

Practical takeaway: For most public building applications — offices, schools, healthcare, transit — an NRC of 0.65 to 0.85 is more than adequate. Chasing NRC values above 0.90 is generally only warranted for specialist acoustic spaces and almost always comes with significantly higher material and installation costs.

Sound Absorption Coefficient α: The Frequency-Specific Detail Behind NRC

If NRC is the final exam grade, the sound absorption coefficient α (alpha) is the individual subject score. It tells you how well a material absorbs sound at one specific frequency, expressed as a decimal between 0 and 1.

Every porous sound absorber has a frequency-dependent absorption profile. A typical acoustic nonwoven might absorb 30–50% of sound at low frequencies (125–250 Hz), jump to 70–90% in the mid-range (500–1000 Hz), and exceed 90% at high frequencies (2000–4000 Hz). This is why relying on NRC alone can be misleading: if your project's primary noise problem is low-frequency rumble from HVAC equipment or traffic vibration, you need to look at the alpha values in the 125–250 Hz bands, not just the headline NRC number, which averages them out with three other frequencies where the material may perform brilliantly.

Frequency-application mapping:

Practical takeaway: Always ask suppliers for the full absorption coefficient curve (the graph), not just the NRC number. A supplier who cannot provide frequency-specific data alongside the NRC value should raise a flag.

STC: Sound Transmission Class — How Well That Wall Blocks Sound

STC measures the ability of a building element — a wall, floor, ceiling, door, or window — to block airborne sound transmission. Critically, STC is about isolation, not absorption. Absorbing sound within a room (NRC) and preventing sound from traveling between rooms (STC) are two entirely different physical processes, and materials that excel at one may contribute nothing to the other.

The higher the STC, the better the isolation. An STC 25 partition lets you understand most of a normal conversation in the next room. At STC 45, loud speech becomes unintelligible. At STC 60 and above, even loud music is barely perceptible through the assembly.

STC experience reference:

It is worth noting that lightweight porous absorbers like acoustic nonwoven fabric contribute virtually nothing to STC on their own — a sheet 0.27 mm thick will not stop sound from passing through a partition any more than a piece of paper would. The role of absorption materials in sound isolation assemblies is supplementary: they are placed inside wall cavities or floor-ceiling plenums to dissipate the acoustic energy that leaks through the structure, thereby boosting the overall STC of the assembly. But if your core problem is sound bleeding between rooms, you need a sound isolation strategy — not just an absorption upgrade.

Practical takeaway: If the complaint is "I can hear every word from the meeting next door," focus on STC and partition design. If the complaint is "I cannot hear the person across my own table because the room echoes so badly," that is where NRC and absorption materials come in.

STI: Speech Transmission Index — How Clearly People Hear the Announcement

Unlike NRC and STC, which describe physical properties of materials and assemblies, STI describes a perceptual outcome: the intelligibility of speech in a given space. It answers the question, "If someone speaks or a PA system broadcasts in this room, how much of the message will the listener actually understand?"

STI runs from 0 (completely unintelligible) to 1 (perfect transmission). For public spaces — airports, railway stations, metro platforms, emergency announcement systems, large lecture halls — STI is arguably the most consequential acoustic metric of all. When speech clarity fails in these environments, the consequence is not mere annoyance; it can mean missed flight connections, passenger confusion, or, in the worst case, a failure of emergency communication.

STI scale (IEC 60268-16):

Two room-acoustic factors dominate STI performance: reverberation time (RT) and background noise level. The longer the reverberation, the more each syllable bleeds into the next, smearing the speech signal. The higher the background noise, the more the signal is masked. The single most effective way to improve STI is to control reverberation through strategically placed sound-absorbing materials — and this is precisely where acoustic nonwoven fabric, deployed behind perforated ceiling or wall panels, delivers its value.

Consider a typical underground metro platform: ceiling heights of 5–8 meters, walls clad in hard tile and concrete, and reverberation times that easily exceed 2–3 seconds. In such an environment, even the loudest PA system will produce a fog of reflected sound that passengers struggle to decode. By embedding acoustic nonwoven as the backing material in the platform's perforated ceiling system, reverberation times can be brought below 1 second, lifting the STI from "poor/marginal" into "good" territory — a transformation that passengers experience as the difference between a garbled mumble and a crystal-clear announcement.

Practical takeaway: If your project faces complaints about unintelligible announcements, the solution is not to turn up the volume. The solution is to reduce reverberation time and manage background noise. The former is achieved through absorption (NRC, α); the latter through equipment specification and source placement.

Fire Classification: The Non-Negotiable Metric for Public Buildings

Fire rating is unique among acoustic metrics: it is not a question of "better or worse" but of regulatory compliance. In occupied public buildings, a material that fails the required fire classification is disqualified regardless of how well it absorbs sound.

Under the European classification system (EN 13501-1), materials are rated from A1 (non-combustible, the highest) through F (easily flammable, essentially untested). The Chinese national standard GB 8624-2012 follows a similar hierarchy:

National building codes leave no ambiguity. Per GB 50222-2017 (Code for Fire Protection in Building Interior Decoration Design), the ceilings of airport terminals, metro stations, and railway stations must use Class A materials. Hospital wards and operating suites require Class A ceiling materials. Schools and kindergartens are subject to the same Class A requirement. This is what makes A2 fire classification a gatekeeping credential for any acoustic material targeting the public building market.

The Silence Tech acoustic nonwoven is certified to Class A2 under GB 8624-2012, meaning it meets the most stringent fire safety requirements for public building ceilings and walls. For specifiers working on transport, healthcare, or education projects, this certification is not a differentiator — it is a prerequisite.

Practical takeaway: Determine the fire classification requirement at the project kickoff stage, not during material selection. Always request the original fire test report from an accredited third-party laboratory (look for CMA or CNAS accreditation in China, or ISO/IEC 17025 internationally). Vague assurances like "recognized by the fire department" should not be accepted as substitutes for certified documentation.

Quick Reference Matrix

Conclusion: The Metric Serves the Mission

A test report can be dense with numbers, but every specification decision ultimately comes back to one question: what acoustic problem is this project actually trying to solve? Is it material-level absorption (NRC)? Is it speech intelligibility in a reverberant hall (STI)? Is it sound leaking through poorly designed partitions (STC)? Or is it the non-negotiable requirement of passing a fire safety inspection (Fire Class)? Each scenario calls for a different prioritization of metrics. No single material can be the champion of every rating simultaneously.

A practical three-step approach for project specification: 

First, define your primary acoustic problem and the metric that matters most. 

Second, demand both the NRC value and the full frequency-specific absorption curve plus the fire test report from every supplier — do not settle for partial data. 

Third, where feasible, conduct an on-site mock-up test, because laboratory ratings are measured under idealized conditions that never perfectly replicate the complexities of a real installation.

Good acoustic design is not about chasing the highest numbers. It is about understanding what each number means and finding the optimal balance of performance, safety, budget, and constructability for the space at hand.