The paper is concerned with the simulation of the action of a force environment on a multi-mode structure by means of a substituted force environment and is relevant to vibration testing of shell-like structures. The use of broad-band average spectral densities as response parameters in the simulation is justified. It is assumed that only simulation in some average sense of the modal responses in frequency bands can be contemplated because of the numbers of modes involved and their indeterminacy. By introducing a simple probabilistic model of the structure-environment interactions, some general results concerning the accuracy of simulation are deduced. Predictions of the simulation accuracy are shown to be possible if the relevant parameters can be estimated. This is confirmed by an experimental investigation.

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