Sound absorption Sound absorption

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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

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Materials: sound insulating & sound absorbing 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 “W_t”.

Sound Absorbing material:

Soft and porous, minimizes the reflected power “W_r”.

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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

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Regarding sound absorption IT 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 < r_c),

• reducing the reflected energy (r > r_c).

The latest effect is obtained by increasing the equivalent absorption area A, which is given by:

• A =  _i S_i ( m² )

where S_i 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 >> r_c), 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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Absorption coefficient vs. frequency & thickness

Increasing the thickness of a porous layer attached directly on a rigid wall

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Effect of distance from a rigid wall

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Acoustic resonators

 

 



  



r l

V

r f c

2 2

2 0

0

 

c₀ : sound speed (m/s) r : neck’s radius (m) l : neck’s length (m)

V : volume of rear cavity (m³)

Scheme of an Helmoltz’s resonator

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Vibrating panels

60

Scheme of a vibrating panel

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Hybrid systems

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Vibrating panels

Resonators

Hybrid systems

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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 component W_spec=W_inc·(1-α) · (1-s) Diffused component

W_dif=W_inc·(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

s W

_diff

W

_diff

W

_spec

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Acoustic diffusors 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 Acoustic diffusors

Curved panels are often used for building acoustical shells in

theatres

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Acoustic diffusors Acoustic diffusors

Curved panels are often used for building acoustical shells in theatres

Reference brand: Wenger (http://www.wengercorp.com)