Determination of dynamic storage modulus of viscoelastic and poroelastic materials using a simplified time of flight approach

Viscoelastic and poroelastic materials are widely used in multilayer panels for noise control. They usually are utilized as inner decoupling layer in double wall systems in order to increase the sound transmission loss of a bare plate. In order to correctly simulate the acoustical behavior of such systems it is necessary to measure elastic properties of those materials (storage modulus, Poisson's ratio and loss factor) and for open cell structures also physical properties need to be determined. Most of the materials utilized in trimmed panels could exhibit elastic properties varying with frequency thus a quasi-static measurement technique could not be accurate enough for taking into account such viscoelasticity effects. The present paper focuses on the estimation of storage modulus as a function of the frequency of visco and poro-elastic materials. In particular tested material is positioned between two plates, one of them being excited by an electromagnetic shaker. Using a sine burst as excitation signal the accelerometric response in time domain is measured at the bottom and upper plates. Using a cross-correlation function the time of flight and the amplitude ratio between accelerometers are finally measured. At the end a time domain transfer matrix model of the experimental set-up is utilized to inversely estimate complex modulus of the materials once remaining mechanical and physical properties have been fixed. Results will be presented and discussed for different viscoelastic and poroelastic materials and compared with well established quasi-static and dynamic techniques.