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dvances in numerical methods have made it possible
to calculate the absorption and transmission of multilayered
structures, taking an increasing number of physical effects into
account like finite size effects, the presence of inhomogeneitics
or inclusions or the effect of anisotropy of the material proper-
ties [1,2]. However, accurate predictions depend i-on the availa-
bility of material data. During the last fifteen years, considera-
ble work has been done on developingmeasuring techniques to
evaluate the revelant parameters of poro-elastic materials[3].
We will give an overview of the techniques that are currently
available.
Propagation of sound in poro-elastic materials
Materials used for sound absorbing and dampening appli-
cations are often much softer than solid materials and
have of porosity higher than 90%. As already predicted by
Zwikker and Kosten [4], two longitudinal waves can propa-
gate in such a medium (apart from a shear wave when the
material is insonified at oblique incidence). Due to the high
difference in density between the frame and the air in the
pores, the properties of one wave (the mechanical wave)
is mainly determined by the apparent elastic moduli and
the apparent density of the material, the air in the pores
having only a minor effect of the wave properties. The
second wave (the air wave) mainly propagates in the air
in the pores, but its properties are strongly determined
by the pore geometry due to inertial, viscous and thermal
effects in the pores. The material parameters can thus be
divided in «mechanical parameters» and «acoustic para-
meters». Which wave carries the most energy depends on
the method of excitation : if the frame is directly excited,
for instance through contact with a vibrating plate, almost
all energy is carried by the mechanical wave. If the mate-
rial is insonified from air, the air wave is the most intense
and the acoustic parameters determine the behaviour. One
should realise that the above described simplification is
not always valid for every material or for every frequency
interval. For instance at low frequencies or for materials
with low permeability, movement of the air in the pores
may generate a vibration in the frame and vice versa and
the full physics of sound propagation in poro-elastic mate-
rials should be taken into account [5].
Materials parameters
Acoustic parameters
The «acoustic» parameters of the material are :
- Porosity
j
- Flow resistivity
s
- Tortuosity
a∞
- Viscous characteristic length
L
- Thermal characteristic length
L′
Characterisation of sound absorbing
materials
Abstract
During the last 10 years, different techniques have been developed to measure the
material parameters of poro-elastic materials. Measuring tortuosity and characteristic
lengths using the ultrasonic technique have become a procedure that is available on
several laboratories now. Until recently, data of the elastic properties of poro-elastic
materials were obtained using quasi-static techniques that only provide data at low
frequencies. Since a lot of damping and sound absorbing materials are visco-elastic,
the elastic coefficients may depend stringly on frequency and temperature. In this walk,
we will give an overview of the diferent techniques available for caractherisation of sound
absorbing materials.
Résumé
Au cours des dix dernières années, plusieurs techniques ont été développées pour
mesurer les paramètres des matériaux poro-élastiques. Actuellement, mesurer la
tortuosité et les longueurs caractéristiques en utilisant des ultrasons est devenu un
procédé disponible dans de nombreux laboratoires. Jusqu’à recemment, les données
des propriétés élastiques des matériaux poro-élastiques étaient obtenues grâce à
des techniques quasi-statiques qui fournissaient uniquement des données à basses
fréquences. Depuis qu’il existe des matériaux acoustiques amortissants ou absorbants
visco-élastiques, les coefficients d’élasticité dependent fortement de la fréquence
et de la température. Dans cet article, nous allons passer en revue les différentes
techniques disponibles pour caractériser les matériaux acoustiques absorbants.
Walter Lauriks, Jan Descheemaecker,
Arne Dijckmans, Gerrit Vermeir
Katholieke Universiteit Leuven
Celestijnenlaan 200D
Hervelee
Belgique
A