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# Sound absorption

Sound 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.

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## Sound absorption

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1. Sound absorption Sound Absorption

2. 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 Sound Absorption

3. 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”. Sound Absorption

4. Sound absorption = weakreflection • If the surface is large compared to wavelength, the reflection happens specularly, as a light ray (Snell’s Law). S S’ Sound Absorption

5. Sound absorption = weakreflection Regarding sound absorption IT DOES NOT MATTER if sound energy is dying inside the wall or is passing through Sound Absorption

6. Sound absorbingmaterials: generalities (1) • When the the noise source is in the same room as the receiver, the noiselevel can be reduced in three ways: • reducing the sound powerradiated by the source, • bringing the receiver far away from the source ( r < rc), • reducinhg the reflectedenergy (r > rc). • The latesteffectisobtained by increasing the equivalentabsorption area A, whichisgiven by: • A = iSi ( m2 ) • where Si and i are respectively area and absorptioncoefficient of the i-thsurfacesurrounding the room. Sound Absorption

7. Sound absorbingmaterials: generalities (2) • Inside the purelyreverberant sound field (r >> rc), the sound levelreduction DL caused by the increase of sound absorptionisgiven by: • DL (f) = 10 log (A2/ A1) (dB) • where 1 and 2 refer to the valuesbefore and after the introduction of the absorbers. • Sound absorbingmaterials are usuallyclassified in the followingcategories: • a) porousmaterials, • b) acousticresonators, • c) vibratingpanels, • d) hybridsystems. Sound Absorption

8. Suspendedceiling, hanging under the structure Foamattacheddirectly to the structure SOUND ABSORBING MATERIALS Porousmaterials Sound Absorption

9. SOUND ABSORBING MATERIALS Absorptioncoefficient vs. frequency & thickness Increasing the thickness of a porouslayerattacheddirectly on a rigidwallimprovesabsorptionatlowfrequency Sound Absorption

10. SOUND ABSORBING MATERIALS Effect of distance from a rigidwall Sound Absorption

11. SOUND ABSORBING MATERIALS Acousticresonators Scheme of an Helmoltz’s resonator c0 : sound speed (m/s) r : neck’s radius (m) l : neck’s length (m) V : volume of rear cavity (m3) Sound Absorption Effect of dampingmaterial inside

12. SOUND ABSORBING MATERIALS Vibratingpanels Scheme of a vibrating panel • : surface density of panel (kg/m2) d : distance panel – wall (m) Sound Absorption

13. SOUND ABSORBING MATERIALS Hybridsystems Schemes of hybridsystems Sound Absorption

14. Vibratingpanels Resonators SOUND ABSORBING MATERIALS Hybridsystems Sound Absorption

15. Example - Casa dellaMusica Helmoltz Resonators Porous panels Vibrating panels “Bass Trap” Sound Absorption

16. Sound ScatteringCoefficients • On a roughsurface, a fractions of the reflectedenergywill be radiateddiffusely, while the remainingfraction1-swill be radiatedspecularly Specular componentWspec =Winc·(1-α) · (1-s) Diffused componentWdif =Winc·(1-α) · s Sound Absorption

17. Acousticdiffusors • Mostdiffusingpanels are done by: • pseudo-random cavities • 2D and 3D scattering versions • Curved surfaces Reference brand: RPG Diffusor Systems (http://www.rpginc.com) Sound Absorption

18. Acousticdiffusors Curvedpanels are oftenused for building acousticalshells in theatres Reference brand: Wenger (http://www.wengercorp.com) Sound Absorption

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