Sound absorption
Sound absorption refers to the ability of materials to absorb sound waves, instead of reflecting them back into the room. When sound hits a sound-absorbing surface, part of the energy is converted into heat within the material – the room sounds quieter, clearer, and less echoing.
The higher the sound absorption coefficient (α) of a material, the more effectively it reduces reverberation. Acoustic panels, textiles, or special ceiling and wall elements thus ensure a noticeably better room acoustics – ideal for offices, educational spaces, gastronomy, or private living areas.
Sound absorption coefficient
The sound absorption coefficient, abbreviated α (Alpha), describes how much sound a material can absorb - instead of reflecting it. The value ranges between 0 and 1:
α = 0 means: no sound is absorbed (100% reflection)
α = 1 means: all sound is absorbed (0% reflection)
A high value thus indicates a strong sound-absorbing effect �a0�a0important for pleasant room acoustics. Materials with α values of about 0.6 are considered efficient sound absorbers. Depending on the room type, frequency range, and location, a targeted selection of suitable absorption coefficients is recommended.
Alpha w – the weighted absorption coefficient
The b1w-Value describes the average absorption performance across all relevant frequencies and is therefore a practical comparison value.
The b1w-Value is determined according to a European standard (EN ISO 11654).
It is often included in product data sheets, catalogues, or performance comparisons.
Additionally, an absorber class (A–E) is often specified – with A representing the highest absorption performance.
The sound absorption coefficient b1 is used in acoustic planning and detailed measurements, while b1w (Alpha w) serves as a simplified characteristic value for product labelling, comparison, and standards testing.
Equivalent Sound Absorption Area
The equivalent sound absorption area A (in m²) describes the overall effect of an absorber – that is how much sound it effectively "absorbs". Products from Molto Coustic such as ceiling panels or absorbers are rated with absorption class A, while baffles, lamella ceilings, and acoustic ceilings are rated with absorption class B.
| Sound Absorption Class | Rated Sound Absorption Factor α | Description |
|---|---|---|
| A | 0.90 to 1.00 | Very high absorption |
| B | 0.80 to 0.85 | High absorption |
| C | 0.60 to 0.75 | Moderate absorption |
| D | 0.30 to 0.55 | Low absorption |
| E | 0.15 to 0.25 | Very low absorption |
Reverberation Time (RT60)
The reverberation time, also called RT60, describes the time span it takes for a sound event to decay by 60 decibels in the room. It is a central measurement in room acoustics and significantly influences how clear or diffuse a room sounds.
The shorter the reverberation time, the better the speech intelligibility — especially in conference rooms, classrooms, or restaurants.
Longer reverberation times are typical for churches or concert halls, where sound is intentionally allowed to reverberate.
With targeted use of acoustic elements such as absorbers or acoustic luminaires, the reverberation time can be individually adjusted to the requirements of the room.
Post-noise Effect
The Post-noise Effect describes the fact that our performance capacity does not recover immediately when a disruptive noise stops.
Even after a brief acoustic interruption – e.g., by loud conversations, phone ringing or street noise – concentration remains impaired. The brain needs several minutes up to half an hour to regain full mental performance.
Tasks that require high concentration, problem-solving or creative thinking are particularly affected, such as in the office or educational institutions.
The Post-noise Effect shows how important a permanently quiet acoustic environment is – because not only does the noise itself disturb, but also its sustainable impact on mental performance.
Product Design: Ben Elmecker
NRC, SAA, and Alpha W - Sound Absorption Coefficients Overview
| Coefficient | NRC (Noise Reduction Coefficient) | SAA (Sound Absorption Average) | α w (Alpha w) – weighted absorption coefficient |
|---|---|---|---|
| Standard | ASTM C423 (USA) | ASTM C423 (USA) | EN ISO 11654 (Europe) |
| Frequency basis | 4 octave center frequencies: 250, 500, 1000, 2000 Hz | 12 third-octave bands from 200 to 2500 Hz | Octave bands from 250 to 4000 Hz |
| Calculation | Arithmetic mean, rounded to 0.05 | Arithmetic mean (uncorrected) | Rated & rounded with reference curves |
| Absorber classes | No | No | Yes (A–E according to EN ISO 11654) |
| Application | Product labelling, simple comparison | More detailed measurement basis than NRC | EU-wide classification, planning & comparison |
| Distribution | USA, Canada | USA, increasingly international | Europe, worldwide in architecture & planning |
Now design room sound - with Molto Coustic
Would you like to learn more about acoustically effective room design? We are happy to advise you.
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MORITZ JUNGWIRTH
Business Development
+43 664 88846944
jungwirth@moltoluce.com