What is the Rw dB rating on an acoustic barrier, and why does it matter?

Acoustic barrier labels and spec sheets are full of numbers: “Rw 26 dB”, “up to 43 dB”, “STC 26”, “Rw 22 (C;Ctr -1;-4)”. If you’re a resident trying to work out whether the barrier next door is any good, or someone choosing barriers for a site, it’s hard to know which number to trust.

This guide explains the one that matters most, Rw, in plain English.

In this article you’ll learn:

• What Rw means and how it’s measured
• Whether there’s such a thing as an “Rw class”
• What C and Ctr mean after the Rw number
• Why “up to 43 dB” and “Rw 16 dB” can describe the same panel
• What real temporary barriers are rated at
• Why you’ll hear less reduction on site than the Rw suggests

What is the Rw rating?

Rw, the weighted sound reduction index, is a single number in decibels (dB) that says how well a material stops sound passing straight through it. It is measured in a laboratory to ISO 10140-2 and calculated to ISO 717-1. The higher the number, the less sound gets through.

An Rw 32 dB barrier blocks more sound through the panel than an Rw 26 dB barrier, which blocks more than an Rw 22 dB barrier.

How is Rw measured?

The test happens between two sealed rooms in an acoustic laboratory:

1. The sample is fitted in an opening between a “source” room and a “receiving” room.
2. Loud test noise is played in the source room.
3. Sound levels are measured in both rooms across 16 frequency bands, from 100 Hz (a low hum) to 3150 Hz (a higher-pitched sound). This is ISO 10140-2.
4. The difference between the rooms, corrected for the receiving room’s acoustics, gives the sound reduction at each frequency.
5. Those 16 results are compared with a standard reference curve from ISO 717-1. The curve is shifted until it fits the results under a set rule. The value of the shifted curve at 500 Hz is the Rw.

The shifting step is why Rw is fair. One very good result at a single high frequency can’t inflate it, because the score depends on how the panel performs across the whole range.

Is there an Rw class?

People often talk about a barrier’s “Rw class” or “dB class”, but Rw doesn’t have official classes. It’s simply a number. A barrier is “Rw 26 dB”, not “Class B”.

The lettered classes A to E you may see on spec sheets belong to sound absorption under ISO 11654, a different property: how much sound a surface soaks up rather than reflects. For permanent roadside noise barriers there’s also a separate system under BS EN 1793-2, which uses a different test and its own categories.

What do C and Ctr mean?

Sometimes you’ll see Rw written as, for example, Rw 22 dB (C;Ctr -1;-4). The two extra numbers are spectrum adaptation terms from ISO 717-1. They adjust the Rw for different kinds of noise:

• C adjusts for noise with a fairly even mix of frequencies, such as speech, music or general activity.
• Ctr adjusts for low-frequency-heavy noise, such as road traffic, diesel engines and large plant.

You add them to the Rw. Siderise’s NRB barrier is rated Rw 22 dB (C;Ctr -1;-4), so for rumbling engine-type noise the effective figure is about 22 - 4 = 18 dB. A large negative Ctr is a warning sign for sites with heavy plant, piling or breakers.

“Up to 43 dB” vs Rw: a real example

“Max” or “up to” figures are usually the best reduction in a single frequency band. They’re always higher than the Rw, sometimes by a long way.

Echo Barrier’s H9 spec sheet quotes “max noise reduction 43 dB”. Its lab test report (University of Salford, UKAS-accredited, January 2019) shows where that comes from:

The 43 dB is the result at 4000 Hz, a high-pitched sound. Rated across the whole range to ISO 717-1, the same panel is Rw 16 dB (C;Ctr -1;-3). Both numbers are genuine; they just answer different questions. Only the Rw lets you compare one barrier with another.

It’s the same story with RVT Group’s SOUNDEX Quilt: “up to 28.2 dB” is its result at 5000 Hz, and its Rw is 15 dB.

STC (Sound Transmission Class) is the American equivalent of Rw, measured to ASTM standards. The numbers are often similar but aren’t identical, so compare Rw with Rw where you can.

What are temporary acoustic barriers rated at?

From manufacturer documents and test reports, November 2026. This site is supported by Hushtec.

As a rough guide to what the differences feel like: a 3 dB change is just noticeable, 5 dB is clearly noticeable, and 10 dB sounds roughly half as loud. So for sound passing through the panel, an Rw 32 barrier lets through noticeably less than an Rw 22 barrier.

In general, heavier (denser) materials have higher Rw ratings. In theory, doubling the weight per square metre adds about 6 dB. That’s the trade-off with very light panels: the Echo H9, for example, weighs 2.0 kg per square metre, which makes it quick to handle but limits how much low-frequency sound it can block.

Why does the barrier near me not reduce noise by its Rw?

Because Rw only describes sound going through the panel. On a real site, sound also:

• bends over the top (diffraction), especially to upstairs windows
• goes round the ends of the barrier
• leaks through gaps at joints and under the base

The reduction you actually get at a given spot is called insertion loss: the difference in noise level with and without the barrier. BS 5228-1, the UK code of practice for construction noise, uses a rule of thumb: roughly 5 dB if the barrier just hides the top of the machine from view, and roughly 10 dB if it completely hides it.

That doesn’t make Rw pointless. A barrier needs a decent Rw so that the sound coming through the panel is well below the sound coming over the top. If a panel only blocks 8 dB of low-frequency engine noise, much of that noise comes straight through, however well the barrier is installed. See our worked example: Is an Rw 26 dB barrier enough? and our explainer What is Echo Barrier?

How to read a barrier spec sheet

• Check the exact product name. Some brands sell several products with very different ratings.
• Look for “Rw” followed by a number, and the standard: ISO 10140-2 (test) and ISO 717-1 (rating).
• Check for C and Ctr, especially Ctr if the site has engines, piling or breakers.
• Treat “up to” figures with caution: ask what the Rw is, and ask for the frequency results.
• Ask for the test report if the barrier forms part of a noise assessment under BS 5228-1. A UKAS-accredited lab report is the gold standard.

Questions residents can ask the contractor

• Which barrier is installed, and what is its Rw to ISO 717-1?
• Was it chosen as part of a BS 5228-1 noise assessment?
• Can it be raised, sealed at the base or moved closer to the noisiest plant?
• Can the loudest machines be shrouded or enclosed instead?

If the answers aren’t good enough and the noise continues, keep a record using our guide to recording noise evidence and follow our complaint guide.

This article is general guidance, not engineering advice. For a specific site, consult a qualified acoustic consultant or your council’s Environmental Health team.