Acoustic Design Series: Sound absorption classes

Understanding Acoustic Absorption Classes

When choosing acoustic finishes, you’ll notice helpful data sheets that showcase a letter rating from Class A to Class E, based on the EN ISO 11654 standard. This rating simplifies detailed acoustic lab tests into an easy-to-understand scale that reflects a product’s sound-absorption effectiveness. By grasping these ratings, you’ll be well-equipped to select the perfect materials for any project, whether it’s a bustling commercial area or a specialised listening space.

Whether designing a high-traffic commercial space or tuning a critical listening environment, understanding these ratings ensures you choose the right material for the job.

How Sound Absorption Classes Work

Acoustic testing evaluates absorption across standard octave bands from 250 Hz to 4000 Hz. Under ISO 11654, these values are fitted against a standardised reference curve to generate a single weighted figure $⍺$_$w$.

Values range from 0.00 (total reflection) to 1.00 (total absorption).

Class	Weighted Absorption $⍺$$w$	Performance Level	Typical Applications
Class A	0.90 – 1.00	Highest absorption	Open-plan offices, call centres, studios, high-traffic venues (e.g., 50 mm+ dense mineral wool panels, acoustic baffles)
Class B	0.80 – 0.85	High absorption	Meeting rooms, conference spaces (e.g., 30–40 mm absorbers, micro-perforated acoustic panels with acoustic fleece)
Class C	0.60 – 0.75	Medium absorption	Classrooms, corridors, general commercial areas (e.g., standard suspended ceiling tiles, slotted timber boards)
Class D	0.30 – 0.55	Low absorption	Spaces needing speech reflection support (e.g., rear walls of lecture halls, heavy draped fabrics, thin carpets)
Class E	0.15 – 0.25	Very low absorption	Light fabrics, textured wall coverings
Unclassified	< 0.15	Reflective	Hard surfaces like bare plasterboard, glazing, polished concrete, and solid timber

Don’t Ignore Shape Indicators: L, M, and H

A single-number rating may not fully reflect a material’s performance. For example, a panel might show average mid-range performance but excel in high-frequency absorption. To address this, ISO 11654 adds an alphabetical shape indicator when a material’s absorption exceeds the reference curve by 0.25 or more in a specific frequency band.

To account for this, ISO 11654 appends an alphabetic shape indicator whenever a material’s measured absorption exceeds the reference curve by 0.25 or more in a specific band:

• L (Low-frequency): Excess absorption around 250 Hz (typical of resonant membranes or panel absorbers).
• M (Mid-frequency): Excess absorption at 500 Hz or 1000 Hz (often seen in perforated panel systems with specific cavity depths).
• H (High-frequency): Excess absorption at 2000 Hz or 4000 Hz (standard in thin foams, mineral fibres, or acoustic fabrics).

Example: A specification of $⍺$_$w$ = 0.65(MH) denotes a Class C absorber that performs significantly above average across the critical human speech and upper frequency bands.

Sound absorption classes are relevant in both music and noisy environments, but their usefulness depends entirely on the acoustic problem being addressed.

Absorption classes are directly relevant and standard practice here.

• Speech-dominated energy: General background chatter, cafeteria clamour, and office noise live primarily between 500 Hz and 4000 Hz—the exact frequency window weighted by EN ISO 11654.
• Direct correlation: Installing Class A or Class B absorbers (such as suspended acoustic ceilings, acoustic baffles, or 50 mm wall panels) reliably lowers overall ambient reverberation (‭RT₆₀), controls the “cocktail party effect,” and boosts speech intelligibility (STI)

• Regulatory alignment: Building regulations and acoustic performance standards for commercial and institutional interiors routinely mandate specific surface areas of Class A or B absorption.

Absorption classes are useful only as a rough starting point, and relying on them exclusively is a common pitfall.