STC vs. Rw vs. NIC: How Sound Insulation Is Actually Rated

STC (US) and Rw (European/ISO 717-1) are both single-number lab ratings of how well a partition blocks airborne sound, close to each other but not identical; NIC is a field-measured rating of the isolation actually achieved between two real rooms, and it typically comes in around 10 points below the lab STC of the same partition because it captures doors, gaps, flanking and workmanship that a lab test excludes. If you’re comparing a US spec sheet to a European one, or wondering why an “STC 50” wall doesn’t sound like STC 50 once it’s built, the answer almost always comes down to which of these three ratings — plus their close cousins DnT,w, IIC and L’nT,w — you’re actually looking at.

In our project work we see this confusion constantly: an architect on an international project reconciling a European glazing manufacturer’s Rw datasheet against a US-market STC requirement, an owner convinced their “STC 50” operable wall is failing because it doesn’t isolate like the lab number promised, or a facility manager who has never heard of NIC being handed a field report that quotes it instead of the STC they expected. None of these are errors in the ratings themselves — they’re errors in treating different rating systems, measured under different conditions, as if they were the same number. This guide sets out exactly what each rating measures, where each is used, why lab performance and field performance diverge, and how to compare products fairly when a project spans both the US and European systems — or spans a lab datasheet and a real, occupied building.

What is STC, and what does it actually measure?

STC (Sound Transmission Class) is a single-number laboratory rating of how well a partition — a wall, floor/ceiling assembly, door, or window — blocks airborne sound from passing through it. It is measured under ASTM E90 (the test method, run in a certified two-chamber lab with the partition mounted in the opening between them) and rated per ASTM E413 (the calculation that reduces the partition’s per-frequency transmission-loss curve to one number by fitting it against a standard reference contour). Higher STC means less airborne sound gets through.

As a rough anchor for what the number means to a listener on the other side:

STC is the dominant rating system in the US and much of the Americas, and it’s the number quoted in the IBC’s Group R occupancy requirements — walls and floor/ceiling assemblies separating hotel guest rooms, apartments and similar sleeping units must meet STC 50 minimum in the lab. We cover that requirement, and the practical margin premium hotel brands build in above it, in hotel sound insulation and the STC rating guest rooms need. Critically, STC is an airborne-only metric: it says nothing about impact noise (footfall), and — a limitation worth remembering for the rest of this article — the ASTM E413 rating curve only extends down to 125 Hz, so it can miss low-frequency energy that a listener still clearly perceives as bass thump or rumble.

What is Rw, and how is it different from STC?

Rw (Weighted Sound Reduction Index) is the ISO/European equivalent of STC, standardized under ISO 717-1. Conceptually the two ratings do the same job: both take a partition’s measured transmission-loss curve across a range of frequency bands and reduce it to a single index number by comparing it against a reference contour, and both are lab-only measurements taken under controlled, standardized test-chamber conditions. For most everyday partitions, Rw and STC land within a point or two of each other — close enough that they’re often discussed loosely as interchangeable.

They are not, in fact, identical. STC (ASTM E413) and Rw (ISO 717-1) use different reference curves, different frequency ranges, and different weighting/fitting procedures, so a partition rated Rw 52 will not necessarily test at exactly STC 52 if it were run through the other standard’s calculation. On an international project — a European manufacturer’s glazing or wall-system datasheet being specified against a US-market STC requirement, for example — treating the two ratings as interchangeable to the decimal is a common and entirely avoidable specification error. The safer practice: quote whichever rating the project’s applicable code or client market actually uses, and if you must convert or compare across systems, treat the other figure as an approximation, not a precise substitute.

One important refinement that comes with Rw and doesn’t have a direct STC equivalent: Rw + Ctr (or Rw + C, depending on the noise spectrum being modeled). Ctr is a spectrum adaptation term, a negative correction added to Rw to account for low-frequency-heavy noise sources — traffic, bass-heavy music, aircraft — that the base Rw curve, much like STC’s 125 Hz floor, doesn’t fully capture. A partition might be quoted at Rw 55 but Rw + Ctr 48, and that 7-point difference is exactly the low-frequency energy that a resident actually complains about when the neighbor’s subwoofer or the road outside is the real noise source. UK Approved Document E, for instance, sets its residential separating-wall minimum using DnT,w + Ctr (the field equivalent, covered below), specifically because ordinary Rw or DnT,w alone would understate real-world low-frequency complaints. We give the fuller regulatory picture, UK and beyond, in sound insulation building regulations.

STC vs. Rw at a glance

What is NIC, and why does it run so much lower than STC?

NIC (Noise Isolation Class) is a field-measured single-number rating of the actual sound isolation between two real, occupied rooms, tested under ASTM E336. This is the critical distinction from STC: NIC is not a normalized, partition-only measurement taken in a controlled lab chamber — it’s measured with a sound level meter in the actual finished rooms, using the room’s own absorption characteristics as they exist, without correcting for the room’s size or reverberation the way a lab test does. That means NIC inherently captures everything a lab STC test excludes: the door and its seals, gaps at the wall’s perimeter, flanking transmission through ceiling plenums or shared ductwork, and ordinary construction workmanship.

The practical consequence, and the number worth remembering: installed NIC typically measures around 10 points below the lab STC of the nominally “same” partition. A wall system tested at STC 50 in a lab often performs closer to NIC 40 once it’s actually built into a real room with a real door. This isn’t a sign that the wall was built badly, and it isn’t a testing error — it’s simply what happens when a pristine, sealed, edge-controlled lab mock-up is replaced by an actual door, actual seals, and an actual ceiling plenum running overhead. For an operable or movable partition wall specifically, NIC 40 is considered an excellent field result, and it’s the number that actually predicts what an occupant standing in one meeting room hears from the one next door — not the wall’s lab-rated STC 40-53 datasheet figure. We cover this trade-off between flexibility and isolation in more depth in our movable partition wall service page and in movable partitions vs. fixed walls.

NIC also inherits STC’s blind spot at the low end: because it’s typically reported using a similar frequency-weighting approach, it can still miss bass-heavy leakage that occupants notice even when the headline NIC number looks acceptable. If a complaint is specifically about a subwoofer, HVAC rumble, or road noise rather than voices, ask for the octave-band data behind the NIC number, not just the single figure — the same advice we give for reading any single-number acoustic metric in reading an acoustic report.

Why lab and field numbers diverge: flanking and workmanship

The gap between a lab rating (STC, Rw) and a field rating (NIC, ASTC, DnT,w) isn’t noise in the measurement — it’s a real, physical, and largely predictable phenomenon with two main causes:

1. Flanking transmission. Sound doesn’t need to pass through a demising wall to reach the next room if it can travel around it: a shared ceiling plenum above a suspended ceiling, back-to-back electrical outlets or junction boxes set into the same stud cavity, HVAC ductwork running continuously between spaces, or a shared structural slab carrying vibration laterally. A lab test isolates the partition specimen from all of these paths by design; a real building never does.
2. Workmanship and weak links. A door, its seals, gaps at the wall’s perimeter, penetrations for cabling or plumbing, and even paint or caulking quality all chip away at a partition’s rated performance. This is the weak-link principle: overall insulation is capped by the weakest element in the entire path, not by the average of all the good ones. A wall rated STC 55 tells you almost nothing about what an occupant hears if the door under it is hollow-core, or if there’s a half-inch gap at the threshold. We go deep on this specific failure mode in why acoustic doors are the weak link.

Field-adjusted equivalents exist precisely to report this reality rather than hide behind a pristine lab number: ASTC (field-tested STC-style rating, typically 3–5 points below lab STC) and, in ISO markets, DnT,w (the direct field equivalent of Rw, measured per ISO 16283 and rated per ISO 717-1). NIC sits at the far end of that spectrum — it’s the least normalized, most “as-built” of the group, which is exactly why the gap it reports (~10 points) is larger than the ASTC gap (~3-5 points): NIC folds in room absorption and geometry as well as flanking, where ASTC applies more correction to approximate a like-for-like lab comparison. For the underlying physics of how sound actually gets from one space to another — airborne versus structure-borne paths — see airborne vs. impact noise.

Why STC alone misses bass — and why that matters for real complaints

One limitation shared by STC, Rw, and (by extension) NIC deserves its own callout, because it explains a specific and recurring category of real-world complaint: the ASTM E413 STC rating curve only extends down to 125 Hz. Anything below that — the low-frequency rumble of HVAC equipment, road traffic, a home cinema’s subwoofer channel, or an operable wall’s boom from an adjacent event space — falls outside what the single STC number is designed to capture.

This is precisely why a wall or operable partition can carry a respectable STC rating on paper and still generate complaints that sound, to the people living with it, like the rating was wrong. It usually wasn’t wrong — it just wasn’t measuring the frequency range where the actual problem lives. Two consequences follow:

• For home cinema, restaurant/bar zoning, or any space with heavy bass content, request the octave-band transmission-loss data behind the STC number, not just the single figure, and specifically check performance at 100 Hz and below.
• This is exactly why Ctr exists on the ISO side. Rw + Ctr (or DnT,w + Ctr in the field) applies a spectrum adaptation specifically weighted toward low-frequency, traffic-like noise sources, which is why European and UK codes increasingly specify the Ctr-adjusted figure rather than bare Rw. If your project is US-based and low-frequency noise is a known risk (an adjacent mechanical room, a nightclub venue, a home theater), it’s worth requesting Rw + Ctr-equivalent octave-band data from the manufacturer even though the code itself will still cite STC.

The impact-noise family runs in parallel: IIC and L’nT,w

Everything above concerns airborne sound — voices, music, traffic — passing through a partition. A completely separate rating family governs impact/structure-borne noise: footsteps, dragged furniture, dropped objects, transmitted through a floor structure rather than through the air.

• IIC (Impact Insulation Class), the US lab rating, tested under ASTM E492, with a code minimum of IIC 50 for floor/ceiling assemblies in Group R occupancies — the same buildings that require STC 50 for the walls.
• L’nT,w, the ISO/European field rating for impact noise (note the inverted scale logic: lower L’nT,w is better, since it measures the level of transmitted impact noise reaching the room below, not a blocking index like STC or Rw). UK Approved Document E sets a new-build maximum of L’nT,w ≤ 62 dB.

The reason this matters here: a floor assembly can score excellently on STC or Rw (blocking voices and television from the room below) while transmitting every footstep, because airborne blocking and impact isolation depend on entirely different design measures — resilient underlayment, floating floor construction, and structural decoupling address impact noise; they do little for airborne transmission, and vice versa. STC/Rw and IIC/L’nT,w must both be checked; neither substitutes for the other. We cover this distinction, and the practical floor treatments that address it, in impact noise, footfall and floors and the broader mechanics of the two transmission paths in airborne vs. impact noise.

When is each rating actually quoted — and how do you compare fairly?

In practice, the rating you encounter depends heavily on where the project is, what stage it’s at, and who wrote the spec:

• US new-build spec sheets and code compliance documents quote lab STC (walls, doors, glazing) and lab IIC (floors), because that’s what ASTM E413/E492 and the IBC require on paper.
• US field verification — a completed hotel corridor, an operable wall after installation, an existing building being diagnosed for a complaint — is reported as ASTC, AIIC, or, for operable/movable partitions specifically, NIC, because these capture what’s actually built rather than a mock-up specimen.
• European and ISO-market spec sheets quote lab Rw (often with Ctr), and field verification is reported as DnT,w (airborne) and L’nT,w (impact), tested per ISO 16283 and rated per ISO 717-1.
• International or mixed-market projects — the most common source of confusion — will have some components (say, a European-manufactured acoustic glazing system) quoted in Rw, and the local code or client requirement stated in STC. Treat any cross-system conversion as approximate, not exact, and where the stakes are high (a flagship hotel, a healthcare confidentiality wall), request field verification in whichever local system actually governs the project, rather than relying solely on a converted lab number.

The single most useful habit across all of this: when comparing two products or two spec sheets, confirm you’re comparing the same category of number — lab-to-lab, field-to-field, same standard family — before drawing any conclusion about which performs better. An STC 52 lab rating and an NIC 45 field rating are not competing claims about the same wall; they’re two different measurements of two different things, and a wall can perfectly well carry both without contradiction. This is the same “read the fine print” discipline we recommend for any acoustic metric in reading an acoustic report.

Practical guidance for specifying across STC, Rw and NIC

1. Identify your governing code first, not the manufacturer’s preferred rating system. If the project sits under IBC, your compliance target is STC (and IIC for floors); under most European national codes, it’s Rw/DnT,w (and L’nT,w for impact).
2. Build in margin for the lab-to-field gap before construction, not after. A wall specified at exactly the code-minimum lab STC 50 has essentially zero margin once the ~3-10 point field gap is applied. Design several points above the code floor, matching the logic covered in hotel sound insulation and STC in guest rooms.
3. For operable and movable partitions, ask for both the lab STC and an expected or tested field NIC. A manufacturer quoting only a lab STC 50 rating without a field NIC figure is giving you half the picture for a product type where the field-to-lab gap is largest.
4. Don’t convert Rw to STC (or vice versa) to the decimal. Use the local-market rating that governs compliance, and treat the other system’s figure as a rough cross-check only.
5. Check for Ctr / low-frequency adaptation whenever bass-heavy noise is a realistic risk — home cinema, adjacent mechanical plant, nightclub or bar zoning, road-facing façades — even on a US project where STC is the code-cited number.
6. Verify with field measurement on completed work, not just the assembly’s lab datasheet, particularly for any space where a specific quiet outcome — not just code compliance — is the actual business requirement. Our wall sound insulation and broader sound insulation solutions work is built around exactly this measured, verified approach rather than relying on catalog ratings alone.

Setting realistic expectations

None of these ratings — STC, Rw, NIC, IIC, or L’nT,w — describe a “silent” room, and none of them should be read as a guarantee independent of everything else in the assembly. A partition’s rating, whatever system it’s quoted in, is a ceiling on what the assembly can achieve under ideal conditions, not a promise of what it will deliver once a door, a flanking path, or a poorly sealed penetration is added to the picture. Reading a rating correctly means asking not just “what number is this,” but “lab or field, which standard, and what isn’t this number telling me” — the low-frequency content STC and Rw both under-report, the impact noise that STC/Rw don’t address at all, and the flanking paths that only a field rating like NIC or ASTC will ever reveal.

Ready to verify what your walls, floors and doors actually achieve?

Spec-sheet ratings tell you what a partition can achieve in a lab; they don’t tell you what your building is actually delivering. Our team measures real, installed sound isolation on site — field STC/ASTC, NIC for operable partitions, IIC/L’nT,w for floors — identifies which weak link (door, seal, flanking path, glazing) is dragging performance below the rated figure, and specifies a fix sized to your actual target, not just a datasheet number. Request a site survey or acoustic measurement to get a clear, verified picture of where your spaces currently stand. You can also explore our sound insulation solutions, wall sound insulation, and movable partition wall pages for related project capabilities.

Frequently Asked Questions

Is Rw the same as STC?

They’re closely related but not identical. Both STC (ASTM E413) and Rw (ISO 717-1) reduce a partition’s frequency-dependent transmission-loss curve to a single number, and for most everyday partitions the two values sit within a point or two of each other. But they use different reference curves and weighting procedures, so an Rw 52 rating is not automatically the same as STC 52 — don’t treat them as interchangeable to the decimal, especially on international projects mixing both standards.

What is NIC and how is it different from STC?

NIC (Noise Isolation Class) is a field-measured rating, tested under ASTM E336, of the actual sound isolation between two real, occupied rooms — not a normalized lab measurement of the partition alone like STC. Because it’s measured in the finished building, NIC captures doors, seals, flanking transmission and workmanship that a lab STC test excludes, which is why installed NIC typically runs around 10 points below the lab STC of the nominally same partition.

Why does my STC 50 wall not sound like STC 50 once it’s built?

Almost always because of the lab-to-field gap: STC is measured in a controlled two-chamber lab isolated from flanking paths, while your finished building includes a door, threshold gaps, shared ceiling plenums, and back-to-back electrical boxes that all bypass the wall. The field-measured equivalent (ASTC, or NIC for an operable partition) is typically several points to around 10 points lower than the lab STC figure, which is exactly why specifications should build in margin above the code minimum rather than targeting it exactly.

What is Rw + Ctr and why does it matter?

Ctr is a spectrum adaptation term added to Rw to account for low-frequency-heavy noise sources such as traffic, bass-heavy music, or mechanical rumble, which the base Rw (or STC) rating — measured only down to about 125 Hz — doesn’t fully capture. A partition rated Rw 55 might only achieve Rw + Ctr 48 against a low-frequency source, and codes like the UK’s Approved Document E specifically require the Ctr-adjusted figure for exactly this reason.

Is NIC or IIC used for impact noise?

Neither — they measure different things entirely. NIC and STC/ASTC rate airborne sound isolation (voices, music) between two rooms. IIC (Impact Insulation Class, tested under ASTM E492) and its ISO field equivalent L’nT,w rate impact or structure-borne noise, such as footsteps, transmitted through a floor. A floor or wall assembly needs both ratings assessed separately, since a good airborne rating says nothing about impact performance.

Which rating should I ask for when comparing a European product against a US code requirement?

Ask for both, and don’t convert one to the other by eye. Request the product’s lab Rw (and Rw + Ctr, if low-frequency noise is a risk) from the manufacturer, and separately confirm the actual compliance requirement under the applicable code — STC and IIC if the project falls under IBC. Where the decision is high-stakes, request field verification (ASTC/NIC, or DnT,w) in whichever system governs the project rather than relying on a cross-system conversion.