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Porous Facing#

INSUL can predict the sound reduction performance in two ways:

  1. As a layer of porous absorber only
  2. As a porous absorber used as a facing on a partition.

Flow Resistivity#

In many theoretical models for porous materials, flow resistivity is a key physical parameter for estimating acoustic performance ~ both for sound insulation and also acoustic absorption.

Flow resistivity is a measure of how resistant a porous material is to airflow.

In INSUL, flow resistivity is described with the unit Rayl/m. This is equivalent to the SI unit, Pa.s/m2.

Finding flow resistivity values...

Flow resistivity information can sometimes be sourced from the manufacturer of the porous material/acoustic blanket.

Alternatively, it can be predicted by relating the flow resistivity to the density using relationships such as those given in Chapter 8 of Beranek's (1971) Noise and Vibration Control.

INSUL also provides a Flow Resistivity Estimator calculation tool for predicting flow resistivity values for several common material types based on the material density.

Equivalent Fluids#

INSUL implements equivalent fluid models to estimate the sound insulation performance of porous materials.

These models assume that sound propagates through a porous material as though that material was a fluid, somewhat like air. They use key properties of a porous material, most notably its flow resistivity, to estimate the characteristic impedance and wave number for the equivalent fluid.

An important assumption of equivalent fluid models is that the frame of the porous material (for example, the fibres of a glass wool blanket) are rigid and that the sound propagating through the material does not cause the frame to move or vibrate.

Equivalent fluid models have been shown to be reasonably reliable for common porous materials like glass wool and mineral wool, provided the material is not too dense or thick or with very high flow resistivity.

Dense materials, high flow resistivity...

INSUL porous material predictions are less reliable for flow resistivity values greater than about 22,000 Rayl/m.

For glass wool, this corresponds to materials with a density of about 33 kg/m3.

For mineral wool, this corresponds to materials with a density of about 54 kg/m3.