THE USE OF THE RAYLEIGH DISK IN THE DETERMINATION 
OF RELATIVE SOUND INTENSITIES, 
By Harold Stiles. 
In the second volume of his Theory of Sound, Lord Rayleigh presents 
a method for determining mathematically the intensity of sound at very 
great distances from a rigid sphere, the source of sound being confined 
to a small area on its surface. Prof. G. W. Stewart* has extended the 
work of Rayleigh and has calculated the relative sound intensities for 
sets of points lying on circles concentric with the sphere, the planes of 
these circles also passing through the source' of sound. The points on 
any one circle were 15 degrees apart and the diameters of the four 
circles used were respectively 2, 3, 4, and 5 times the diameter of the 
sphere. 
During the summer of 1912 at the State University of Iowa, Prof. 
Stewart and myself undertook to verify experimentally the theoretical 
determination and to test the Rayleigh disk as a means for determining 
relative sound intensities. 
Although Rayleigh suggests the possibilities of the disk, we were 
unable to discover any report of its use in the actual determination of 
sound intensities in the open. A modification of the Rayleigh disk ap- 
paratus which we used consisted of a brass tube 5 cm. in diameter and 
in length about three-fourths of the wave length of the sound produced. 
The tube is drawn to scale in Fig. 2. At the point t is located a 
thin paper diaphragm one-fourth of a wave length from the open end 
of the tube. In the constricted portion B a circular mirror 6 mm. in 
diameter is delicately suspended by a quartz fiber so as to make an 
angle of 45 degrees with the axis of the tube. Light from an illumi- 
nated scale passes through a small window at B and is reflected by 
the mirror along the axis of the tube through another window at C to 
the observing telescope. Alternating currents of air in the resonating 
tube deflect the disk, the deflection being proportional to the energy. 
*Phys. Rev. vol. XXXVIII, No. 6, December, 1911. 
