Sound%20absorption

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Sound 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 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
  (Snells Law).

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Sound absorption weak reflection
Regarding sound absorption IT DOES NOT MATTER if
sound energy is dying inside the wall or is
passing through
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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 lt rc),
• reducing the reflected energy (r gt 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)

• Inside the purely reverberant sound field (r gtgt
  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 ABSORBING MATERIALS
Porous materials
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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 ABSORBING MATERIALS
Effect of distance from a rigid wall
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Acoustic resonators
SOUND ABSORBING MATERIALS
Scheme of an Helmoltzs resonator
c0 sound speed (m/s) r necks radius (m) l
necks length (m) V volume of rear cavity
(m3)
Effect of damping material inside
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Vibrating panels
SOUND ABSORBING MATERIALS
Scheme of a vibrating panel

• surface density of panel (kg/m2)
• d distance panel wall (m)

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Hybrid systems
SOUND ABSORBING MATERIALS
Schemes of hybrid systems
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Hybrid systems
SOUND ABSORBING MATERIALS
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Example - Casa della Musica
Helmoltz Resonators
Porous panels
Vibrating panels
Bass Trap
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Sound Scattering Coefficient 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

Specular componentWspec Winc(1-a) (1-s)
Diffused componentWdif Winc(1-a) s
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Acoustic 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 diffusors
Curved panels are often used for building
acoustical shells in theatres
Reference brand Wenger (http//www.wengercorp.co
m)