Holst-Jensen O., Sorokin S.

The purpose of this study is to investigate the possibilities of reducing the transfer of energy in certain frequency regions from vibration sources like pumps or compressors to large piping systems. In order to reduce the energy, the tube is fitted with blocking masses in certain periodic pattern. This structure at a certain optimized geometry may have the ability to block the propagation of waves from the source into the tube in certain frequency bands. The theoretical model is formulated as a system of boundary equations, which describe propagation of flexural, axial and torsion waves within each segment of a tube between inclusions, and continuity conditions at the points, where masses are attached. The presence of masses couples propagation of waves of all these types. An exact solution of this system is obtained and it is found that appropriate location of three identical equally spaced masses can dramatically decrease the power input into the system in some frequency ‘stop bands’ regardless the excitation conditions. This effect, well known for an infinite chain of periodic attachments, is demonstrated here for only three ‘periodicity cells’. Theoretical analysis is expanded to parametric studies of location of ‘stop bands’ in a given frequency range and to sensitivity analysis of wave attenuation inside the ‘stop bands’ to possible imperfections of mounting and spacing of attachments. Amplitudes of forced vibrations of a span, where an excitation force is applied, are also assessed and strong localization (a trapped mode effect) is observed. The experimental model is made of a straight pipe with eccentric blocking masses. The pipe is driven by a shaker and it is connected to a radiating surface. The insertion loss achieved by the periodic blocking masses is measured acoustically.