Wall Cladding Acoustic Certifications

A submittal package crosses our desk with a request we see on nearly every project that touches an occupied conference room, courtroom, or open-plan office floor: the specifier wants proof, not a marketing claim, that the wall cladding under review actually performs the way the cut sheet says it does. The product data lists a single absorption number and a passing reference to sound testing, but the architect of record wants the underlying test report, the laboratory that produced it, and the exact mounting condition the sample was tested in before the product moves from proposed substitution to approved equal.

We see this most often on healthcare corridors, courtrooms, open-plan offices, and higher-education buildings, where an acoustic consultant has already modeled a required absorption coefficient or transmission class for a wall surface and handed a number to the design team rather than a brand name. When that happens, the manufacturer’s job stops being about aesthetics and starts being about paperwork: matching the acoustic wall and ceiling systems we manufacture to a documented, third-party-verified performance claim that survives a peer review.

That paperwork lives inside a small set of test standards most specifiers reference by number rather than by name, and the difference between them determines what a wall cladding system can and cannot claim on a spec sheet. Getting that distinction right the first time saves a resubmittal cycle later, and it is the difference between a project team trusting our documentation on the next job or treating every future claim with suspicion.

What a Certified Acoustic Test Report Actually Contains

A real acoustic test report is not a single number printed on a brochure. It is a document with a specific structure, and specifiers who review submittals for a living know exactly which fields to check before they accept a claim. At minimum, a usable report identifies the testing laboratory and its accreditation, the standard the test was run under, the date of the test, a description of the exact specimen construction, and the mounting condition the sample was tested in.

That last point matters more than most people outside the testing world expect. A panel tested mounted directly against a rigid wall behaves differently than the same panel tested over an air gap or a furring channel, and the resulting rating can shift meaningfully between the two conditions. A single blanket rating covering an entire decorative wall panel and tile catalog is a red flag, not a convenience, because different substrates, thicknesses, and facing materials inside that catalog do not share one acoustic result. A specifier who receives only one number for a whole product line should ask which specific construction was actually placed in the test chamber.

The report should also show frequency-by-frequency data, typically across the 125 Hz to 4,000 Hz range, before it collapses that data into the single number most people recognize. That underlying curve is what a peer reviewer or acoustic consultant actually wants to see, because a product can hit an acceptable single-number rating while performing poorly at the specific frequencies that matter most for a given space, such as low-frequency mechanical noise or the speech-frequency range that drives privacy in a clinic or courtroom.

ASTM E90 and Airborne Sound Transmission Loss

ASTM E90 is the laboratory method used to measure how much airborne sound a wall assembly blocks as it travels between two adjoining spaces. The test places a sound source in one chamber and measures the resulting level in an adjacent receiving chamber, then calculates the difference in decibels at each of a series of frequencies. That difference is transmission loss, and it is measured for the full assembly under test, not for a decorative surface material considered on its own.

This distinction causes most of the confusion in practice. A wall cladding product installed over an existing partition changes the acoustic behavior of that partition somewhat, but the cladding itself was very likely never tested as a stand-alone sound barrier, because a thin decorative surface is not the primary path sound takes through a wall. A different category of product illustrates why that matters: items built and marketed specifically as acoustic office partitions are tested and rated as a complete sound barrier under E90 in their own right, publishing transmission data rather than borrowing a rating from a generic wall type.

When a specifier asks a wall cladding manufacturer for an E90-based rating, the honest answer is usually that the cladding was not designed or tested to block sound between rooms at all. It was designed and tested to absorb sound within the room where it is installed, which is a different property measured under a different standard entirely.

ASTM E413 and STC Classification

ASTM E413 takes the raw transmission-loss curve produced under E90 and converts it into the single number most people know as STC, or Sound Transmission Class. The method fits the measured curve against a standard reference contour and derives one rating that summarizes performance across the full frequency range, weighted toward the frequencies most relevant to speech and everyday building noise.

The number itself only means something in context. A partition rated around STC 30 typically allows normal speech to be understood clearly through the wall. Around STC 40, loud speech is audible but largely not intelligible. By the time an assembly reaches STC 50 or higher, loud speech generally cannot be understood from the adjacent space at all, which is the range building codes and acoustic consultants often target for courtrooms, conference rooms, and similar spaces where confidentiality matters.

STC classification belongs to the wall assembly as a whole, not to a decorative layer applied over it. That is a fact worth stating plainly to a specifier, because it heads off a request for an STC number that a wall cladding line was never built to earn in the first place.

ASTM C423 and the NRC Rating for Sound Absorption

ASTM C423 measures a completely different property: how much sound energy a material absorbs rather than how much it blocks from passing through. The test is run in a reverberation room, a space engineered to reflect sound with minimal absorption of its own. A sample is placed in the room, mounted according to a specific, documented mounting type, and the rate at which sound decays after it stops is measured with and without the sample present. The difference reveals how much sound energy the material absorbed.

That result is converted into the Noise Reduction Coefficient, or NRC, a single number between 0.00 and 1.00 rounded to the nearest 0.05. A material rated near 0.05 reflects nearly all of the sound that reaches it, similar to a hard, flat surface. A material rated near 0.90 or higher absorbs most of the sound energy that hits it, similar to a heavy fabric-wrapped panel. A felt wall panel considered for a busy commercial space is typically evaluated primarily on this scale, since its job is to reduce reverberation and echo within the room rather than to stop sound from crossing into the next one.

The mounting type recorded in a C423 report is not a footnote. A panel tested mounted flat against a hard backing will generally show a lower NRC than the same panel tested over an air space, because the air gap behind the panel adds low-frequency absorption the flat mounting does not capture. A specifier comparing two competing products should confirm both were tested under the same mounting type before treating the two NRC numbers as comparable.

STC vs. NRC: Two Different Numbers on the Same Cut Sheet

Because STC and NRC both show up as a single number on the same page, they are the two figures specifiers confuse most often, even though they describe unrelated performance properties measured under different standards. The table below lays out how each one is produced and what it actually tells a design team.

Standard	What It Measures	Test Method	Resulting Rating	What It Tells a Specifier
ASTM E90	Airborne sound transmission loss through a full assembly	Two-chamber lab test measuring level difference across 125–4,000 Hz	Transmission loss (dB) per frequency	Raw data used to calculate STC; rarely quoted alone on a submittal
ASTM E413	Single-number sound isolation rating	Fits the E90 transmission-loss curve to a standard reference contour	STC (Sound Transmission Class)	How well a complete wall assembly blocks sound moving between two spaces
ASTM C423	Sound absorption of a material or surface	Reverberation-room decay-rate measurement, sample mounted per a documented mounting type	NRC (Noise Reduction Coefficient), 0.00–1.00	How much sound a wall surface absorbs inside the room where it is installed

A wall cladding product can be an excellent performer under one of these standards and irrelevant to the other. Neither result substitutes for the other on a submittal, and a specifier who asks for “the acoustic rating” without naming a standard is usually asking for whichever one actually applies to how the product will be used.

Third-Party Test Labs and Verifying an Acoustic Claim

A test report is only as trustworthy as the lab that produced it. Specifiers reviewing a submittal for a project like a university seminar hall or a healthcare facility typically want to see that the testing was performed by an independent laboratory accredited to the relevant scope of acoustic testing, not generated in-house by the manufacturer without outside verification. Accreditation bodies audit a lab’s equipment calibration, chamber construction, and procedural compliance against the referenced ASTM method, which is what gives a reviewer confidence the reported number reflects an actual, repeatable measurement.

Specifiers also compare the tested specimen against what is actually being purchased. If a product line has been reformulated, re-faced, or produced with a different core material since the report was issued, the old test result no longer describes the product on the truck. A responsible manufacturer retests when a construction changes materially, rather than continuing to publish a number that predates a change to the product it is supposed to describe. On a project like a university seminar hall built around sound absorbing panels, an acoustic consultant may have already specified an exact NRC value, and mismatched or outdated documentation is one of the more common reasons a submittal gets bounced back for resubmission.

Chain of custody matters here too. A sample sent for testing should be representative production material, not a hand-built prototype that differs from what ships to a jobsite. Reviewers who catch a mismatch between the tested sample description and the actual product data sheet will reasonably question every other claim in the same package.

LEED and WELL Building Standard Documentation for Acoustic Credits

Green building certification programs add another layer of documentation on top of the base ASTM reports. Projects pursuing an acoustic-related credit under LEED, or targeting the sound concept under the WELL Building Standard, generally require the underlying third-party test data rather than a summary claim, because the certifying body’s reviewers are checking the same fields a specifier would: the standard referenced, the lab, the mounting condition, and the date of test.

In practice, this means the same ASTM E90, E413, and C423 reports already produced for a standard submittal usually satisfy a certification reviewer, provided the tested construction matches the installed condition and the report is current. When a design team is chasing a sound-related credit, the reviewer wants to see the actual reverberation-room test data, produced under the ASTM C423 sound-absorption test method, not a single NRC figure copied onto a spec sheet without its supporting report attached.

Healthcare projects add a further wrinkle, since speech privacy requirements in patient areas are often driven by both code language and a facility’s own internal acoustic standards. A wall cladding chosen for a corridor in a healthcare vertical market project needs documentation that speaks to both the absorption performance a designer wants and any code-referenced privacy requirement layered on top of it.

Conclusion

Acoustic certification is ultimately a documentation discipline as much as a testing discipline. The physics behind STC and NRC ratings are well established, and the standards that measure them are settled and widely referenced across the industry. What separates a submittal that sails through review from one that bounces back twice is whether the manufacturer can produce a complete, matched, and current test report the moment a specifier asks for it, rather than a marketing number with nothing behind it. Manufacturers who treat that documentation as a permanent part of the product, not an afterthought, are the ones who stop fielding the same follow-up request on every project.

FAQ

Can one NRC number apply to an entire wall cladding product line?

No. Facing material, core density, thickness, and mounting condition all affect absorption, so a single NRC figure printed across an entire catalog is rarely accurate for every configuration in it. A specifier should ask which exact construction and mounting type produced the number in question.

Is a high STC rating the same thing as a high NRC rating?

No. STC describes how well an assembly blocks sound from passing between two spaces, while NRC describes how much sound a surface absorbs inside the room where it sits. A product can perform very differently on each scale, and neither number predicts the other.

Who is qualified to issue the acoustic test report for a submittal?

An independent laboratory accredited for the relevant acoustic test method should perform and issue the report. In-house data can support product development, but most specifiers and certification reviewers expect third-party verification before they will accept a claim on a formal submittal.

Do LEED or WELL certification reviewers require a separate acoustic test beyond the standard ASTM reports?

Usually not. Existing ASTM E90, E413, or C423 reports typically satisfy documentation requirements for a relevant credit, as long as the tested specimen and mounting condition match what is actually installed and the report has not been superseded by a product change.