256 Building acoustics
6.5.4 Transmission through porous materials
As pointed out in the introduction to this chapter, high sound insulation is based on high
reflection from a dividing partition, not on dissipating the sound energy in the partition
itself. Applying a porous material for good sound insulation is therefore not appropriate.
This does not imply, however, that estimating the sound reduction index for such
materials is not relevant. It is certainly of interest to be able to estimate the added
insulation when mounting a porous absorber on to a wall or below a ceiling.
Measured data for the sound reduction index of mineral wool of different densities
are available; e.g. by Homb et al. (1983). An example is given in Figure 6.29 where
measured data for rock wool samples of density 50 kg/m^3 are compared with
calculations. In these calculations we obviously cannot use the formulae in the present
chapter, apart from the one performing the averaging over the incidence angle (Equation
(6.99)). Calculating the transmission factor of the porous material we shall have to use
the equations given in Chapter 5 (section 5.7.1), where we calculated the impedance and
absorption factor for such materials. It should be noted that we assume that the material
is of infinite extent in the lateral direction, but we have reasons to believe that the
boundary conditions are of minor importance in this case. (Set up an expression for the
transmission factor e.g. by normal incidence using these equations.)
Figure 6.29 Sound reduction index of rock wool of density 50 kg/m^3. The thickness in mm is indicated on the
curves. Measured data from Homb et al. (1983). Dashed curves are predicted results using the model of Mechel
for describing a porous material having flow resistivity 12 kPa·s/m^2 and porosity 95 %.
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Frequency(Hz)0
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Sound reduction index (dB)40020010050