The calculation of damping from the results of resonance and free oscillation experiments has hitherto been based on elementary theory in which the springs or other elastic supports are represented by lumped stiffness parameters. This “ light spring theory” is generalised here to include the elastic stiffness terms in distributed parameter form; i.e. the mass of the springs or other supports is taken account of by including wave equations for the propagation of stress waves in the springs and so on. A record of the free motion of a model in a wind tunnel, shown in Fig. 1., indicates the inertial effects of the spring supports. To minimise this “surging” of the supports one may design them to have high natural frequencies compared with the system frequencies. However, this gives no indication of the error entailed in using light spring theory, nor does it cover experiments which must be performed at high frequencies.