Two Systems, One Idea
Both Rw (ISO) and STC (ASTM) are single-number ratings of airborne sound insulation. They answer the same question — how well does an element block sound — but use different standards, reference curves and frequency ranges, so they are not automatically equal.
Key Differences
| STC | Rw | |
|---|---|---|
| Standard | ASTM E90 / E413 | ISO 10140 / 717-1 |
| Frequency range | 125–4000 Hz | 100–3150 Hz (extendable) |
| Rating procedure | ASTM reference contour | ISO reference curve |
| Spectrum terms | — | C and Ctr (traffic/low-frequency) |
Why They Differ
The two ratings use different reference curves and slightly different frequency ranges and fitting rules. Consequently, a given construction usually produces Rw and STC values that are close — often within a decibel or two — but not identical, and there is no fixed conversion factor.
When to Use Which
- STC is common in projects following North American / ASTM conventions.
- Rw (with C and Ctr) is common internationally and in projects following ISO conventions.
- Always quote the rating that matches the project specification and the tested data available.
Important Cautions
- Do not write "Rw 50 = STC 50" as a general rule.
- Both are assembly ratings, not material properties.
- Neither describes low-frequency performance fully; the Ctr term helps for traffic noise.
Relevant Standards
- ASTM E90 / E413 — STC measurement and classification.
- ISO 10140 / ISO 717-1 — Rw measurement and rating.
Related Yamamah Solutions
For help specifying insulation to the correct rating system, contact us for a free acoustic consultation.
Frequently Asked Questions
Is Rw 50 the same as STC 50? Not automatically. They are close for many constructions but use different procedures.
Which is higher, Rw or STC? It varies by construction and frequency content; there is no fixed relationship.
Verified Sources
- ASTM E90 / E413.
- ISO 10140 / ISO 717-1.