Room Sound Absorption - Sound Absorption Coefficient

Room sound absorption and absorption coefficients for common materials like plaster walls, brickwork, plywood panels etc...

The sound absorption coefficient is the ratio of absorbed sound intensity in an actual material to the incident sound intensity and can be expressed as

α = Ia / Ii        (1)

where 

α = sound absorption coefficient

Ia = sound intensity absorbed  (W/m2)

Ii = incident sound intensity  (W/m2)

 Absorption coefficient - α - for some common materials can be found in the table below:

MaterialSound Absorption
Coefficient
- α -
Acoustic belt, 12 mm 0.5
Acoustic tiles 0.4 - 0.8
Asbestos, sprayed 25 mm 0.6 - 0.7
Brickwork, painted 0.01 - 0.02
Brickwork, unpainted 0.02 - 0.05
Carpet, heavy on concrete 0.3 - 0.6
Carpe, heavy on foam rubber 0.5 - 0.7
Concrete block, coarse 0.3 - 0.4
Concrete block, painted 0.05 - 0.07
Cork sheet, 6 mm 0.1 - 0.2
Fiberboard on battens, 12 mm 0.3 - 0.4
Floor, concrete or terrazzo 0.02
Floor, linoleum, asphalt, rubber or cork tiles on concrete 0.03
Floor, wood 0.06 - 0.1
Hardwood 0.3
Glass, large panes heavy plate 0.03 - 0.05
Glass, ordinary windows 0.1 - 0.2
Gypsum board, 12 mm 0.04 - 0.07
Mineral wool, 100 mm 0.65
Persons, each 2.0 - 5.0
Plaster walls 0.01 - 0.03
Plywood panel, 3 mm 0.01 - 0.02
Polystyrene, expanded on 50mm battens 0.35
Polystyrene, expanded rigid backing 0.15
Polyurethane foam, flexible 0.95
Rubber sheet, 6 mm porous 0.1 - 0.2
Slag wool or glass silk, 50 mm 0.8 - 0.9
Snow 0.75
Wood wool cement on battens, 25 mm 0.6 - 0.07

Note! - the sound absorption coefficient varies with the frequency. 

Total Room Sound Absorption

The total sound absorption in a room can be expressed as

A = S1 α1 + S2 α2 + .. + Sn αn

   = ∑ Si αi            (2)

where 

A = the absorption of the room (m2 Sabine)

Sn = area of the actual surface (m2)

αn = absorption coefficient of the actual surface

Mean Absorption Coefficient 

The mean absorption coefficient for the room can be expressed as:

αm = A / S      (3)

where

αm = mean absorption coefficient

S = total surface in the room (m2)

The acoustic characteristics of a room can be calculated with the formulas above, or estimated for typical rooms.

Example - Room Acoustic Absorption

The total sound absorption in a room with

  • 10 m2 concrete floor with absorption coefficient 0.02
  • 10 m2 concrete roof with absorption coefficient 0.02
  • 40 m2 fiberboard walls with absorption coefficient 0.3

can be calculated as

A = (10 m2) (0.02) + (10 m2) (0.02) + (40 m2) (0.3)

   = 12.2 m2 Sabine

The mean absorption coefficient can be calculated as

αm = (12.2 m2 Sabine) / ((10 m2) + (10 m2) + (40 m2))

     = 0.2

Related Topics

  • Acoustics - Room acoustics and acoustic properties - decibel A, B and C - Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation
  • Noise and Attenuation - Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems

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