The sound absorbing material is one of the important countermeasures for the control of noise, especially by using passive means. In order to use the absorbing materials most efficiently for given design conditions, the physical parameters and its acoustic characteristics of the materials should be known a priori. In this study, a method based on both of the experiment and the theory has been studied for the estimation of the acoustic propagation constant and the characteristic impedance of porous sound materials, which are most fundamental acoustic properties of materials. The rigid frame model of the porous material was adopted in the underlying governing equations. First, a new measurement method was proposed by using the two loads and three microphones in the duct: the normal incidence of sound was assumed. Two types of porous materials were tested in the experiments: the glass fiber and the polyurethane foam. The measured propagation constant and characteristic impedance were compared with those by the direct method using the conventional two microphone method and by the two cavity method. The results of the proposed method showed very good agreements with those by the existing techniques and could be regarded as very reliable. Second, the measured two acoustic parameters were compared with the theoretical rigid frame models of Biot-Allard, Johnson and Attenborough, and by using the iterative curve fitting to the measured two acoustic parameters, the basic physical parameters of the porous material could be estimated. In this way, the unmeasurable physical parameters such as tortuosity and shape factors could be determined by using the measurable quantities such as porosity, flow resistivity, density, etc. Finally, from the estimated complex density and the bulk modulus of the material, the sound absorption coefficient as a function of the frequency could be predicted and the good agreements with the directly measured results could be observed. The method studied in this thesis is thought to be useful in the efficient application of the sound absorbing porous materials to many practical noise control problems.
