410 BELL SYSTEM TECHNICAL JOURNAL 



of equally spaced rods. The component waves are brought to a focus 

 at different points along a focal surface analogous to the dispersion of 

 light waves by an optical grating. A high-frequency microphone is 

 moved back. and forth along the focal plane through an amplitude large 

 enough to cover one order of the spectra. This microphone is con- 

 nected to an appropriate meter which records optically the intensity at 

 various parts of the spectrum, which have a 1 : 1 correspondence with 

 the component frequencies in the original current. 



Measurement of Pitch 



For acoustical studies, where it is of no particular importance to 

 know the wave form but where interest lies in the variation of pitch 

 with time, as in the study of the vibrato in musical tones, or in the 

 inflections of the speaking voice, several types of instruments have been 

 devised. Perhaps of these the most widely known is the tonoscope 

 developed by C. E. Seashore ^^ and his associates, which operates on the 

 stroboscopic principle. This instrument has rows of uniformly spaced 

 dots on a rotating cylinder, the number of dots increasing in successive 

 rows. A neon light is made to flicker in synchronism with the funda- 

 mental of the tone under investigation. The particular row which 

 under the light appears stationary gives the pitch of the tone at any 

 instant. By the aid of a suitable camera the time variations of pitch 

 may be recorded photographically, giving a so-called strobophotograph. 



A frequency recorder operating on a different principle has been 

 described by Hunt.^^ By a special circuit arrangement, employing 

 gas-filled discharge tubes in combination with a spark recorder, the 

 pitch of a tone can be recorded on paper. The scale is linear up to 8,000 

 cycles. This instrument is capable of following changes in pitch at a 

 high rate of speed. 



High-Speed Level Recorder 

 In some important types of sound measurements we are not inter- 

 ested in a detailed analysis of the sound wave but merely in the varia- 

 tion with time of the average level of the sound, as in the measurement 

 of the rate of decay in a room or the flow of energy in speech, music, or 

 noise. In some cases this average is preferably taken over long and in 

 others over short time intervals. For long time averages, a thermo- 

 couple or rectifier and an ammeter may be used, but for short time 

 averages an instrument is required which can follow changes in in- 

 tensity at a higher rate of speed. Frequently also the range of intcn- 



" Jour. Aeons. Sac. Amer. 2, 77 (1930). 

 ^^Rev. Sci. Inst. 6, 43 (1935). 



