On the Whispering-Gallery phenomenon

Abstract

The theory of the curious and interesting effects observed in the Whispering Gallery at the base of the dome in St. Paul’s Cathedral was discussed long ago by the late Lord Rayleigh in his treatise on Sound, and more fully, from the point of view of wave-propagation, in two recent papers. Expressed in general terms, the explanation put forward is: the sound-waves cling to the concave surface of the wall and travel circumferentially along it, suffering a diminution of intensity relatively small in comparison with that of waves spherically diverging from the source. The theory was supported by the observation that the effects under consideration are most noticeable when a directed source of sound is used and placed close to the curved surface, with its maximum emission in a direction tangential to it. An experimental illustration of the theory was also put forward, using a semicircular reflector of sheet metal, with a bird-call as a source of sound of short wave-length, and a high-pressure sensitive flame as detector. With this apparatus, a small obstacle placed close to the concave surface is competent to intercept most of the effect. Some recent experiments of Barton and Kilby, illustrating the matter further, by the aid of dust-figures produced by electric sparks, may also be mentioned. On account of the great interest of the subject, not only from a purely acoustical standpoint, but also in view of analogous effects in other branches of physics, e. g. , the propagation of electric or seismic waves round the earth, it occurred to the authors that it would be worth while to make a closer investigation of the phenomena and of the explanations that have been advanced, and this paper describes the results obtained.