Siren harmonics and a pure tone siren

1. Introduction .—This paper deals with the purity of tone of the sound produced by an ordinary Seebeck siren, and with the problem of designing a siren to produce a pure, or comparatively pure, tone. The results were obtained in the course of certain technical experiments on the location of air­craft by sound; they seem, however, to be of more general interest, as a source of sound giving a fairly pure tone of some intensity, whose frequency may be varied at will, is not easily obtainable. The results here described were obtained by the authors in the course of their work as technical officers of the Munitions Inventions Department, and are published by permission of the Department. The experiments were carried out at the Acoustical Besearch Section of the Department with the help of Major W. S. Tucker, R. E. The authors’ best thanks are due to Major Tucker and his staff for their assistance. The Seebeck siren, as described in books on sound, consists essentially of a revolving disc bored with holes, against which a stream of air is driven from a delivery pipe. The holes are equally spaced round a circle, and the pipe is fixed so that as the disc revolves the holes pass the mouth of the pipe in succession; when no hole is opposite the pipe the flow of air is partially or completely interrupted, and as a hole approaches and passes the flow of air waxes and wanes. When the disc is rotated sufficiently fast, a musical note is produced, whose frequency is equal to the number of holes passing the pipe per second.