16 november 2012sound absorption1 sound absorption
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16 November 2012 Sound Absorption 1
Sound absorptionSound absorption
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Sound against a wall
• 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
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Materials: sound insulating & sound absorbingMaterials: 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”.
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Sound absorption = weak reflection • If the surface is large compared to wavelength, the
reflection happens specularly, as a light ray (Snell’s Law).
S
S’
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Regarding sound absorptionIT DOES NOT MATTER
if sound energy is dying inside the wall or is passing through
Sound absorption = weak reflection
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Sound absorbing materials: generalities (1)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.
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Sound absorbing materials: generalities (2)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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Foam attached directly to the structure
Suspended ceiling, hanging under the structure
SOUND ABSORBING MATERIALS
Porous materials
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Absorption coefficient vs. frequency & thickness
SOUND ABSORBING MATERIALS
Increasing the thickness of a porous layer attached directly on a rigid wall improves absorption at low frequency
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Effect of distance from a rigid wall
SOUND ABSORBING MATERIALS
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Acoustic resonators
rlV
rcf
22
20
0
c0 : sound speed (m/s)r : neck’s radius (m)l : neck’s length (m)V : volume of rear cavity (m3)
SOUND ABSORBING MATERIALS
Scheme of an Helmoltz’s resonator
Effect of damping material inside
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Vibrating panels
: surface density of panel (kg/m2)d : distance panel – wall (m)
SOUND ABSORBING MATERIALS
Scheme of a vibrating panel
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Hybrid systems
SOUND ABSORBING MATERIALS
Schemes of hybrid systems
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Vibrating panels
Resonators
Hybrid systems
SOUND ABSORBING MATERIALS
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Example - Casa della Musica
Porous panels
“Bass Trap”
Helmoltz Resonators
Vibrating panels
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Sound Scattering Coefficient s
Specular componentWspec =Winc·(1-α) · (1-s)
Diffused componentWdif =Winc·(1-α) · s
• 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
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Acoustic diffusorsAcoustic diffusors
• Most diffusing panels are done by:
• pseudo-random cavities
• 2D and 3D scattering versions
• Curved surfaces
Reference brand: RPG Diffusor Systems (http://www.rpginc.com)
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Acoustic diffusorsAcoustic diffusors
Curved panels are often used for building acoustical shells in theatres
Reference brand: Wenger (http://www.wengercorp.com)