On measurement of mechanical properties of sound absorbing materials

This paper is an overview of quasistatic measurement technique used to determine mechanical properties of sound absorbing materials. Experimental context and behaviour of polymer foam and rock wool are summarized. Concerning parameters are: static and dynamic strains, dynamic effects such as skeleton resonance and coupling with saturating fluid. Material anisotropy and frequency dependance of characteristics are also addressed. It is shown that this technique is well suited for usual material up to 100 Hz. Restrictions occurs for thin samples of very high air flow resistivity and for fibrous material that do not exhibit a linear behaviour with static strain. ha l-0 03 31 37 2, v er si on 1 16 O ct 2 00 8 Author manuscript, published in "2nd Biot Conference on Poromechanics, Grenoble : France (2002)" 2 Biot Conference on Poromechanics – August 26-28, 2002 – Grenoble, France 2 3.1 Influence of static and dynamic strain amplitude Sample stiffness is studied as a function of static s ε and dynamic d ε strains, so that imposed strain is ) sin( ) ( t t d s ω ε ε ε + = . (3) First, the influence of the amplitude of dynamic strain is studied. Measurement is performed at 100 Hz for a static strain of 2%. Figure 2 shows results for polymer foam (grey squares): stiffness and loss factor are constant under 0.05 % dynamic strain. Then stiffness decreases and loss factor increases. The same behaviour is observed on figure 3 for rock wool. This confirms Pritz work (Pritz 1986, Pritz 1994) that mentions non linearity past 0.05 %. The influence of the amplitude of static strain is now studied. Measurement is performed at 100 Hz for 0.02% dynamic strain. Figure 2 shows results for polymer foam (black circles). First, stiffness increases quickly with static amplitude, as long as surface cells, damaged by cutting, are compressed. Then stiffness reaches a plateau, corresponding to linearity zone: related value is considered to be the actual stiffness of material. For higher amplitudes, stiffness decreases due to buckling of cells. In the same way, loss factor first decreases and then reaches a stable value considered as actual material loss factor. Figure 3 shows a different behaviour for fibrous material. Stiffness always increases without reaching a stable value, due to collapsing of fibres. No clear tendency is observed for loss factor. No linearity zone can be found. Measurement of stiffness and loss factor are performed using a dynamic strain less than 0.05 %. For polymer foam, static strain is increasing step by step in order to reach linearity zone. This procedure is repeated for every sample. For fibrous material, measurement is performed using a static strain as close as possible to use conditions. Figure 2. Influence of dynamic (at static strain = 2%) and static strain (at dynamic strain = 0.02%) on measured stiffness and loss factor of polymer foam. Figure 3. Influence of dynamic (at static strain = 1.2%) and static strain (at dynamic strain = 0.085%) on measured stiffness and loss factor of rock wool.