The Number on the Data Sheet

A felt-finish PET panel on an office wall looks like an interior design decision. It is actually an acoustic one, and the single number printed on its data sheet, the Noise Reduction Coefficient, tells you less about how that room will sound than most people assume. Reading that number properly is the difference between a room that is fixed and a room that is merely decorated.

What NRC actually measures

NRC comes from laboratory testing to ASTM C423, the reverberation room method. It is the average of a material's sound absorption coefficients at four frequencies, 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05. The same test also produces a lesser-known but more detailed figure, the Sound Absorption Average (SAA), which averages twelve one-third octave bands from 200 to 2500 Hz and is rounded to the nearest 0.01. Two practical consequences follow directly from those definitions. NRC does not include the 125 Hz band at all, so it says nothing about low-frequency hum. And because of the 0.05 rounding, two products with the same NRC can still differ measurably in the bands that matter for a specific room.

Why the mounting changes the result

ASTM E795, the companion standard that defines how specimens are mounted during absorption testing, states the point plainly: the sound absorption of a material covering a flat surface depends not only on the material's physical properties but also on the way it is mounted over that surface. A panel tested with an air space behind it can report a different figure from the same panel fixed directly to a wall. When comparing products, the question to ask is not only "what is the NRC?" but "what mounting was it tested with, and is that how it will be installed here?"

Understanding sound absorption vs sound blocking helps you select the right acoustic solution for your space.

What the fibre is doing

PET panels are made from polyester fibre, which can be produced from recycled plastic bottles, and they absorb sound as porous absorbers: sound energy entering the fibre network is converted into heat through friction and thermal exchange. A 2026 peer-reviewed study in Results in Engineering, which manufactured panels from upcycled waste PET bottles, confirmed this typical porous, viscous-resistive behaviour, with performance strongest in the mid-to-high frequencies. The same study found absorption was strongly governed by thickness and bulk density, with an optimum at an intermediate density of around 180 kg/m³ — too loose or too dense both reduced performance — and a layered 30 mm fibre plus 10 mm flake configuration reached an SAA of 0.73, outperforming single-layer panels of comparable thickness.

How much absorption is enough

The amount of absorption a room needs is a calculation, not a guess. The Sabine equation, derived by Wallace Sabine from measurements in rooms at Harvard University, relates reverberation time to room volume and total absorption: T = 0.161V / A in metric units, where T is the time in seconds for sound to decay by 60 dB. Targets then come from the room's use. For classrooms, ANSI/ASA S12.60 sets a maximum reverberation time of 0.6 seconds in an unoccupied, furnished classroom under 10,000 cubic feet (about 283 m³), 0.7 seconds for classrooms between 10,000 and 20,000 cubic feet, and a maximum background noise level of 35 dBA. Panel quantity follows from the gap between the room as built and that target.

Learn more about acoustic absorber vs soundproofing material to understand its role in acoustic design.

Absorption is not the only rating that matters

Wall and ceiling finishes also carry a fire classification. Under ASTM E84, interior finishes are grouped by flame spread index: Class A covers 0 to 25, Class B 26 to 75 and Class C 76 to 200, each with a smoke-developed index of no more than 450. Whatever the acoustic figure, the fire classification on the test report should be checked against what the project's authority and specification require for that space.

The acouStick approach

acouStick supplies Acoustic PET Panels in thicknesses from 9 to 24 mm for offices, restaurants, schools and studios. Selection starts from the room rather than the catalogue: its volume, its reverberation target, the frequencies that need controlling, and the mounting the panels will actually be installed with — so the number on the data sheet and the result in the room describe the same thing.

Professional acoustic treatment helps control sound reflections and improve room acoustics.

If you are specifying absorption for an office, classroom, restaurant or studio, acouStick can help you match panel type, thickness and quantity to the room before you order.

Email: info@acoustick.co | Call: +966 51 130 809 | Web: www.acoustick.co

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REFERENCES

• ASTM International. ASTM C423, "Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method" — NRC and SAA definitions as summarised by Intertek: intertek.com/building/standards/astm-c423

• ASTM International. ASTM E795-16, "Standard Practices for Mounting Test Specimens During Sound Absorption Tests" — scope statement: store.astm.org/e0795-16.html

• Khoshouei, P., Soltani, P., Khoshouei, P., & Taban, E. "Upcycling waste PET bottles into high-performance fiber/flake composite panels for sustainable acoustic and thermal insulation in buildings." Results in Engineering (ScienceDirect), 2026.

• ScienceDirect Topics. "Sabine Equation" — metric form T = 0.161V/A and origin of the equation: sciencedirect.com/topics/engineering/sabine-equation

• Acoustical Society of America / ANSI. ANSI/ASA S12.60-2010/Part 1 (R2020), "Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools" — limits as summarised by Armstrong World Industries, "Classroom Acoustics and ANSI Standard S12.60": armstrong.com/en-us/articles/classroom-acoustics-ansi-standard.html

• ASTM International. ASTM E84, "Standard Test Method for Surface Burning Characteristics of Building Materials" — Class A/B/C thresholds as summarised by Intertek: intertek.com/building/standards/astm-e84