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Sound insulation

Sound insulation. 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 2012.

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Sound insulation

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  1. Sound insulation Sound insulation

  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 16 November 2012 Sound insulation Sound Absorption 2

  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”. 16 November 2012 Sound insulation Sound Absorption 3

  4. The Sound Reduction Index R • With regard to a sound imping over a wallwedefinetas: • transmissioncoefficient: • Itis the ratio between the transmittedpowerWt and the incidentpowerWo. • The Sound Reduction IndexR of a wallcharacterized by a transmissioncoefficienttisgiven by: • Sound Reduction Index: (dB) Sound insulation

  5. Change of R with frequency 4 differentfrequencyranges can be identified: • Rigidityregion, Rdrops by 6 dB/octave. • Resonanceregion (the whole panel isaffected by resonances and antiresonances). • Massregion, Rincreses by 6 dB/octave. • Coincidenceregion (coincidencebetweenwavelength in air and inside the flexuralvibrations of the panel make the Sound Reduction Index to drop). Sound insulation

  6. The mass law • The value of Rincreses by 6 dB whendoubling the frequency. • The value of Rincreases by 6 dB whendoubling the mass of the wall Single Wall R = 30 dB Double Wall R = 36 dB Two separate walls R = 60 dB Sound insulation

  7. Coincidencefrequency • Example: steel, Fcr=97700 Hzm2/kg, s’ = 8.1 kg/(m2mm) • s = 10mm, hence s = s’s = 8.110 = 81 kg/m2 • fcoinc = Fcr/s = 97700/81 = 1206 Hz Sound insulation

  8. Sound Insulation D vs Sound Reduction Index R • The Sound Reduction Index Risdefined by: • The Sound Insulation D isdefined by: • We can make an energy balance of the energypassingthrough the separatingwall, havingsurfaceSdiv, and reverberating in room 2, having an equivalentabsorption area A2: • After some mathpassages, weget the relationshipbetweenR and D: Sound insulation

  9. 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 Sound insulation

  10. At thispoint, the weightedvalue of the Sound Insulation Index, Rw,isread on the reference curve at the frequency of 500 Hz. Weighted Sound Reduction Index RW • A reference curve isshifted down at 1 dB steps, until the sum of unfavourabledeviationsbecomessmallerthan 32 dB Sound insulation

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