Reduction index modeling by finite elements

Abstract

When a sound is traveling from one room to another through a wall, it changes character. In most cases the wanted change is to reduce the level of the sound and, different walls do that differently well. A measure of how much the sound is lowered in level is the so-called reduction index, which is dependent on the material properties and geometry of the separating structure, (i.e. the wall). The aim of this work has been to investigate the possibility to construct a computer model for calculating the reduction index of a structure. The finite element method has been used to investigate the vibrations in a structure cased by air-borne sound waves. The vibrations propagate through the structure leading to a radiation of sound on the opposite side. The difference in sound pressure level on both sides of the structure determines the value of the reduction index for specific structure. When a 2D finite element model was developed, a parameter study was performed where the material properties of the structure was changed in a controlled manner. Conclusions of the results from the parameter study was drawn, both to get a confirmation of the model and the influence material properties have on the reduction index. The results from the model agreed well with earlier calculation methods for double walls. Therefore, the model is expected to deliver good results also for more complex structures. The parameter study show some interesting results when looking at two differently stiff plates coupled to each by air and studs. At some frequencies the reduction index is dependent of in which direction the sound is traveling through the structure. If the sound hits the stiffer plate first, a higher reduction index can be seen than if the sound hits the softer plate and is radiated from the stiff plate.