Summary of the Doctoral Thesis
"Study of the acoustic behavior of multilayer structures by means of NAH"
Eva MĒ Escuder Silla

    The object of this thesis is the study of the acoustic behavior of multiple panels, using like tool, among others, the denominated NAH (Nearfield Acoustical Holography) technique. The obtained conclusions and the tools developed in this study have implications in two scopes, the sound insulation with multilayer structures, and the optimization of the radiation of electro-acoustic systems based on the vibration of a panel caused by an exciting element of dynamic type (case of the flat loudspeakers). The process to obtain guessed right predictions of the behavior of multilayer structures, as much from the point of view of the acoustic application of the construction, like in the electro-acoustic (improvement of benefits of loudspeakers), must start off, necessarily, of a precise knowledge of the acoustic characteristics of the components, that is to say, each one of the layers. This study constitutes first stage of this investigation. The validation of the theories on the sound insulation needs the test of different materials and configurations. The objective is to increase to the maximum the index of isolation of a certain configuration, trying to diminish the costs as far as possible. The study of the acoustic behavior of different configurations usually is made in transmission chamber. The results of the measures in these conditions do not have why to agree with the obtained ones with measures "in situ", since the conditions of assembly are until certain point uncontrollable. Diverse works exist, where the different models, theories and techniques of measurement are developed with this aim. There are models and theories that characterize absorbents materials measuring in the tube of Kundt. Also exist methods based on mechanisms of "investment" so that, from measures of a determined acoustics variable, they allow to obtain the necessary parameters of entrance for the simulation programs. One is not due to forget that the results of a simulation are for ideal partitions with elastic properties that do not vary based on the incidence direction. In many of the materials used normally in the construction, such as the DM, the conglomerate, the brick or the glass cannot be assured isotropy. The elastic properties, or the density of the material can vary based on the angle of incidence. The assembly characteristics influence much in the type of model to take to make the prediction. At the moment, it is interesting the implementation of conditions of contour that respond to the conditions of fixation with the purpose of improving the predictions of the isolation of finite multilayer structures. 

    Respect to the application of the improvement of the efficiency of radiating systems, a distributed modes loudspeaker is used like experimental reference. It consists of a flat surface that is in connection with a transducer, and radiate generated acoustic energy when vibrating. The efficiency of these transducers depends, to a great extent, of the connect between the electromechanical part and the mechanical-acoustics, and of the mechanical properties of the materials of which the flat surface is constituted. In addition, the different configurations from assembly contribute conditions of contour to the problem, that turn out from interest to study. In addition to the measures of vibrations and pressure level, the NAH technique is used. It is based on the measurement of sound pressure level of the study surface, excited acoustically on the other hand later, in a flat parallel and near this surface. Known these values to certain distance, by means of techniques of digital processing, the acoustic magnitudes in the object surface can be obtained by means of the back-propagation of the field. 

    In this thesis the following constribuciones are made: 
1) to optimize the configuration (Hardware and Software) of NAH. 
2) to analyze the efficiency of the radiation and the characteristics of vibration of different surfaces based on different conditions of fixation. 
3) it is deepened in methods to determine the acoustic impedance of radiation 
4) methods of optimization in the design of slight partitions multilayer are applied to improve the isolation contributed in the construction, as well as inverse methods of characterization of materials from measures of transmission loss.