16 November 2012Sound Absorption1 Sound absorption

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)