Overview
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Sohbi SAHRAOUI: University of Maine Acoustics Laboratory, UMR CNRS 6613
INTRODUCTION
The artificial porous materials commonly used in industry are simple in structure, although they vary widely. Among these, materials whose porosity is (mainly or entirely) "open" often exhibit interesting sound absorption properties in airborne acoustics
An incident sound wave induces relative air movements in the material's "pores". These movements are then effectively dissipated by simple internal friction of the air, whose velocities relative to the structure must cancel out at the air/solid skeleton contact surface or "pore surface" S p : these are the viscous effect losses. Acoustic phenomena also induce small temperature oscillations in the saturating air, which cancel out on contact with the solid (which remains at room temperature). This results in irreversible heat exchange between the air and the solid: thermal effect losses, which are generally lower than the former. Finally, when the solid itself is set in motion, viscoelastic losses occur, linked to internal friction in the solid matrix undergoing deformation (its vibration may result from inertial and viscous actions –– exerted at the S p walls by air movements, or more directly, from its contact with a plate, screen, etc.). These structural losses can play an important role, particularly in the vicinity of resonances.
In principle, a distinction can be made between intrinsic acoustic properties, on the one hand, and the overall acoustic properties of the treatment, on the other. Thus, in paragraph 1 , the geometric parameters associated with the microgeometry of the porous structures under consideration are introduced and the intrinsic acoustic properties of the materials are deduced within...
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Measuring the acoustic properties of porous materials
Appendix: equivalent fluid
The "effective density" and "effective modulus of compressibility" functions of the equivalent fluid must be suitably represented by the following approximate models:
References
Standardization
French Standards Association (Afnor)
- Fiberboard – Determining surface absorption – 2: test methods for hardboard. - NF EN 382-2 - (03-1994)
- Acoustics – Materials for acoustic applications – Determination of air flow resistance. - NF EN 29053 - (08-1993)
- Thermal performance of buildings – Air permeability of...
Organizations
Laboratoire d'acoustique de l'Université du Maine http://laum.univ-lemans.fr
Centre de transfert de technologie du Mans (CTTM) http://www.cttm-lemans.com
Laboratoire de sciences de l'habitat (LASH). École nationale des travaux publics...
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