Testing & Standards
Every acoustic figure we publish comes from a named test method. This page explains what each method measures, what the resulting number does and does not tell you, and how to read a report before you write it into a specification. The reports themselves are on the certifications page.
Two different jobs
Almost every acoustic mistake we see on site comes from confusing two things that sound similar and are not related at all.
- Absorption
- How much sound energy a surface takes out of the room it faces. It changes how the room sounds to the people in it: less echo, shorter reverberation, clearer speech. It is measured as a coefficient per frequency and summarised as NRC.
- Transmission loss
- How much sound a construction stops from getting to the other side. It changes what the neighbours hear. It is measured in decibels per frequency and summarised as STC.
A high NRC says nothing about how much sound a material blocks, and a high STC says nothing about how a room will sound inside. Foam on the wall will not stop a neighbour hearing the television, and mass in the wall will not stop a conference room echoing. Most real projects need both, in different layers of the same build-up.
Absorption and NRC
The methods
ASTM C423 and EN ISO 354 both measure absorption in a reverberation room. A known area of the material is placed in the room, the decay of sound is measured with and without it, and the difference gives an absorption coefficient at each frequency band. The two standards use the same physical principle and produce comparable numbers.
What NRC is
NRC is a single number derived from the coefficients at 250, 500, 1000 and 2000 Hz, averaged and rounded to the nearest 0.05. It runs from 0.00, a perfect reflector, to around 1.00, which absorbs effectively all the sound that reaches it. A value above 1.00 can appear in a report because of edge effects in the test chamber; it does not mean more than total absorption.
What the single number hides
NRC covers only the mid frequencies. A 9 mm PET felt panel rated NRC 0.30 and a 100 mm foam rated NRC 0.90 behave very differently below 250 Hz, and NRC will not show you that. If the problem is bass, ask for the full coefficient table rather than the NRC, and look at the 125 Hz and 250 Hz bands. That is why a low-frequency absorber such as BassBloc, effective from roughly 100 to 500 Hz, exists as a separate product rather than as a thicker panel.
Mounting changes the result
The same material tested flat against the wall and tested with an air gap behind it will give different numbers, and the gap almost always improves low-frequency performance. A report is only meaningful alongside the mounting it was tested in. Our published figures state the thickness they apply to: PET felt reads NRC 0.30 at 9 mm, 0.40 at 12 mm, 0.50 at 18 mm and 0.60 at 24 mm, and reaches 0.90 when the same panel is mounted over 20 mm of BassBloc.
Transmission loss and STC
The method
ASTM E90 measures airborne sound transmission loss between two isolated rooms with the test partition built into the opening between them. EN ISO 10140 is the European equivalent and reports a weighted index, Rw, rather than STC. The two are close but not interchangeable, and a specification should name the one it means.
What STC is
STC is a single number fitted to the transmission loss curve across sixteen frequency bands. Roughly, STC 30 means loud speech next door is audible but not intelligible; STC 45 means loud speech is barely audible; STC 55 and above is the range specified where confidentiality matters. Each additional 10 points is a large practical step, not a small one.
What the single number hides
STC is weighted towards speech frequencies and tells you very little about bass. A partition rated STC 50 can still pass the low end of a subwoofer or a compressor clearly. For home theatres, clubs and plant rooms, the low-frequency part of the curve matters more than the STC label, and the build-up has to be designed around mass and isolation rather than around a headline number.
A rating belongs to a construction, not a layer
This is the most common misreading we see. STC is measured on a complete laboratory partition. A single membrane has a transmission loss curve of its own, but the rating that matters is the one for the whole wall it sits in. Our SoundBlanket mass loaded vinyl is published at STC 30 at 2.5 mm and STC 34 at 4 mm to ASTM E90, at 2100 kg/m³. Those are figures for the material as tested; the wall it goes into will have its own rating, usually higher, and that is the number a specification should call for.
Fire and surface burning
- ASTM E84
- The surface burning characteristics test, giving a flame spread index and a smoke developed index. Class A is the best of the three classes and is what most commercial interior specifications in India and the US ask for.
- EN 13501-1
- The European reaction-to-fire classification, written as a letter plus smoke and droplet sub-classes, for example B s1 d0. It is a different test regime from ASTM E84, so a Class A result does not automatically convert into a European class or the other way round.
- UL 94
- A flammability classification for plastics, applied to the specific products that carry it rather than to the range as a whole.
Where a project specification names a fire class, ask for the report for the exact product and thickness being supplied. Fire performance does not transfer between thicknesses, backings or finishes the way people often assume.
Other certification
- ISO 9001:2015: the quality management system the plant runs to. It certifies how we make and check material consistently, not how any one product performs acoustically.
- OEKO-TEX Standard 100: tests textiles for harmful substances. It applies to the fabrics that carry it.
- UL GREENGUARD: low chemical emissions for indoor air quality, applicable to the specific products certified under it.
- Testing houses: our third-party acoustic and fire work is carried out by laboratories including SGS and ARAI. The report names the laboratory that issued it.
How to read a report
Before you accept a figure from us or from anyone else, check five things on the report itself.
- The standard: named in full, with its year. "Tested to ASTM C423" is a claim; "ASTM C423-22" on a laboratory letterhead is a fact.
- The laboratory: an independent, accredited testing house, named, with a report number you can quote in a submittal.
- The sample: the exact product, thickness, density and finish. A figure for 24 mm does not apply to 9 mm.
- The mounting or construction: for absorption, the mounting type; for transmission loss, the full partition build-up.
- The full data: the per-frequency table, not only the single-number summary. The summary is a convenience, the table is the measurement.
If any of those five is missing, the number is not yet usable in a specification.
Third-party and in-house figures
We run an in-house laboratory at the Palwal plant. It is used for development work and for checking production batches against their records. It is not used to generate the certified figures we publish.
Where a value is an in-house or indicative figure rather than a third-party certified one, we label it as such on the product page and in the datasheet. Our acoustic carpet, for example, carries an indicative NRC 0.25 that is an in-house measurement and is described that way rather than presented as a tested result. We would rather tell you a number is indicative than have a consultant find that out at submittal stage.
Batch traceability connects the two. A supplied batch can be traced to its production run and its test record, which is what lets a consultant confirm that the material delivered to site is the material that was tested.
Why site results differ from laboratory results
A laboratory measures one variable under controlled conditions. A building has several working at once, and the gap between the two is normal rather than a defect.
- Flanking: sound travelling around a partition through the floor slab, the ceiling void, a shared duct or a continuous window mullion. A perfect wall with an open ceiling void above it performs like a much weaker wall.
- Gaps and penetrations: an unsealed edge, a back-to-back socket or an undercut door will undo most of a construction's rating. Doors alone account for a large share of the unwanted noise in a typical room.
- Workmanship: layers that are not continuous, fixings that bridge an isolation gap, and sealant that was never applied.
- Coverage and placement: absorption works on the area actually treated and on where it is placed. Covering every surface evenly over-deadens a room; treating only the back wall leaves the reflections that matter.
- Room volume and use: the same treatment behaves differently in a small studio and in a double-height atrium.
Predicting the result in place, and verifying it afterwards, is an acoustic consultant's scope. We manufacture and specify material; we do not carry out on-site measurement or installation, and we will say so rather than imply otherwise.
Writing a specification that can be met
A clause is enforceable when it names the quantity, the method and the sample. Compare:
- Hard to enforce
- "Acoustic panels with high absorption and fire-rated construction."
- Enforceable
- "PET felt acoustic panels, 24 mm, minimum NRC 0.60 measured to ASTM C423 by an accredited laboratory, mounting as tested to be stated; surface burning Class A to ASTM E84; test reports for the supplied thickness to be submitted with the material."
Two further points are worth writing in. State whether the required rating applies to the material or to the completed construction, because for transmission loss those are different numbers. And where the room has a low-frequency problem, specify the required performance at 125 Hz and 250 Hz explicitly rather than relying on NRC or STC to carry it.
Documents we issue
For a tender or a submittal we issue a compliance pack matched to the products actually specified: third-party test reports against the standards named in the specification, fire classification certificates, certifications that apply to those products, manufacturer authorisation, and our statutory registration documents.