ACOUSTICAL INSTRUMENTS 407 



ment may at times find an application in the study of acoustical prob- 

 lems. We must, therefore, necessarily restrict ourselves to a discus- 

 sion of the kind of instruments which can give types of information of 

 general acoustical interest. 



The Oscillograph 



If we wish to obtain a complete picture of the sound wave the micro- 

 phone amplifier output is connected to an oscillograph, which is a 

 device for translating a time pattern of the electric current into a 

 corresponding space pattern. If an undistorted pattern is to be 

 obtained, not only must the various harmonic components of the 

 current and the recorded wave have the same relative amplitudes, but 

 their phase relationships must be preserved. One form of instrument 

 very closely satisfying these conditions up to about 10,000 cycles 

 is a Curtis string oscillograph,^^ which is a modified form of the Ein- 

 thoven galvanometer. The arrangement of this instrument is shown 

 diagrammatically in Fig. 8. It records the wave form optically on 

 photographic paper, which is automatically developed and fixed 

 within a fraction of a minute after exposure. 



In certain types of problems one of the various recording devices 

 employed in the production of sound pictures may be used advanta- 

 geously. These instruments have, in general, not been designed to be 

 free from phase distortion but the records are in a form suitable for 

 reproduction so they lend themselves particularly to the study of the 

 subjective aspects of sound. 



When the sound wave to be studied is steady, the wave form can be 

 conveniently observed or photographed by means of a cathode ray 

 oscillograph. These instruments are now to be had in convenient form. 

 When used with an automatic sweep circuit, as suggested by Bedell 

 and Reich ,^^ the wave form of any steady state current is shown as a 

 stationary pattern on a screen. These oscillographs are generally 

 free from both frequency and phase distortion up to the highest audio 

 frequencies. 



Harmonic Analyzers for Steady Currents 



If we wish to study the composition of a sound wave in terms of its 



harmonic components we may, of course, analyze the oscillographic 



records by means of any one of the well known methods of harmonic 



analysis, but this is at best a laborious process. Also, it is usually 



difficult to read an oscillogram with sufficient accuracy to determine 



the magnitude of any component that is much smaller than that of the 



^' Bell Sys. Tech. Jour. XU, p. 76. 

 ^^ Science 63, 619 (1926). 



