23 November 2012Sound insulation1

23 November 2012Sound insulation216 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

23 November 2012Sound insulation316 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.

23 November 2012Sound insulation4 The Sound Reduction Index R With regard to a sound imping over a wall we define t as: transmission coefficient: It is the ratio between the transmitted power Wt and the incident power Wo. The Sound Reduction Index R of a wall characterized by a transmission coefficient t is given by: Sound Reduction Index: (dB)

23 November 2012Sound insulation5 Change of R with frequency 4 different frequency ranges can be identified: Rigidity region, R drops by 6 dB/octave. Resonance region (the whole panel is affected by resonances and antiresonances). Mass region, R increses by 6 dB/octave. Coincidence region (coincidence between wavelength in air and inside the flexural vibrations of the panel make the Sound Reduction Index to drop).

23 November 2012Sound insulation6 The mass law The value of R increses by 6 dB when doubling the frequency. The value of R increases by 6 dB when doubling the mass of the wall Double Wall R = 36 dB Single Wall R = 30 dB Two separate walls R = 60 dB

23 November 2012Sound insulation7 Coincidence frequency Example: steel, F cr =97700 Hz m 2 /kg, = 8.1 kg/(m 2 mm) s = 10mm, hence = s = = 81 kg/m 2 f coinc = F cr / = 97700/81 = 1206 Hz

23 November 2012Sound insulation8 Sound Insulation D vs Sound Reduction Index R The Sound Reduction Index R is defined by: The Sound Insulation D is defined by: We can make an energy balance of the energy passing through the separating wall, having surface S div, and reverberating in room 2, having an equivalent absorption area A 2 : After some math passages, we get the relationship between R and D:

23 November 2012Sound insulation9 Apparent Sound Reduction Index R Theory – definition of t and R Practice – lab measurement (R) no flanking transmission Practice – in situ measurement (R) significant flanking transmission

23 November 2012Sound insulation10 Weighted Sound Reduction Index R W A reference curve is shifted down at 1 dB steps, until the sum of unfavourable deviations becomes smaller than 32 dB At this point, the weighted value of the Sound Insulation Index, Rw, is read on the reference curve at the frequency of 500 Hz.