A correction to the theory of the rayleigh disc as applied to the measurement of sound-intensity in water

Abstract

Konig's theory of the Rayleigh disc assumes the disc to be motionless, apart from its slow rotation. This assumption is unjustifiable in a relatively heavy medium such as water. The disc partakes of the motion of the medium to an extent depending on the density of the medium and the mass and dimensions of the disc. Consequently the fluid-velocity ν in König's equation for the torque on the disc must be modified by the introduction of a factor β involving the velocity of the disc relative to the medium. The value of β for ellipsoidal discs is calculated from hydrodynamic theory and determined experimentally. A comparison is made of the results for ellipsoidal discs and for flat cylindrical discs. A method of measuring sound-intensities under water by means of small heavy metal discs is described.