A continuous damped vibration absorber to reduce broad-band wave propagation in beams

In order to attenuate structural waves in beams, a damped mass–spring absorber system is considered that is attached continuously along the beam length. Compared with other measures, such as impedance changes or tuned neutralisers applied at a single point, it is effective for excitation at any location along the beam. Although it is a tuned system, it can also be designed to be effective over a broad frequency range by the use of a high damping loss factor and multiple tuning frequencies. It has the advantage over constrained layer damping treatments that it can be effective even when the structural wavelength is long. The parameters controlling its behaviour are investigated and simple formulae developed, allowing optimisation of its performance. The effective frequency bandwidth increases as the mass ratio of the absorber and the beam is increased and, for moderate-to-high damping, it also increases as the damping loss factor is increased. The maximum decay rate is independent of mass and damping for light damping, but for higher damping it reduces as loss factor increases and increases as the mass ratio increase. A particular application is the reduction of noise from a railway track, which requires the attenuation of structural waves along the rail to be increased over a frequency band of two or more octaves.