Room walls in which people reside are often built as panels with periodical stiffeners because the aim is to have walls with sufficient rigidity and minimum mass. That kind of panels have specific acoustic response which can be modelled with simplified methods because of periodic repetition of boundary conditions which causes minimising of calculation time with optimal accuracy. In the case when the panel is excited with plane acoustic waves and when the height of the panel period is minimally three times larger than its width, infinite panel periodically supported in one direction can be modelled as infinite periodically supported beam which is the case in this dissertation. Acoustical response is calculated with the method od virtual work with displacement defined with series of space-harmonic waves. Concerning that the model is infinite and that a goal is to calculate finite panel the acoustic and structural coupling effects are limited to the area of finite panel using spatial windowing method. In this dissertation a theoretical description and numerical results of influence of damping in periodical stiffeners on the panel acoustic response is given. The influence of the number of space-harmonic waves on the calculation accuracy of the panel acoustic response and panel free vibrations is analysed. When there is sufficient damping in the system the maximum number of space-harmonic waves for gaining an appropriate accuracy appear on the maximum frequency as it is published in the literature but if the damping is low, the maximum number of space-harmonic waves appear on lower frequency. Damping in the sistem also influences the number of space-harmonic waves bellow which there amplitudes of free vibrations are faster calculated with the method of virtual work than with the series expansion. Space-harmonic waves in the literature are presented in diagrams with amplitudes and phase velocities for one frequency. In this dissertation a diagram with amplitude and phase velocities for an frequency spectrum is used where wave amplitudes are defined with color. With this approach the changes in the frequency domain can be easily seen. Also, free vibrations of infinite periodically supported beam on simple supports are calculated where a new shape function is derived and its accuracy is confirmed with the method of virtual work. In accordance with numerical simulation within the current computer possibilities experimental boundary conditions are chosen and the experiments where done. The results of eksperimental model are in a good agreement with the results of numerical simulation.