ACOUSTICAL INSTRUMENTS 389 



the designer of acoustical instruments some twenty years ago through 

 the invention of the vacuum tube telephone amplifier. Previously 

 his chief concern lay in making a device sufficiently sensitive to give a 

 measurable response; now sensitivity became of secondary importance, 

 and attention could be focused on the design of instruments which 

 should be capable of performing their function without distortion. 



As a result of this invention, instruments depending upon an 

 amplifier for their utility have come so to dominate the field of acoustic 

 measurements that we might easily be led to disregard all others. 

 A number of such other instruments have, however, in recent years 

 been brought to a high state of development which are peculiarly 

 suited either for the calibration of other devices, or for the study of 

 certain special problems. This paper attempts to give a brief critical 

 survey of the various types of acoustical instruments which at the 

 present time are finding applications in technical fields and in acoustical 

 research studies. 



General Principles 



The Rayleigh Disc 



That under certain conditions a torque is exerted on a thin disc 



suspended by a fine fibre in a stream of air was first observed by the 



late Lord Rayleigh,^ who recognized in this phenomenon a means for 



measuring the intensity of sound. 



The following quantitative relationship between the torque and the 

 stream velocity was derived by W. Koenig : ^ 



4 

 T = -por^u^ sm 2d, 



where po is the mean density of the medium, r the radius of the disc, 

 u the stream velocity, and d the angle between the undisturbed stream 

 and the normal to the disc. The assumptions underlying the deriva- 

 tion of this formula are that the fluid is incompressible, that the disc 

 is an infinitely thin ellipsoid and that there are no forces due to vis- 

 cosity or to discontinuities of flow at the edges of the disc; i.e., the 

 velocities are derivable from a potential. In view of these assump- 

 tions how far may we rely on the above formula in applying it to a 

 suspended plane flat disc as commonly used for measuring the particle 

 velocity of a sound wave, where none of these assumptions are strictly 

 fulfilled? In most acoustical problems it is perfectly safe to assume 

 a potential field as the forces due to viscosity and eddies are of the 



^Phil. Mag. 14, 186 (1882). 

 * Wied. Ann. 43, 43 (1891). 



