A standard source of sound and the measurement of minimum audibility

Abstract

In the Discourse on Audition, held by the Physical Society in 1931, one of the present authors put forward the suggestion that a standard source of sound might be made from a sounding tube open to the air at one end, the amplitude being measured by observing the traces of smoke particles at an antinode. The air vibrations were to be maintained by a vibrating diaphragm closing one end of the tube, and the open end was to be provided with a baffle board, to confine the disturbance to the half space (Andrade 1931 a ). The present paper deals with a development of a source of known amplitude, based on this principle, and the application of it measure the minimum audible energy. Photographs of the traces of fine cork particles at a node in a vibrating air column had been made at the time of the original suggestion, but no success had been attained in photographing the traces of smoke particles. The extent to which particles follow the motion of a rapidly vibrating medium in which particles follow the motion of a rapidly vibrating medium in which they are suspended depends upon their size, their density and the frequency of the vibration. Examples are given in a paper on the vibration of air in tubes (Andrade 1931 b ). Application of the formula there cited shows, for instance, that particles of magnesium oxide smoke, of diameter 0⋅3 μ , acquire 0⋅9999 of the full amplitude of the air vibration at a frequency of 500 c./sec. Such smoke particles are, therefore, suitable for use as tracing points, and were, in fact, adopted in this work.