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This paper presents a case study on the full inverse characterization of the material properties of an open-cell poroelastic foam using impedance tube measurements. It aims to show the importance of controlling the lateral boundary condition in the impedance tube, and selecting an appropriate acoustic model to obtain the most accurate material properties. The case study uses a four-inch thick melamine foam and a 100-mm diameter tube. The foam is mechanically cut to fit within the circular tube. However, the cutting process is not perfect and a tiny lateral air gap exists between the material and the tube (i.e. the foam diameter is 99.5 mm for a 100-mm diameter tube). The typical characterization procedure is to mix direct and indirect measurements to retrieve the material properties of the foam. First, open porosity, bulk density, and static airflow resistivity are directly measured.

A poroelastic characterization of open-cell porous materials using an impedance tube is proposed in this paper. Commonly, porous materials are modeled using Biot’s theory. However, this theory requires several parameters which can be difficult to obtain by different methods (direct, indirect or inverse measurements). The proposed method retrieves all the Biot’s parameters with one absorption measurement in an impedance tube for isotropic poroelastic materials following the Johnson-Champoux-Allard’s model (for the fluid phase). The sample is a cylinder bonded to the rigid termination of the tube with a diameter smaller than the tube’s one. In that case, a lateral air gap is voluntary induced to prevent lateral clamping. Using this setup, the absorption curve exhibits a characteristic elastic resonance (quarter wavelength resonance) and the repeatability is ensured by controlling boundary and mounting conditions.

A method is presented to determine the bulk elastic properties of isotropic elastic closed-cell foams from impedance tube sound absorption tests. For such foams, resonant sound absorption is generally observed, where acoustic energy is transformed into mechanical vibration, which in turn is dissipated into heat due to structural damping. This paper shows how the bulk Young's modulus and damping loss factor can be deduced from the resonant absorption. Also, an optimal damping loss factor yielding 100% of absorption at the first resonance is defined from the developed theory. It is shown how this optimal factor can be used to properly design efficient sound absorbing treatments. The method is experimentally tested on one expanding closed-cell foam to find its elastic properties. Using the found properties, sound absorption predictions using an equivalent solid model with and without surface absorption are compared to measurements.

Nowadays, expanding sealing parts in automotive hollow body networks are widely used. These parts are usually made up from expanding foams or an assembly of expanding foams and solid materials. The use of these sealing parts has demonstrated an influence on the noise inside the car. These findings proved the necessity of designing sealing parts especially to reduce the propagation of sound through the frame cavities and hollow bodies. In this work, experimental investigations have been conducted to characterize the acoustic performances (absorption, transmission loss) of the individual materials constituting the parts and their assembly. Some design rules have been extracted to improve their efficiencies. Also, to better understand the acoustic behavior of the expanding foams, existing theoretical models for closed or open foams have been tested and compared to measurements.

There are many software tools in use today that are implementing the Biot, or complementary, method for the evaluation of foam and fiber materials. The justification of this process is to understand which mechanisms of the noise control material are contributing to the noise reduction and to optimize the material based on its acoustic properties. The disadvantage of this method is that it is quite complex and time consuming to test a material in order to extract the underlying properties that govern the acoustic performance. An alternative inverse method for material characterization based on simple impedance tube measurements has been developed lately. This paper recalls the physics and mathematics behind the method and concentrates on its validation.