Presentation on theme: "16 November 2012Sound Absorption1 Sound absorption."— Presentation transcript:
16 November 2012Sound Absorption1 Sound absorption
16 November 2012Sound Absorption2 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
16 November 2012Sound Absorption3 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.
16 November 2012Sound Absorption4 Sound absorption = weak reflection If the surface is large compared to wavelength, the reflection happens specularly, as a light ray (Snells Law). S S
16 November 2012Sound Absorption5 Regarding sound absorption IT DOES NOT MATTER if sound energy is dying inside the wall or is passing through Sound absorption = weak reflection
16 November 2012Sound Absorption6 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 ), reducinhg 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 ( m 2 ) where S i and i are respectively area and absorption coefficient of the i-th surface surrounding the room.
16 November 2012Sound Absorption7 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 (A 2 / A 1 ) (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.
16 November 2012Sound Absorption8 Foam attached directly to the structure Suspended ceiling, hanging under the structure SOUND ABSORBING MATERIALS Porous materials
16 November 2012Sound Absorption9 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
16 November 2012Sound Absorption10 Effect of distance from a rigid wall SOUND ABSORBING MATERIALS
16 November 2012Sound Absorption11 Acoustic resonators c 0 : sound speed (m/s) r : necks radius (m) l : necks length (m) V : volume of rear cavity (m 3 ) SOUND ABSORBING MATERIALS Scheme of an Helmoltzs resonator Effect of damping material inside
16 November 2012Sound Absorption12 Vibrating panels : surface density of panel (kg/m 2 ) d : distance panel – wall (m) SOUND ABSORBING MATERIALS Scheme of a vibrating panel
16 November 2012Sound Absorption13 Hybrid systems SOUND ABSORBING MATERIALS Schemes of hybrid systems
16 November 2012Sound Absorption14 Vibrating panels Resonators Hybrid systems SOUND ABSORBING MATERIALS
16 November 2012Sound Absorption15 Example - Casa della Musica Porous panels Bass Trap Helmoltz Resonators Vibrating panels
16 November 2012Sound Absorption16 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
16 November 2012Sound Absorption17 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)
16 November 2012Sound Absorption18 Acoustic diffusors Curved panels are often used for building acoustical shells in theatres Reference brand: Wenger (http://www.wengercorp.com)