An investigation of the performance of the Rayleigh disk

Abstract

The paper describes an investigation of the nature of the law which governs the torque on a Rayleigh disk suspended in a sound field. The torque is found to follow an expression of the form obtained theoretically by König, provided that factors such as the finite thickness of the disk and the lack of infinite inertia of the disk are taken into account. The measurements were made at frequencies spread over the range 250-4000 cyc./sec. The correction to the torque in König’s formula made necessary by the thickness of the disk is shown to be large and to have a sign opposite to that determined theoretically by König for ellipsoidal disks. The influence upon the torque of the proximity of the wall of a tube has been investigated. The nature of the precautions which should be observed in choosing the form of a Rayleigh disk are discussed with reference to reducing to a minimum the effects on the torque of the thickness and mobility of the disk and of diffraction of sound by the disk. An investigation has been made of the numerical factor which relates the magnitudes of the torque to the other variables of König’s formula. This part of the investigation reduces essentially to the measurement of the oscillatory velocity of the field in which the disk is situated. The smoke-particle method of Andrade and of Carrière was adapted for the above purpose, and it was found possible to make determinations of oscillatory velocity within about 1 % for frequencies up to 4000 cyc./sec. Measurements have been made of the torque on a Rayleigh disk of known dimensions in terms of the oscillatory velocity for frequencies in the range 250-4000 cyc./sec. Discrepancies of 3·5 % in velocity (7 % in torque) are shown to follow from the use of König’s formula, and these results confirm, and extend into the audio-frequency range, the investigations of Herrington & Oatley in the band 9-22 cyc./sec. The experimental work confirms that the stability of behaviour of the Rayleigh disk justifies its continued use as a reference standard of acoustical intensity but shows that in setting up the reference standard, corrections amounting in extreme cases to 0·3 db. must be applied to the intensity as determined from König’s formula.