On the Theory of the Inertia and Diffraction Corrections for the Rayleigh Disc

A thin circular disc suspended by a quartz fibre tends to set itself broadside on to the direction of the propagation of incident sound waves, and its use in combination with resonators is well known as a means of measuring relative intensities of acoustic radiation fields. In a progressive or stationary wave in which the velocity amplitude is │ξ│, the average couple on a circular disc of radius a is usually given by the well-known formula L = - ⅔ρ a 3│ξ│2 sin 2α, (1) where ρ is the density of the medium and α is the angle between the direction of propagation of the wave-front and the normal to the disc, while the negative sign indicates that the couple tends to diminish α.