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Vibration neutralizers are effective vibration control devices at a single frequency. If they can compensate for drift in the excitation frequency by adjusting their stiffness the performance can be improved, and the range of problems to which they can be applied is broadened. This paper considers a beam-like adaptive vibration neutralizer, and it is shown that the stiffness of the device and hence its natural frequency can be significantly altered by varying the beam cross-section. Several different beam configurations are investigated and the rate of change of stiffness as a function of beam separation is calculated for each configuration. The results are validated by some simple experiments. Real-time stiffness control of a beam-like tuneable neutralizer is also demonstrated both by computer simulation and experiment. The neutralizer is subjected to swept sine excitation over a six-second period and the tuned condition is maintained throughout the excitation period. The efficacy of using a nonlinear fuzzy logic controller is compared with the use of a simple proportional controller.

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Control of interior noise levels in aircraft has been a significant research area over the last two decades. Vibration absorbers have often been researched as more efficacious solutions to this problem than absorbent blankets or fully active systems. In this paper we review the large body of work performed in this field and we offer an indication of the remaining areas for fruitful research. Surprisingly few installations of vibration absorbers for interior noise control have been realized, and we believe this is due to the pervasive belief in industry that vibration absorbers can only be applied to control resonant, not forced behavior in a structure. The potential of adaptive passive control using vibration absorbers has been shown by many researchers, and we believe that this direction may yield several practical solutions to the problem of interior noise in aircraft.