Acoustic Ratings Explained: αw, NRC and Sound Absorption Classes for Wall Panels
Every acoustic panel datasheet quotes numbers: αw 0.65, NRC 0.85, absorption Class B. These ratings are the most reliable way to compare products (far more reliable than marketing language), but only if you know what they measure. This guide explains the three rating systems you will encounter when choosing acoustic wall panels, and what values actually matter for different rooms.
αw: the weighted sound absorption coefficient
The αw value (alpha-w) is the European standard measure of sound absorption, defined in EN ISO 11654. It expresses how much sound energy a material absorbs rather than reflects, on a scale from 0.00 (total reflection, like glazed tile) to 1.00 (total absorption). A panel rated αw 0.65 absorbs roughly 65 percent of the sound energy that reaches it across the standard frequency bands.
Because αw is weighted across frequencies, two products with the same αw can behave differently at very low or very high frequencies. For typical speech-range echo problems (offices, restaurants, classrooms), αw is a dependable single-number comparison.
Sound absorption classes A to E
EN ISO 11654 also groups αw values into classes, which is what you will often see on specification documents:
- Class A: αw 0.90 to 1.00, the highest absorption
- Class B: αw 0.80 to 0.85
- Class C: αw 0.60 to 0.75, strong general-purpose performance
- Class D: αw 0.30 to 0.55
- Class E: αw 0.15 to 0.25
Architects specifying acoustic treatment for public buildings typically require Class A to C materials over a defined percentage of the wall or ceiling area. For domestic use, class matters less than coverage: several Class C panels usually outperform one small Class A panel because total absorbing area dominates the result.
NRC: the Noise Reduction Coefficient
NRC is the North American equivalent, averaging absorption at 250, 500, 1000 and 2000 Hz. It runs on the same 0-to-1 scale and tracks αw closely for most materials. In-Acoustic 18mm PET felt panels achieve NRC values between 0.70 and 0.90 depending on shape and mounting. For comparison, bare plasterboard sits around 0.03, which is why an untreated room with hard parallel walls sounds hollow.
What the ratings look like in real products
Material structure determines absorption. Porous, fibrous materials absorb well because sound energy dissipates as friction inside the fibre matrix. Wood wool cement boards such as Sonablock combine open wood-wool structure with cement binding: the 25mm panels deliver strong mid-frequency absorption suitable for offices, studios, sports halls and workshops, with the added benefits of B-s1,d0 fire classification and impact resistance. PET felt panels excel in the speech frequency range where video-call echo lives. Slatted wood panels on felt backing combine absorption (through the felt and the gaps) with diffusion from the slat geometry.
Hard decorative materials (MCM stone tiles, ceramics, solid timber boards) absorb very little. They are chosen for appearance and durability, and paired with absorbing surfaces elsewhere in the room when acoustics matter.
How much absorption do you actually need?
Reverberation time, not panel rating, is what you experience. The practical rule: treat 15 to 25 percent of the total wall area of an echoey room with Class B to C or better material and the change is clearly audible. Open-plan offices and restaurants usually need more; bedrooms and furnished living rooms need less because soft furnishings already absorb.
Frequently asked questions
Is a higher αw always better?
Not necessarily. Past a certain point a room becomes acoustically "dead," which feels unnatural for living spaces. For homes, generous coverage with αw 0.6 to 0.8 material gives a comfortable, controlled result. Maximum absorption is mainly for studios and call-intensive offices.
What is the difference between αw and NRC?
They measure the same property under different standards. αw (EN ISO 11654) is weighted across a frequency curve; NRC (ASTM C423) is a four-frequency average. Values for the same product are usually within a few hundredths of each other.
Do acoustic ratings mean a panel soundproofs my room?
No. Absorption ratings describe echo reduction inside a room. Blocking sound between rooms requires mass and construction changes, not surface treatment. No wall panel with a high αw will silence a noisy neighbour.
What rating should I look for in a home office?
Anything αw/NRC 0.6 or higher works well. Plan for 6 to 10 panels (or equivalent area) on the main reflective wall of a 12 to 16 m² room. Coverage area matters more than chasing the highest single rating.
Why do thickness and mounting change the rating?
Thicker material and an air gap behind the panel both extend absorption into lower frequencies. This is why 18mm felt outperforms 9mm, and why 25mm wood wool panels are specified where deeper absorption is required.
Which Wall Panels Pro products have the highest absorption?
Sonablock wood wool panels (25mm) and In-Acoustic 18mm PET felt panels lead the range, followed by acoustic wood slat panels on felt backing. Decorative MCM and wood panels are appearance-first products with minimal absorption.