Expressions are given for estimating the rms response of a general mechanical structure, vibrating in individual modes, when exposed to noise. The noise field is characterized by the free-field rms sound pressure. Linearity is assumed in air and structure. The principles of reciprocity and superposition are employed. As an extreme case, it is assumed that all damping is due to radiation to the far field. The mean square velocity for a uniform distribution of sources (alternatively, the ensemble average for all angles of incidence) is particularly simple; for given sound spectrum level, it varies inversely with modal stiffness but is independent of the acoustic-mechanical coupling. Under the same assumptions, the mean square dynamic strains in fundamental modes of plates. designed to the same static strain under fixed pressurization, are approximately proportional to spectral density of free-field noise multiplied by mass and divided by the square of surface area. Modifications for other conditions are discussed. (Supported by Wright Air Development Division.)