360 



BELL SYSTEM TECHNICAL JOURNAL 



If / is expressed in microwatts and p in bars, this reduces to 



415 



(5) 



The intensity level / is defined by 



/ = log.o / (6) 



and is expressed in bels. These relations hold for any complex sound 

 as well as for a pure tone if p is interpreted as the root mean square 

 value of the pressure change. 



It is seen then that all of these quantities can be determined by 

 making experimental measurements of the pressure change. For 

 accomplishing this the following methods were used. 



/ 



FILTER 



8,000 



CYCLES TO 



INFINITY 



FILTER 

 8,000 TO 



11,300 

 CYCLES 



FILTER 

 5600 TO 



8000 

 CYCLES 



FILTER 

 4000 TO 



5600 

 CYCLES 



EIGHT 

 OTHER 

 FILTERS 



I FLUX- \ 

 I METER j 



15-SECOND SYNCHRONOUS 

 CONTACTS MOTOR 



FOR TAKING IS-SECOND 



READINGS WHICH AVERAGE 



ALL FREOUENCIES 



Fig. 12 — Schematic of electrical circuit for measuring the average power-frequency 



distribution of sounds. 



The speech to be analyzed is picked up by a Wente condenser 



microphone and sent into a vacuum tube circuit. This circuit is 



arranged so that any one of 14 band pass filters can be inserted. 



After passing through the filter the electrical speech wave is then 



sent through a rectifier and finally into a meter. A schematic ^ of 



^ See paper entitled "A New Analyzer of Speech and Music" by H. K. Dunn 

 (Bell Laboratories Record, November, 1930) and also paper entitled "Absolute 

 Amplitudes and Spectra of Certain Musical Instruments and Orchestras" by Sivian, 

 Dunn & White, Jour. Acotis. Soc. of America, Jan., 1931. 



