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BELL SYSTEM TECHNICAL JOURNAL 



Some of the more detailed considerations which have led to the 

 setting of these standards will now be given. 



Frequency Band 



Figure 2 gives some data in regard to the frequency range required 

 for different musical instruments as well as speech. These data were 

 obtained in the Bell Telephone Laboratories using an arrangement 

 capable of picking up and reproducing practically the whole audible 

 frequency range. Certain very low-frequency instruments, such as 

 organ pipes and bass drums, were not included in the tests owing to 

 laboratory limitations. A number of observers listened to the repro- 

 duced material, first, when practically the whole frequency range was 

 transmitted and, second, with either the high frequencies or low 



ACTUAL TONE RANGE 



ACCOMPANYING NOISE RANGE 



''CUTOFF FREQUENCY AT WHICH 80%OF 

 THE OBSERVERS COULD DETECT THE FILTER 



TROMBONE 



BASS CLARINET 



BASSOON 



BASS TUBA 



SNARE DRUM 



14 " CYMBALS 



KETTLE DRUM-TYMPAr|ll 



BASS DRUM 



VIOLIN 



CELLO 



BASE VIOL 



FEMALE SPEECH 



MALE SPEECH 



30 50 



100 



200 



5000 10000 20000 



500 1000 2000 

 CUTOFF FREQUENCY 



Fig. 2 — Summary of important ranges required for dififerent instruments. 



frequencies cut off by means of filters. The observers endeavored to 

 note whether there was any perceptible effect when the filters were 

 introduced but did not attempt to determine whether introducing 

 the filters made the reproduced material sound more or less pleasing. 



Referring to the figure, it will be noticed that at the lower frequen- 

 cies little appears to be lost by cutting off frequencies below about 

 50 cycles. At the upper frequencies, however, with certain of the 

 musical instruments something is lost by cutting off frequencies above 

 8,000 cycles. Hissing sounds, sounds of a percussion nature and 

 sounds of jingling keys, rustling paper, etc., appear to be most affected 

 by cutting off the high frequencies. 



Tests have shown, however, that when the frequency range 50 to 



