How to read a noise barrier test report without an acoustics degree: the three numbers that matter, what each one actually measures, how the two transmission ratings relate, and the red flags to catch before a panel under-performs on site.
Rw and STC both rate how much sound a panel blocks (transmission); they differ only by standard (ISO 717-1 vs ASTM E413) and usually agree within 1–2 dB. NRC rates how much sound the source face absorbs — a separate measurement that cannot be converted into Rw or STC. A compliant highway sound barrier needs both: absorption to stop reflected noise and enough mass for the transmission rating. For China's JT/T 646-2025, demand Rw ≥ 26 dB and NRC ≥ 0.60 in writing.
Before the definitions, fix the mental model: a barrier does two acoustic jobs.
A noise barrier works by blocking the direct line of sight between a source (traffic, rail, plant) and a receiver. That job splits into two measurable behaviours. First, transmission — how much sound energy the panel prevents from passing through to the far side. Second, absorption — how much sound the source-facing surface swallows instead of bouncing back across the road. Rw and STC describe transmission; NRC describes absorption. A report that shows only one of the two is incomplete for traffic work.
The practical upshot for a buyer: never compare panels on a single acronym. Confirm the panel is rated for both transmission and absorption, on the same standard basis, and against the threshold your project's acoustic design actually requires. Our standard panels are specified to meet or exceed JT/T 646-2025, with typical NRC 0.80–0.95 and STC 25–35 dB depending on construction.
Use this table as a quick reference when a report lands on your desk.
| Figure | What it measures | Standard / basis | Unit & meaning | Zhaobang typical |
|---|---|---|---|---|
| Rw | Weighted sound reduction index — sound blocked passing through the panel | ISO 717-1 (international) | dB; higher = better insulation | Meets Rw ≥ 26 dB (JT/T 646-2025) |
| STC | Sound Transmission Class — same physical job as Rw, US basis | ASTM E413 / tested per ASTM E90 | dB; higher = better insulation | 25 – 35 dB by design |
| NRC | Noise Reduction Coefficient — sound absorbed by the source face | Average of absorption at 250/500/1k/2k Hz | 0–1; higher = more absorption | 0.80 – 0.95 (absorptive) |
Rw and STC are not interchangeable with NRC — they answer different questions. A panel can score well on one and poorly on the other, which is exactly why a full report shows both.
The two transmission ratings come from the same measurement, not two separate tests.
Both Rw and STC are computed from the panel's airborne sound insulation curve — the laboratory measurement of transmission loss across frequencies, taken per ISO 10140 or ASTM E90. The ratings then apply a reference contour (ISO 717-1 for Rw, ASTM E413 for STC) and reduce it to one number. Because the underlying curve is identical, the two ratings track each other closely, typically within 1–2 dB, with STC often reading a touch lower.
The procurement trap is inconsistency. A supplier may quote Rw to make a panel look strong, then quote STC for a competitor to make that one look weak. Insist that every panel in a comparison is rated on the same basis. If numbers are close, ask for the raw 1/3-octave transmission curve — it reveals whether performance holds at the low frequencies that dominate traffic rumble.
Full panel constructions, materials and measured ranges are on the noise barrier specifications page.
They measure different physics, so a calculator cannot bridge them.
NRC is the arithmetic average of a panel's sound-absorption coefficients at four frequencies (250 Hz, 500 Hz, 1 kHz, 2 kHz), expressed on a 0–1 scale. It tells you how much incident sound the source-facing surface takes in rather than reflecting. Rw and STC, by contrast, describe sound energy travelling through the panel to the far side. A perforated absorptive face can score NRC 0.90 yet still transmit a lot of low-frequency energy if it lacks mass; a solid heavy panel can transmit very little (high Rw) while reflecting noise straight back at the road (low NRC).
That is why effective traffic barriers combine the two: an absorptive face to suppress reflected noise and reverberation at the barrier, plus enough panel mass to deliver the required transmission rating. When you read a report, treat NRC and Rw/STC as complementary, not substitutable.
The numbers are only useful if the report is credible and comparable.
See the noise barrier product page for our standard supply envelope, MOQ and lead times, and bring these five checks to your quotation request.
Rw and STC both describe how much sound a panel stops from passing through it — transmission. Rw is the ISO 717-1 single-number rating used in most of the world; STC is the ASTM E413 rating used in the United States. NRC is different: it measures how much sound the source-facing surface absorbs rather than reflects. A complete noise barrier report should show both a transmission figure (Rw or STC) and an absorption figure (NRC), because each addresses a separate acoustic job.
Rw and STC are derived from the same underlying transmission-loss curve using slightly different reference curves and rounding rules under ISO 717-1 and ASTM E413. For a given panel they usually fall within about 1 to 2 dB of each other, with STC often reading marginally lower. The important point for procurement is consistency: compare panels only when they are rated on the same basis, and ask for the raw 1/3-octave curve if a supplier switches between Rw and STC to make a weaker panel look stronger.
No. NRC is an absorption average and Rw/STC are transmission ratings, so they measure different physical behaviours and cannot be converted into one another. A panel with a very high NRC may still transmit sound if it lacks mass, while a heavy reflective panel may have strong transmission performance but poor absorption. Effective traffic barriers need both: an absorptive face to limit reflected noise plus enough mass to deliver the required transmission rating.
Under JT/T 646-2025, the Chinese transport-industry standard for highway noise barriers, non-transparent panels must reach a weighted sound reduction index Rw of at least 26 dB, and absorptive panels must reach a noise reduction coefficient NRC of at least 0.60. Zhaobang Metal specifies its standard panels to meet or exceed these thresholds, with typical constructions delivering NRC 0.80-0.95 and STC 25-35 dB. Always request the test report number and issuing laboratory when the standard is a contract requirement.