NAMS 2002 Workshop - ICOM 2008

can be deduced from the imaginary part of the surface impedance Z of the
material according to the relation below:
Phi=(P0/we). Im(1/Z)
with, P0 the atmospheric pressure (Pa), w the angular frequency (rad.s-1) and e
the sample thickness.
The porosity is obtained by an average over a selected optimal frequency interval
where ¦ does not depend on the frequency. This acoustic method allows
accessing other porous media properties such as permeability, tortuosity&
Experimental procedure and data treatment need to be validated and calibrated.
For calibration, two products are used: cork and melamine. For porous media
characterisation, two different porous materials were investigated and different
results are obtained: open-celled metal foam and microfiltration plane membrane.
These two kinds of materials are very different in the cell topology and the
material made of, but exhibit similar porosity values ranged from 80 to 90%.
Metals foams are used as reference material for experimental set up validation.
Indeed, the geometrical parameters as well as the physicals properties (pore
diameter, porosity, material, permeability&) of each studied metal foam sample
were already finely characterized. In addition with the set up validation function,
data obtained lead to an improved understanding of sound absorption in opencelled
metal foams for noise control applications. With these results, we can
investigated the filtration of particles generated from the combustion of different
products (oil, fuel, municipal waste, biomass) more especially the the influence of
the metallic foam thickness on the retention. In fact, we have observed that the
particles flow is less and less important with the progression through volume of
the metallic foam and the retention becomes less efficient.
Tests were carried out using microfltration plane membrane. As these
membranes are very thin (0.1mm), a stack of several specimens layers are
required to better capture the physical of the material. In order to evaluate the
stack thickness impact on the determination of the membrane porosity, a first
tests series was performed for length varying in the range 1mm to 5mm. A good
agreement was obtained between our results and the porosity value given by the
manufacturer.
A second tests series concerned the impact of new membranes and fouled
membranes on the FRF; it is in progress and will be presented during ICOM
2008.