16 november 2012sound absorption1 sound absorption

16 November 2012 Sound Absorption 1

Sound absorptionSound absorption


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


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”.


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’


Regarding sound absorptionIT DOES NOT MATTER

if sound energy is dying inside the wall or is passing through

Sound absorption = weak reflection


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.


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.


Foam attached directly to the structure

Suspended ceiling, hanging under the structure

SOUND ABSORBING MATERIALS

Porous materials


Absorption coefficient vs. frequency & thickness


Increasing the thickness of a porous layer attached directly on a rigid wall improves absorption at low frequency


Effect of distance from a rigid wall



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)


Scheme of an Helmoltz’s resonator

Effect of damping material inside


Vibrating panels

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


Scheme of a vibrating panel


Hybrid systems


Schemes of hybrid systems


Vibrating panels

Resonators

Hybrid systems



Example - Casa della Musica

Porous panels

“Bass Trap”

Helmoltz Resonators

Vibrating panels


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


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)


Acoustic diffusorsAcoustic diffusors

Curved panels are often used for building acoustical shells in theatres

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

16 november 2012sound absorption1 sound absorption

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