1. Sound absorption
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Sound Absorption

2. Sound against a wall r + a + t = 1
09/13/2003
Balance of sound energy impinging over a wall
The energy balance shows three main fluxes:
Reflected
Absorbed
Transmitted
Hence three coefficients are defined, as the ratios with the impinging energy
r + a + t = 1
16 November 2012
Sound Absorption
AES 115th Convention - New York, NY

3. Materials: sound insulating & sound absorbing
Sound absorbing materials must not be confused with sound insulating materials:
Sound Insulating material:
Heavy and stiff, minimizes the transmitted power “Wt”.
Sound Absorbing material:
Soft and porous, minimizes the reflected power “Wr”.
16 November 2012
Sound Absorption

4. Sound absorption = weak reflection
09/13/2003
If the surface is large compared to wavelength, the reflection happens specularly, as a light ray (Snell’s Law). S S’
16 November 2012
Sound Absorption
AES 115th Convention - New York, NY

5. Sound absorption = weak reflection
Regarding sound absorption
IT DOES NOT MATTER
if sound energy is dying inside the wall or is passing through
16 November 2012
Sound Absorption

6. Sound absorbing materials: generalities (1)
When the the noise source is in the same room as the receiver, the noise level can be reduced in three ways:
reducing the sound power radiated by the source,
bringing the receiver far away from the source ( r < rc),
reducinhg the reflected energy (r > rc).
The latest effect is obtained by increasing the equivalent absorption area A, which is given by:
A =  i Si ( m2 )
where Si and i are respectively area and absorption coefficient of the i-th surface surrounding the room.
16 November 2012
Sound Absorption

7. Sound absorbing materials: generalities (2)
Inside the purely reverberant sound field (r >> rc), the sound level reduction DL caused by the increase of sound absorption is given by:
DL (f) = 10 log (A2/ A1) (dB)
where 1 and 2 refer to the values before and after the introduction of the absorbers.
Sound absorbing materials are usually classified in the following categories:
a) porous materials,
b) acoustic resonators,
c) vibrating panels,
d) hybrid systems.
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Sound Absorption

8. SOUND ABSORBING MATERIALS
Porous materials
Foam attached directly to the structure
Suspended ceiling, hanging under the structure
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Sound Absorption

9. SOUND ABSORBING MATERIALS
Absorption coefficient vs. frequency & thickness
Increasing the thickness of a porous layer attached directly on a rigid wall improves absorption at low frequency
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Sound Absorption

10. SOUND ABSORBING MATERIALS
Effect of distance from a rigid wall
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Sound Absorption

11. Acoustic resonators SOUND ABSORBING MATERIALS
Scheme of an Helmoltz’s resonator
c0 : sound speed (m/s)
r : neck’s radius (m)
l : neck’s length (m)
V : volume of rear cavity (m3)
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Sound Absorption
Effect of damping material inside

12. SOUND ABSORBING MATERIALS Scheme of a vibrating panel
Vibrating panels
Scheme of a vibrating panel
: surface density of panel (kg/m2)
d : distance panel – wall (m)
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Sound Absorption

13. SOUND ABSORBING MATERIALS Schemes of hybrid systems
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Sound Absorption

14. SOUND ABSORBING MATERIALS
Vibrating panels
Hybrid systems
Resonators
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Sound Absorption

15. Example - Casa della Musica
Helmoltz Resonators
Porous panels
Vibrating panels
“Bass Trap”
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Sound Absorption

16. Sound Scattering Coefficient s
09/13/2003
On a rough surface, a fraction s of the reflected energy will be radiated diffusely, while the remaining fraction 1-s will be radiated specularly
Specular component Wspec =Winc·(1-α) · (1-s)
Diffused component Wdif =Winc·(1-α) · s
16 November 2012
Sound Absorption
AES 115th Convention - New York, NY

17. Reference brand: RPG Diffusor Systems (http://www.rpginc.com)
Acoustic diffusors
Most diffusing panels are done by:
pseudo-random cavities
2D and 3D scattering versions
Curved surfaces
Reference brand: RPG Diffusor Systems (
16 November 2012
Sound Absorption

18. Reference brand: Wenger (http://www.wengercorp.com)
Acoustic diffusors
Curved panels are often used for building acoustical shells in theatres
Reference brand: Wenger (
16 November 2012
Sound Absorption