23 november 2012sound insulation1. 23 november 2012sound insulation216 november 2012sound...
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23 November 2012 Sound insulation 1
Sound insulationSound insulation
23 November 2012 Sound insulation 216 November 2012 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
23 November 2012 Sound insulation 316 November 2012 Sound Absorption 3
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”.
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The Sound Reduction Index RThe Sound Reduction Index RWith 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)
Wo
Wtt
t
R1
log10 10
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Change of R with frequencyChange 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).
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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
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Coincidence frequency
• Example: steel, Fcr=97700 Hzm2/kg, ’ = 8.1 kg/(m2mm)
• s = 10mm, hence = ’s = 8.110 = 81 kg/m2
• fcoinc = Fcr/= 97700/81 = 1206 Hz
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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 Sdiv, and reverberating in room 2, having an equivalent absorption area A2:
• After some math passages, we get the relationship between R and D:
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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
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Weighted Sound Reduction Index RW
• 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.