A more air-permeable facer is not automatically a better sound absorber, and its airflow result cannot be converted into a panel NRC. Use the facer result to support material screening, then determine whether existing acoustic evidence covers the proposed panel construction and mounting. Where it does not, test or assess the relevant assembly through an appropriate acoustic route.
The procurement question often arrives in a tempting form: two facers have different airflow results; which one will give the higher acoustic rating? It asks one measurement to answer a different question.
For a GRECHO sample-review discussion, a more useful starting point is the claim needed for the finished product. Does the buyer need comparative material screening, a small-specimen acoustic investigation, or evidence for a saleable ceiling or wall panel? Those tasks can use different tools, but the resulting numbers should not be treated as interchangeable.
“More open” needs a measurement basis
First establish what “more open” means. A higher reported flow has meaning only with the test conditions, specimen construction and relevant measurement basis. ISO 9237 addresses permeability of fabrics to air, including industrial fabrics and nonwovens.[1] It does not assign an acoustic rating to a finished panel.
The method, pressure and unit comparison is already covered separately. Even after that comparison is sound, an important limit remains: the measurement describes air passing through the tested specimen under the stated conditions. It is not a direct measurement of sound energy absorbed by the intended installation.
That limit does not make airflow unhelpful. It makes airflow one input whose role should be stated accurately.
Sound absorption is not simply the ease of passage through a sheet
Sound absorption and air passage are different physical questions. Absorption concerns what happens to incident sound energy; a steady airflow measurement does not by itself describe that response over frequency.
A simple limiting example helps: an unobstructed opening lets air pass readily, but “air passes through” is not a measurement of the energy dissipated in an absorber. This is an explanatory example, not a proposed panel construction or a product comparison.
The material in front of an absorber can influence how the assembly interacts with sound, but there is no universal rule here that the greatest flow value produces the greatest absorption at every frequency. Selecting a facer therefore involves compatibility with the construction, not maximising a single number without a panel context.
ASTM C423 describes sound-absorption measurements in a reverberation room and notes limitations when applying laboratory coefficients to practical conditions.[2] That is a different evidence route from testing a loose facing for air permeability.
Ask for the evidence at the level of the claim
The following decision table is a review aid. It does not replace a testing laboratory's method selection.
| Question to answer | Useful evidence route | What must not be inferred automatically |
|---|---|---|
| Are these facer airflow values comparable? | Identified specimens and the appropriate air-permeability measurement record | A panel NRC or room-performance claim |
| How does a small acoustic specimen respond at normal incidence? | An appropriate impedance-tube investigation | A directly interchangeable reverberation-room rating |
| How does the evaluated panel construction absorb sound under the specified test mounting? | Applicable reverberation-room testing and mounting record | Performance for every core, air gap, joint or installation |
| Does an existing report cover the proposed product variant? | Documented comparison with the report's specimen and permitted scope | Automatic transfer because the loose facer has the same name or gsm |
ISO 10534-2 describes a two-microphone impedance-tube method. Its public scope explicitly distinguishes normal-incidence coefficients from the random-incidence results obtained in a reverberation room.[3] Small-specimen work can be valuable without being relabelled as the final panel result.
ISO 354 provides a reverberation-room route for measuring absorption of relevant acoustic materials or objects.[4] The testing provider should confirm which method and presentation are required for the buyer's intended claim.
A board sample is not yet a defined installation
The word “panel” can conceal an incomplete test description. Record the core, thickness, facing arrangement and any bonding layer, but also identify how the specimen is mounted for evaluation.
ASTM E795 explicitly recognises mounting as a factor in absorption and provides specimen-mounting practices for ASTM C423 testing.[5] A report reference to a mounting type should therefore be understood, not removed from the data when a headline value is copied into a brochure.
At minimum, ask whether the proposed construction and the reported specimen agree on the relevant backing, cavity or air space, edge exposure, joints and attachment. These are comparison questions for the report and testing team, not a claim that every change has the same effect.
The earlier adhesive-layer investigation discusses why a changed bond arrangement can require investigation. The present task is different: deciding what level of acoustic evidence is needed before specifying or promoting the complete product.
Read past the headline rating
A single-number rating is useful for its intended reporting purpose, but it is a compressed description. Retain the underlying frequency-dependent results and the construction record when comparing candidate panels.
Suppose, as a purely illustrative situation, two tested constructions have similar headline ratings but different behaviour in the frequency range important to the project. Treating their ratings as proof of identical practical performance would discard information already present in the reports.
Ask the acoustic consultant or laboratory to interpret the differences, their repeatability and their significance for the application. Do not average a few selected favourable values and label the result NRC, a weighted absorption coefficient or another formal rating unless the applicable method supports that calculation.
Also keep sound absorption separate from sound insulation. Evidence about a panel's absorption does not automatically establish how effectively a partition prevents sound transmission between spaces. A proposal using the term “acoustic performance” should specify which problem it addresses.
Retest when the evidence gap calls for it, not by reflex
This article is not a rule that every minor facer change demands a complete new acoustic programme. The first step is to examine existing evidence and ask whether its scope actually covers the proposed construction.
A report for the relevant assembly may already provide what the buyer needs. A different facing, backing or installation may require further evaluation. The appropriate response depends on the change and the available evidence; it should be determined by the competent product and acoustic-testing team.
What should not happen is replacing that scope review with “this facer is more permeable, so the previous rating must still apply”. That conclusion would bypass the very issue being investigated.
A practical request for acoustic facer options should include the panel section, the acoustic result or requirement, the test mounting and the proposed material change. GRECHO can coordinate facing options and supporting documentation. The finished-product acoustic statement should remain attached to the evidence for that product.
For a wider view of the next decision, use our acoustic facer evaluation guide to move between material comparison, panel-report review and investigation of a changed construction.
Questions behind an acoustic specification
Can NRC be calculated from a facer air-permeability value?
There is no general conversion in this guidance. The airflow measurement and the acoustic rating describe different tests and evidence levels. Use an appropriate acoustic evaluation for the panel claim.
Does an impedance-tube result replace a reverberation-room report?
Not automatically. The methods use different incidence conditions and specimen arrangements. Tube testing can support development, but its result should not be presented as an interchangeable panel rating.
Must every material variant be tested from the beginning?
Not necessarily. Review the existing report, the proposed change and the permitted evidence scope first. Further testing is needed where the relevant claim is not already supported, rather than simply because a variant has a different name.
Discuss the panel evidence needed for your facer selection.
References
[1] ISO. ISO 9237:1995. Public scope for fabric air permeability. Source
[2] ASTM International. ASTM C423-23e1. Public scope and limitations for reverberation-room absorption measurements. Source
[3] ISO. ISO 10534-2:2023, corrected version 2025-08. Public scope and incidence distinction. Source
[4] ISO. ISO 354:2003. Public scope for reverberation-room absorption. Source
[5] ASTM International. ASTM E795-23. Public guidance on specimen mounting for acoustic tests. Source
