ACOUSTICAL INSTRUMENTS 391 



velocities of 5 cm. per second. We might, however, expect that a 

 torque resulting from discontinuous stream flow at the edges would be 

 governed by a similar law. 



No direct tests have been reported on the accuracy of the coefficient 

 in Koenig's formula above 92 c.p.s., at which Zernow's measurements 

 were made. However, sound intensities determined by the Rayleigh 

 disc through the application of Koenig's formula have been found to 

 be in good agreement with those determined with a microphone 

 calibrated by other independent means. All the tests of the formula 

 have been made with plane or spherical sound waves of moderate 

 intensity. This fact should be borne in mind in order to guard against 

 the use of the device under conditions where the formula may not be 

 applicable. Such may be the case, for instance, where the sound 

 intensity is very high. Measurements in non-uniform sound fields 

 recently made by Kotowski ^ showed quite anomalous effects; in some 

 cases the deflection was even in a direction opposite to that expected. 



One great disadvantage of the Rayleigh disc method of measuring 

 sound intensity, as ordinarily applied, is the fact that the disc will 

 deflect under the action of a steady air stream. As the stream velocities 

 in a sound wave are in any case quite small, circulating air currents 

 may easily produce comparable deflections unless the instrument is well 

 shielded therefrom. Under carefully controlled conditions measure- 

 ments can be accurately made at sound intensities corresponding to 

 pressures as low as one bar. 



The effect of circulating air currents is greatly reduced in the method 

 of measurement with the Rayleigh disc adopted by Sivian.^" In this 

 method the intensity of the sound to be measured is modulated at the 

 source at a frequency of about 0.4 cycle per second. The disc with 

 its suspension is proportioned so that its natural frequency is equal 

 to this modulating frequency. The disc will then oscillate under the 

 action of the modulated sound wave at an amplitude proportional to 

 the square of the velocity. As circulating air currents generally have 

 components lying below the modulating frequency they will have but 

 little effect on the amplitude of the oscillations of the disc. 



Determination of Intensity from Static Pressure Measurements 



Another purely mechanical means for measuring sound intensity in 



absolute terms is based upon the fact that when radiant energy falls 



on a reflecting surface a static pressure is exerted on this surface, 



which in the case of sound is equal to ^^ ((7 + l)/2)//c, where / is the 



^E.N.T. 9, 404 (1932). 



lop/w/. AIag.5, 615 (1928). 



11 Lord Rayleigh, Phil. Mag. 10, 365 (1905). 



