STC vs Rw: Why Your 45 dB Window Gets Graded at 39

STC vs Rw is the rare specification pair where the two numbers almost agree, and the agreement is what gets people into trouble. For most walls and windows they land within 1–2 points of each other, so suppliers quote whichever one their test report happens to carry and assume it travels. Then a European or Chinese buyer grades the same window on Rw+Ctr, the traffic-noise correction takes 6 dB off, and the 45 dB window on your datasheet is a 39 dB window on theirs.

Nobody falsified anything. The datasheet quoted a rating; the buyer's code asked for a different rating built from the same test data. Here is the whole landscape in one table, then the parts that actually cost money.

STC vs Rw at a Glance

STC is a single-number rating of how much airborne sound a building element blocks, obtained by fitting a fixed reference contour to 16 one-third-octave transmission-loss values between 125 Hz and 4000 Hz under ASTM E413.

Rw is the ISO 717-1 equivalent, fitted to 16 bands between 100 Hz and 3150 Hz, and it is normally published with two correction terms — C and Ctr — that adjust it for the kind of noise the element will actually face.

The Two Rules That Make Them Diverge

The contour shapes are the same. The differences are small, specific, and they all live at the bottom of the spectrum.

The band range. Rw starts at 100 Hz and stops at 3150 Hz. STC starts at 125 Hz and runs to 4000 Hz. Rw therefore counts one extra low band and drops one high band, which is why the ISO system is generally treated as the stricter of the two for low-frequency performance — bass is where partitions actually fail, and where 4000 Hz performance is irrelevant because almost nothing leaks there.

The 8 dB rule. ASTM E413 refuses to let any single band sit more than 8 dB below the contour. ISO 717-1 has no such veto; it only caps the total of unfavourable deviations at 32 dB. The practical consequence is counter-intuitive: an assembly with one deep, narrow weakness — a resonance dip — can be penalised hard by STC and barely noticed by Rw, while a broadly mediocre assembly can score the reverse.

For most conventional constructions these two effects roughly cancel and you get the 1–2 dB agreement everyone relies on. For anything with a pronounced dip — a double-glazed unit at its mass-air-mass resonance, a lightweight steel-stud partition — they do not cancel, and the direction of the error is not predictable from the number alone.

The Part That Actually Costs You: C and Ctr

The spectrum adaptation terms are the reason a rating survives the test lab and dies at the buyer's specification desk.

• C corrects Rw for broadband, speech-like noise — living activity, offices, corridors. It typically sits between −1 and −3 dB.
• Ctr corrects Rw for low-frequency-dominant noise — urban traffic, bass-heavy music, plant machinery, aircraft at distance. It typically sits between −3 and −9 dB.

A published result looks like Rw 52 (C −1; Ctr −6): 51 dB against general noise, 46 dB against traffic. The bare Rw is the most flattering of the three numbers, which is exactly why it is the one that ends up on marketing material and the one buyers have learned to distrust.

Two market rules make this structural rather than optional:

• China grades building doors and windows by the corrected number, not the bare one. GB/T 8485-2008 classifies exterior doors and windows on Rw+Ctr and interior ones on Rw+C, across six performance grades. GB/T 50121-2005 is the umbrella rating standard that defines the weighted quantities and the spectrum adaptation terms.
• North America's analogue for exterior openings is OITC, not STC. ASTM E1332 rates outdoor-indoor attenuation using a transportation source spectrum down to 80 Hz. For a window facing a road, OITC is routinely several points below STC for the same unit, for the same physical reason Ctr is negative.

The single most useful thing you can do with this section: stop quoting one number. Quote Rw (C; Ctr) and STC together with the report number, and let the buyer pick.

Where the Number on Your Datasheet Goes Wrong

Five failure modes, in the order they show up in disputes.

1. Glass rating quoted as window rating. A glazing unit's Rw is a property of the glass. The window is glass plus frame plus seals plus the installation joint, and the assembly is almost always lower. A supplier quoting the glass maker's figure for a finished window is quoting someone else's product.
2. Lab value quoted as site value. ASTM E90 and ISO 10140 are laboratory tests with suppressed flanking transmission. Field ratings — ASTC, NIC, D_nT,w — are typically several points lower on the same construction. A specification that names a field target and a datasheet that reports a lab value are not the same conversation.
3. Sample size differs from order size. Sound reduction depends on the element's size and its mounting. A 1.2 × 1.5 m test specimen does not licence a claim for a 2.4 m tall unit.
4. The bare Rw travels alone. Drop the (C; Ctr) and you have removed the only information the buyer's code actually uses.
5. Rounding and the missing report. Ratings are whole numbers from a full third-octave curve. Without the report number, test date and laboratory, the figure is unverifiable, and increasingly buyers treat unverifiable acoustic claims as absent ones.

The datum problem in items 1 and 3 is the same one that produces window-size disputes generally: the number is real, but it describes a different object than the buyer thinks — the pattern set out in rough opening vs unit size.

How to Write the Acoustic Line on a Spec Sheet

One line, six fields, no adjectives:

Acoustic performance: Rw 45 (C −1; Ctr −6) dB / STC 46 — whole window assembly 1230 × 1480 mm, tested per ISO 10140-2 and ASTM E90, Report No. XXXX, [lab name], [date].

Acoustic spec checklist

• Both Rw (C; Ctr) and STC published, from the same test where possible
• Stated explicitly whether the figure is glazing only or the complete assembly
• Specimen size given, and the ordered size is within a defensible range of it
• Test standard named (ISO 10140 / ASTM E90) and rating standard named (ISO 717-1 / ASTM E413)
• Laboratory, report number and date on the datasheet, not on request
• Field target vs laboratory value distinguished if the buyer's spec names one
• Destination market's grading index checked — Rw+Ctr for exterior openings in China and much of the EU
• The acoustic figure appears on the product drawing next to the element it describes, not only in a PDF annex

The market-by-market split here works exactly like thermal ratings, where the same wall carries different numbers in different systems — see R-value vs U-value vs lambda. Reference sizes for the units these ratings attach to are on the standard window sizes page.

FAQ

What is the difference between STC and Rw?

STC is the North American single-number sound insulation rating defined in ASTM E413, fitted over 125–4000 Hz with a rule that no band may fall more than 8 dB below the reference contour. Rw is the ISO 717-1 equivalent, fitted over 100–3150 Hz with no single-band limit. For conventional assemblies the two are usually within 1–2 dB, but they are not the same quantity and neither is a conversion of the other.

Can you convert Rw to STC?

Not reliably. The two ratings use different frequency ranges and different fitting rules, so the offset between them depends on the shape of the assembly's transmission-loss curve. Assemblies with a sharp resonance dip diverge most. If a buyer needs both, the correct answer is to report both from the measured third-octave data, not to apply an offset.

What does Rw+Ctr mean, and why is it lower?

Ctr is the ISO 717-1 spectrum adaptation term for low-frequency-dominant noise such as road traffic. It is negative, typically −3 to −9 dB, because most building elements block bass less effectively than speech. Rw+Ctr is therefore the realistic figure for a window facing a street, and it is the index China's GB/T 8485 uses to grade exterior doors and windows.

What acoustic rating do exporters need to publish for windows?

Publish Rw with both correction terms — written Rw (C; Ctr) — plus STC if you sell into North America, and state whether the figure is for the glazing or the whole assembly. Buyers in Europe and China will grade the exterior product on Rw+Ctr regardless of what the headline number says, so publishing only the bare Rw guarantees a downgrade at their end.

How should the acoustic rating appear on a product image?

Next to the element it belongs to, on the drawing or catalogue image, with the index name attached to the value — "Rw 45 (C −1; Ctr −6), whole assembly" rather than a floating "45 dB". Acoustic figures get misread the same way dimensions do when they sit in a caption instead of on the geometry, so the callout has to be pinned to the actual frame and glazing in the image and exported at the size the buyer's catalogue or marketplace expects. That is measurement-and-annotation work rather than photo retouching, and it is a different job again from an AI image generator, which will happily letter a confident "45 dB" onto a window it never tested — a number the buyer cannot distinguish from a real one until the shipment is graded.

Sources & References

• AcousPlan — Understanding sound insulation: STC, Rw, band ranges, contour-fitting rules and C/Ctr values
• ASTM E413 — Classification for Rating Sound Insulation
• Studiobricks — ISO 717-1 vs STC: sound insulation ratings guide
• GB/T 8485-2008 — Graduation and test method of airborne sound insulation for building doors and windows (Rw+Ctr for exterior, Rw+C for interior)
• GB/T 50121-2005 — Rating standard of sound insulation in buildings (weighted quantities and spectrum adaptation terms)
• Press Glass — Evidence of performance showing Rw (C; Ctr) reported format