Sound absorption 16 November 2012 Sound Absorption

Sound against a wall r + a + t = 1 09/13/2003 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 2012 Sound Absorption AES 115th Convention - New York, NY

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”. 16 November 2012 Sound Absorption

Sound absorption = weak reflection 09/13/2003 If the surface is large compared to wavelength, the reflection happens specularly, as a light ray (Snell’s Law). S S’ 16 November 2012 Sound Absorption AES 115th Convention - New York, NY

Sound absorption = weak reflection Regarding sound absorption IT DOES NOT MATTER if sound energy is dying inside the wall or is passing through 16 November 2012 Sound Absorption

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), reducing 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. 16 November 2012 Sound Absorption

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. 16 November 2012 Sound Absorption

SOUND ABSORBING MATERIALS Porous materials Foam attached directly to the structure Suspended ceiling, hanging under the structure 16 November 2012 Sound Absorption

SOUND ABSORBING MATERIALS Absorption coefficient vs. frequency & thickness Increasing the thickness of a porous layer attached directly on a rigid wall improves absorption at low frequency 16 November 2012 Sound Absorption

SOUND ABSORBING MATERIALS Effect of distance from a rigid wall 16 November 2012 Sound Absorption

Acoustic resonators SOUND ABSORBING MATERIALS 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) 16 November 2012 Sound Absorption Effect of damping material inside

SOUND ABSORBING MATERIALS Scheme of a vibrating panel Vibrating panels Scheme of a vibrating panel : surface density of panel (kg/m2) d : distance panel – wall (m) 16 November 2012 Sound Absorption

SOUND ABSORBING MATERIALS Schemes of hybrid systems 16 November 2012 Sound Absorption

SOUND ABSORBING MATERIALS Vibrating panels Hybrid systems Resonators 16 November 2012 Sound Absorption

Example - Casa della Musica Helmoltz Resonators Porous panels Vibrating panels “Bass Trap” 16 November 2012 Sound Absorption

Sound Scattering Coefficient s 09/13/2003 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 Specular component Wspec =Winc·(1-α) · (1-s) Diffused component Wdif =Winc·(1-α) · s 16 November 2012 Sound Absorption AES 115th Convention - New York, NY

Reference brand: RPG Diffusor Systems (http://www.rpginc.com) 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 2012 Sound Absorption

Reference brand: Wenger (http://www.wengercorp.com) Acoustic diffusors Curved panels are often used for building acoustical shells in theatres Reference brand: Wenger (http://www.wengercorp.com) 16 November 2012 Sound Absorption