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10
Characterisation of sound absorbing materials
Fig. 6 : Standing wave pattern and spatial Fourier
Transform for the configuration of figure 5.
Frequency 66 Hz (left) and 800 Hz (right)
By fitting the theoretical dispersion curves to the measured
ones, the shear modulus as a function of frequency can
be extracted. Figure 7 shows a typical result of a shear
modulus of a foam as a function of frequency.
At present, work is continuing on different configurations
that are more easy to realise and on the temperature
dependence of the elastic constants.
A «simplified» version of this technique has been proposed
by Geebelen [17]. Using an acoustic point source made
of a compression driver and a tube, a shear wave can be
generated in a layer of the sample. The quarter wavelenght
resonance of this shear wave can be detected with a laser
Doppler vibrometer pointing at an angle towards the surface.
This method does not require a complicated set-up.
Fig. 8 : Experimental set-up for measuring the shear modulus from
the quarter wavelenght resonance in a layer (from ref [16])
Conclusion
An overview has been presented of the different methods
that can be used to determine the material parameters
that govern the acoustic behaviour of a poro-elastic mate-
rial. It is now possible to obtain the frequency dependent
shear modulus for vibroelastic foams.
Remerciements
This manuscript describes the activities during the last
fifteen year at the «Laboratory foe Acoustics and Thermal
Physics» of the K.U. Leuven, Leuven, Belgium. This work
would not have been possible without the contribution of
numerous doctoral students, postdocs and visitors.
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Fig. 7 : Typical result of a shear modulus as a function of frequency