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Resonant structural vibrations are a common source of disruptive noise, and suppressing these vibrations is often the most direct way to reduce the noise levels. Elastic metamaterials (EMMs) consist of distributed resonant substructures, at a scale which is small compared to the wavelength of vibration. This allows these materials to be used in applications where space is limited, and more traditional vibration suppression techniques would be impractical. Tuned resonators can be designed through selection of geometry or material properties, but an alternative approach, which requires significantly less prototyping, is through the use of shunted electrodynamic inertial actuators. In this paper, a novel electrodynamic metamaterial (EDMM) is proposed consisting of an array of mass-produced inertial actuators, each connected to a tuned shunt impedance. It is considered impractical to measure the dynamic and electrical parameters of a large number of actuators, and so the effect of uncertainties in the actuators is investigated on both the performance and the stability of the EDMM.

Parametric uncertainty in a structure can reduce modelling accuracy, leading to a reduction in the efficacy of a vibration suppression system. The suppression of modal vibration with robustness to parametric uncertainty has been demonstrated using multiple tuned-vibration-absorbers (TVAs) with distributed resonance frequencies. In this paper, an Elastic Metamaterial (EMM) unit cell consisting of multiple single-degree-of-freedom resonators is defined for the absorption of vibration in a cantilever beam. A genetic algorithm (GA), hybrid genetic algorithm (HGA) and particle swarm optimisation (PSO) are compared in their ability to optimise the resonance frequencies of the unit cell to minimise the mean kinetic energy gain of the EMM, on a beam with parametric variation. Firstly, all optimisation procedures are able to produce an EMM unit cell with good robustness to parametric uncertainty. When the optimisations are run until convergence, the PSO is shown to achieve the best fitness value, but with an increase in computation time compared to the GA. The HGA achieves a better fitness value than the GA but computation time is inflated by a factor greater than 50. By also running until a time constraint is met, it is shown that the GA and PSO perform similarly for the same time-limit.

Undesirable resonant vibrations in a structure can be suppressed using various techniques. However, parametric uncertainty can reduce modelling accuracy and lead to a reduction in the efficacy of the designed suppression system. The suppression of multiple modes of vibration with robustness to parametric uncertainty is achievable using a large number of tuned-vibration-absorbers (TVAs) with distributed resonance frequencies. However, the performance of TVAs is also dependent on their positions relative to the mode shapes of the structure, which are not always known. Elastic metamaterials (EMMs) consist of distributed resonant substructures, at a scale which is small compared to the wavelength of vibration. This allows the material to be considered homogeneous with one set of defining physical properties, thus reducing the dependence on mode shapes. In this paper, a unit cell of a single-degree-of-freedom resonator-based metamaterial is defined, and the individual resonance frequencies are optimised using different metaheuristic algorithms for a nominal fixed-parameter beam and for the same structure with uncertainty in one or more of its parameters. The performance of the optimised metamaterial unit cell is then assessed when applied to more complex structures.

The purpose of this paper is to report an accidental finding which was felt to be provocative and worthy of further, more controlled, investigation. The emphasis in this report is on detailed description of the phenomenon, with a minimum of theorizing. While in a very deep hypnotic trance, 5 was induced to have both positive and negative hallucinations. On the following day, he reported spontaneously that he had been struck by the clarity of both the visions and the perceptions of actual objects while hypnotized; he had not, however, been wearing his glasses at the time, though, under normal circumstances he wore his glasses at all times. No suggestions for improved vision or extra effort had been given. 2 careful opthalmo-logical examinations were made during the following 2 weeks, confirming the fact that S's eyesight showed a significant improvement during hypnosis as opposed to the waking state.