Definition
Rw (Weighted Sound Reduction Index) is the single-number rating of a building element's airborne sound insulation — how well a wall, floor, ceiling, door or window reduces sound passing through it. It is expressed in decibels (dB); a higher Rw means better insulation.
Rw is not measured directly. It is derived from the frequency-dependent Sound Reduction Index (R) using the rating procedure in ISO 717-1.
What Does Rw Measure?
Rw measures airborne sound insulation: the reduction in sound energy when sound passes through a building element from one room to another. It is the quantity you care about when you want to stop a neighbour's TV, a street, or a meeting from being heard on the other side of a wall.
Rw does not measure sound absorption (echo inside a room) and does not measure impact noise (footsteps on a floor) — impact noise uses a different rating (Ln,w).
Unit
The unit of Rw is the decibel (dB). Typical values range from roughly Rw 25 dB (a lightweight partition) to Rw 55 dB and above (a well-built double-leaf wall). Higher values indicate better insulation.
Airborne Sound Insulation
Airborne sound insulation concerns sound that travels through the air and then through a building element: speech, music, traffic and similar sources. This is distinct from impact sound, which is generated by direct mechanical contact (footsteps, dropped objects, moving furniture) and is rated separately.
Sound Reduction Index (R)
The foundation of Rw is the Sound Reduction Index R, a frequency-dependent measure of how much sound a building element blocks. It is defined by the ratio of incident to transmitted sound power:
R = 10 × log10(W1 / W2)
where:
- W1 — incident sound power on the element
- W2 — transmitted sound power through the element
R is frequency-dependent (it changes across the spectrum), and is measured at multiple frequency bands. It is important to understand that R and Rw are not the same thing: R is the measured, frequency-dependent index, while Rw is the single-number rating derived from R.
Frequency Dependence
Sound insulation changes with frequency. Most building elements insulate better at mid and high frequencies and worse at low frequencies (bass and traffic rumble). This is why R is measured across a range of one-third-octave bands (typically 100 Hz to 3150 Hz, extendable to 50 Hz–5000 Hz) — a single number at one frequency would be misleading.
Weighted Single-Number Rating
Rw converts the full frequency curve of R into one convenient number. The ISO 717-1 procedure compares the measured R curve to a reference curve, shifts the reference curve until the sum of unfavourable deviations reaches a defined limit (but no more), and reads the shifted reference curve value at 500 Hz as the Rw value. This averaging is what makes Rw a weighted rating rather than a plain average.
How Rw Is Determined
In outline:
- The building element is measured in a laboratory, producing the frequency-dependent Sound Reduction Index R (per ISO 10140).
- The R curve is then rated against the ISO 717-1 reference curve to produce the single-number Rw.
Two additional spectrum-adaptation terms are commonly quoted alongside Rw:
- C — adapts Rw for broadband noise such as speech and general living noise.
- Ctr — adapts Rw for low-frequency noise such as urban traffic and music with strong bass.
These are written as Rw (C; Ctr), for example Rw 52 dB (−1; −5).
Laboratory Testing
Laboratory measurement of airborne sound insulation is defined in ISO 10140. A sample of the building element is installed in an opening between two reverberation rooms (a source room and a receiving room). Sound is generated in the source room, and the levels on both sides are measured across frequency bands to obtain R. Laboratory conditions deliberately eliminate flanking paths so the test isolates the element itself.
Field Testing
Field measurement of sound insulation is defined in ISO 16283. It measures the apparent sound reduction index (R′w), which includes all real-world transmission paths — not only the element itself but also flanking. Because of this, field performance is usually lower than the laboratory Rw. The laboratory value should not be assumed equal to on-site performance.
Rw vs STC
| Rw | STC |
|---|---|
| ISO rating (ISO 717-1) | ASTM / US rating (ASTM E413) |
| Airborne sound insulation | Airborne sound insulation |
| Weighted single-number rating | Weighted single-number rating |
| Different reference curve & frequency range | Different reference curve & frequency range |
| Not automatically interchangeable | Not automatically interchangeable |
Rw and STC are the ISO and ASTM counterparts of the same idea, but they use different rating procedures, reference curves and frequency ranges, so Rw 50 does not automatically equal STC 50. In practice the numbers are often close (usually within a decibel or two for many constructions), but they should not be treated as identical without a documented conversion for the specific element.
Rw vs NRC
Rw and NRC measure different phenomena:
- Rw — airborne sound insulation (blocking sound passing through an element), unit dB.
- NRC — sound absorption (reducing echo inside a room), unitless.
A material with a high NRC (absorbs sound well) can have almost no insulation value, and a wall with a high Rw may absorb almost nothing. They answer completely different questions.
Rw vs RT60
Rw and RT60 are also unrelated:
- Rw — sound transmission through a building element between two spaces.
- RT60 — decay of sound inside a single space (reverberation time, in seconds).
A room can have excellent insulation (high Rw walls) and still be reverberant inside (long RT60), or vice versa.
Wall / Ceiling / Door / Window Systems
Rw is a property of a complete building element or system — a specific wall construction, a floor assembly, a ceiling, a door or a window — not a fixed property of a material in isolation. A statement such as "mineral wool has Rw 50" or "MLV has Rw 45" is meaningless without specifying the tested system, because the result depends on the entire construction (leafs, mass, cavity, damping, seals).
Factors Affecting Rw
- Mass — heavier, more massive constructions generally insulate better (mass law).
- Number of leafs and cavity — decoupled double-leaf systems usually outperform single-leaf systems of the same weight.
- Damping and absorption in the cavity — insulation added inside a cavity improves performance.
- Decoupling — resilient mounts and staggered framing reduce structural coupling.
- Sealing — gaps and leaks at edges and penetrations sharply reduce insulation.
- Frequency — performance varies across the spectrum.
Flanking Transmission
In a real building, sound also travels around a wall through flanking paths — the ceiling void, ducts, electrical penetrations, doors, windows, junctions and structural connections. This is why an excellent laboratory result can still leave a poor on-site result. Flanking is a major reason field performance (R′w) is lower than laboratory Rw, and it must be addressed in the overall design, not just by the partition itself.
Common Misunderstandings
- "Rw 50 means the wall blocks 50 dB of sound" — incorrect; it is a weighted single-number rating, not a literal decibel reduction at every frequency.
- "Rw 50 = STC 50" — incorrect as a general rule; the two ratings use different procedures.
- "High Rw means the room absorbs sound" — incorrect; insulation and absorption are different.
- "A material has a fixed Rw" — incorrect; Rw belongs to a tested system.
- "Laboratory Rw equals on-site performance" — incorrect; field performance is usually lower due to flanking and workmanship.
Limitations
- Rw alone does not describe impact noise (footsteps) — that requires a separate rating.
- Rw can under-represent low-frequency performance; the Ctr term addresses this but is sometimes omitted.
- Rw does not tell you about room reverberation, speech intelligibility or sound absorption.
- A high Rw does not mean the element "soundproofs" the room in an absolute sense; real performance depends on the whole building and installation.
Relevant Standards
- ISO 10140 — Acoustics — Laboratory measurement of sound insulation of building elements (the multi-part standard for laboratory measurement of airborne and impact sound insulation).
- ISO 717-1 — Acoustics — Rating of sound insulation in buildings and of building elements — Part 1: Airborne sound insulation (defines Rw and the C / Ctr spectrum adaptation terms).
- ISO 16283 — Acoustics — Field measurement of sound insulation in buildings and of building elements (defines the apparent sound reduction index R′w measured on site).
Related Yamamah Solutions
Yamamah supplies components used within sound-insulating wall and ceiling systems. Any stated Rw/STC performance applies only to a complete, tested system — not to an individual material:
For sound insulation systems and assemblies, contact us about sound insulation solutions for your project.
See also the Sound Absorption vs Sound Insulation hub.
Read more in the four-part acoustic fundamentals series: What Is NRC? · What Is RT60? · What Is STC?
Frequently Asked Questions
What is a good Rw value? It depends on the application. For example, a basic partition may be around Rw 35–40 dB, while high-performance separating walls can reach Rw 55 dB or more. The required value should come from the relevant building code or project specification for the specific use.
Is a higher Rw always better? For insulation between spaces, yes — but the correct target depends on the noise source and the sensitivity of the receiving room.
Can I compare Rw and STC directly? Not automatically. They use different standards and procedures; treat them as related but not equal unless a specific conversion is documented.
Does Rw include impact noise? No. Impact noise (footsteps) is rated separately (e.g., Ln,w / L′nT,w).
Verified Sources
- ISO 10140 — Acoustics — Laboratory measurement of sound insulation of building elements.
- ISO 717-1 — Acoustics — Rating of sound insulation — Part 1: Airborne sound insulation.
- ISO 16283 — Acoustics — Field measurement of sound insulation in buildings and of building elements.