Dynamic mechanical response of foams for noise control

In this work, the viscoelastic behavior of open cell polyurethane foams used in noise control applications is investigated through dynamic mechanical analysis in compression. Several levels of static strains superimposed on a small dynamic one were considered in order to assess the effect of material non-linearity on the mechanical response. Further, a wide range of frequencies and temperatures was explored. For each static strain a different master curve for conservative component of complex modulus, E', could be determined. Interestingly, the loss factor was the same at all static strains, indicating that the relative contribution of energy dissipation and conservation is unaffected by the static strain. Moreover, shift factors (and thus the bulk material relaxation times) turned out to be independent on static strain level. These results suggest that the non-linearity of the foam is linked to the change in foam structure with strain rather than to a non-linear behavior of the viscoelastic constituent material. The acoustic performance of the considered materials was modelled for a case study with two approaches: i) standard simulations performed taking a single valued complex modulus, measured at 50Hz and room temperature or ii) simulations taking into account the complex modulus frequency dependence obtained from the master curves. The transmission loss prediction obtained in the second case is in better agreement with experiments, especially at high frequencies.