The paper analyzes the eigenmodes of bending vibrations of ecological paperboard as of a plate by taking into account the additional effect of bending vibrations of a string included in the paperboard. The tension of the string can be used to control the eigenmodes of the analyzed paperboard. Experimental investigations were conducted using time-averaged projection moiré techniques on a specially developed experimental setup. The performed experimental and numerical studies demonstrated that the first eigenmodes of vibrations of the paperboard with supplementary stiffness elements occur at higher frequencies when compared with the conventional paperboard. The obtained eigenmodes enable to determine not only the vibration characteristics of the wall of the package by nondestructive methods, but also the strength parameters of the material of the package. The obtained results are used in the process of design of package elements.