What the Ratings Are Actually Measuring
Both STC and Rw measure airborne sound insulation: they express, as a single number, how much sound a construction reduces across the speech frequency range. The single number summarises a curve of performance across frequencies into one value, which is useful for comparing systems but hides what happens at individual frequencies. The rating is a shorthand — the full picture is the performance curve behind the number.
A higher number means more sound reduction, but the comparison is only valid within the same rating system and for similar construction types.
How the Two Systems Differ
STC and Rw are built on the same principle but differ in their measurement standards, frequency weighting and adjustment procedures. STC was developed in the North American context; Rw comes from the international standard system that Australian practice follows. The practical result is that the two numbers are similar but not identical for the same wall, and they cannot be mixed in one specification — a project is rated in one system, and the values are read in that system.
The difference matters when comparing products: a wall described with an STC figure and another with an Rw figure are not directly comparable until both are converted or measured under the same standard.
Which Rating Applies in Australia
Australian building and acoustic practice works in the Rw family — the international airborne sound insulation rating — and Australian requirements are expressed in that system. STC figures appear in imported product data and North American references, and they are useful context, but the specification and verification are done in the system the project’s standard uses. The rating system is confirmed before any number is compared, because a figure without its system is a figure without its meaning.
The same discipline runs through the ceiling and wall acoustic performance guide: the objective is defined in its proper units before materials are compared.
What a Rating Does and Does Not Promise
A laboratory rating promises what the construction achieves under ideal test conditions: no flanking, no penetrations, no workmanship variation. It does not promise what the site will deliver, because site conditions — flanking paths, outlets, gaps, building movement — reduce the result. The rating is the system’s potential; the site result is what is measured after installation. The gap between the two is designed for, not discovered.
The rating is also specific to the construction: changing the frame, lining, insulation or sealing changes the assembly and its performance.
Reading Ratings in Context
Ratings are read in context: the system (STC or Rw), the construction they describe, the test conditions, and the site conditions they will face. A number lifted from a brochure and compared across systems is not a comparison — it is a misreading. The specification names the rating system, the target and the verification method, so the acoustic objective is one document from design to acceptance. The context also includes the source of the number: a rating from a manufacturer’s system sheet describes that system, while one from a field test describes the built assembly. The two are different evidence and are not interchangeable. The specification states which evidence the target is based on, so the objective is defined by something the project can verify.
The verification is the last step of the context: the installed result is measured where the objective matters, and the rating’s promise is checked against the site’s delivery. The context is the rating’s meaning.
The Rating-Context Checklist
- Which rating system does the project’s standard use — Rw or STC?
- Are all product figures expressed in the same system before comparison?
- What construction does the rating describe — frame, lining, insulation, sealing?
- What are the test conditions behind the number?
- What site conditions will reduce the result — flanking, penetrations, workmanship?
- Is the verification method named in the specification?
Frequently Asked Questions
Can I compare STC and Rw numbers directly?
No — the two systems measure and weight differently, so a direct comparison misleads. Figures are converted or re-measured under the same standard before they are compared.
Why is the site result lower than the rating?
Because laboratory ratings assume ideal conditions, and site reality adds flanking paths, penetrations and workmanship variation. The design allows for the difference, and the site result is measured rather than assumed.
Does a higher rating always mean a quieter room?
Not by itself — the rating is one variable, and the room’s result depends on the complete assembly, the flanking paths and the noise source. A high rating on a poorly installed or bypassed wall delivers less than a moderate rating on a sealed, continuous one.
To specify an acoustic rating correctly for your project, confirm the system first and contact CeilingPro with your photos for an inspection assessment and quote. A rating is a number in a system, not a number in isolation.
