ACOUSTICAL INSTRUMENTS 409 



shall indicate the variations with time in the frequencies and ampli- 

 tudes of all the harmonic components. Analyzers of the type just 

 described indicate at a given instant the amplitude of only one com- 

 ponent. In order to follow variations in all the components it would be 

 necessary to sweep the frequency of transmission of the analyzing 

 circuits rapidly back and forth over the frequency range of interest. 

 The selective element of the analyzer, however, possesses a finite time 

 constant; that is, when the selective circuit is set at a given trans- 

 mission frequency a finite time is required for the transmitted current 

 to reach a certain fraction of its steady state value and, similarly, a 

 finite time is required for the current to decay to a certain fraction of 

 this value when the transmission frequency is changed. This time 

 constant depends to some extent upon the shape of the transmission 

 versus frequency characteristic of the analyzing circuit but, in general, 

 it bears an inverse relation to the selectivity. It is therefore not 

 possible with an analyzer of this type, having a single variable selective 

 element, to perform a rapid analysis without sacrificing resolution. 

 This difficulty can, however, be circumvented if the analyzer is pro- 

 vided with a large number of fixed selective elements which are con- 

 tinuously operative. To build up the large number of required circuits 

 from electrical elements would be extremely costly and would result in 

 a bulky piece of apparatus. A compact form of analyzer having a 

 large number of fixed selective mechanical elements has recently been 

 described by C. N. Hickman.'*^ This device has a series of tuned reeds, 

 all driven electromagnetically at the same time by the current to be 

 analyzed. The reeds are tuned so that their resonant frequencies 

 differ progressively by equal pitch intervals. One hundred and twenty 

 reeds are used to cover the range from 50 to 3,200 cycles. The deflec- 

 tion of each reed is made visible by the projection on a screen of a spot 

 of light reflected from a mirror attached to the reed. The strength 

 of each component in the current may thus be observed simultaneously 

 on the screen or, if desired, the deflections may be recorded photo- 

 graphically. 



A difi'erent and ingenious approach to this problem has been made by 

 E. Meyer ^^ in a recently described instrument. By methods well 

 known in communications engineering the frequency of each component 

 in the current to be analyzed is increased by an equal amount. A 

 special high-frequency loud speaker translates the resultant currents 

 into sound waves which are now all of very short wave-length. These 

 waves are reflected from a concave grating made up of a large number 



" Jour. Aeons. Soc. Amer. 6, 108 (1934). 

 ""Zeits.ftir Tech. Phys. 12, 630 (1934). 



