Sound Absorption in Porous Materials.
Sähkötekniikan korkeakoulu | Master's thesis
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Signal, Speech and Language Processing
CCIS - Master’s Programme in Computer, Communication and Information Sciences (TS2013)
AbstractPorous materials have been the most commonly used materials to dissipate sound energy and reduce sound reflections. The popularity of these materials resides in their ability to efficiently absorb sound at mid and high frequencies using relatively thin layers. Dissipation of sound energy in porous materials occurs through the interconnected pores due to viscous, thermal and inertial effects caused by the interaction between the fluid and the solid phases. Measurements of the sound absorption coefficients of sound absorbing materials are generally performed using the reverberation chamber or the impedance tube methods. The two of them are laboratory measurement techniques. The impedance tube methods compute the absorption coefficients only considering normal incident sound waves, whereas the reverberation chamber method assumes diffuse field, that is, sound arriving the absorbing material from all directions. The ability of porous materials to absorb sound strongly depends on their physical characteristics. These include porosity, tortuosity, shape of the pores, and flow resistivity. The physical properties of the total system determine the final sound absorption coefficients. The physical properties of the system include thickness and bulk density of the porous layer, and size of the particles forming the material. Theoretical, semi-phenomenological, and empirical models have been developed to predict the sound absorption properties of porous materials. It has been demonstrated that the most popular models, which were developed to predict the sound absorption properties of mineral wool porous materials, do not accurately predict the sound absorption coefficient of porous materials made of natural cellulose fibres. New parameters have to be added to the models.
Thesis advisorHänninen, Tuomas
porous, absorption, sound, modelling, cellulose, impedance