624 BELL SYSTEM TECHNICAL JOURNAL 



It is felt that the caHber of the playing was such as to render the com- 

 ments and measured frequency ranges generally applicable. These 

 ranges probably represent extreme conditions, for the observers were in 

 effect situated unusually close to the instruments, they were listening 

 under most favorable conditions, and they had only to pick out a 

 particular distortion. 



The piano was the only instrument which did not require the repro- 

 duction of its lowest fundamentals for perfect fidelity. Therefore trans- 

 mission of 40 cycles — the lowest note of the bass viol — was required, 

 and this was found to be ample for the percussion instruments. How- 

 ever, as the 80 per cent marks indicate, little was lost when frequencies 

 below 60 cycles were not reproduced. 



Many of the instruments produced noises that extended to high fre- 

 quencies, but only the oboe, violin, and snare drum were thought to ex- 

 tend their tone ranges to the upper audible limit. The action of bows 

 on the strings and the clatter of reeds in the reed instruments produced 

 very prominent noises of high frequency. When the lips were used as 

 reeds the noises were much less prominent. The noises indicated for 

 the flute and piccolo were produced by the impact of the air from the 

 lips against the embouchure opening. As a group the lipped instru- 

 ments produced only moderately high frequencies; the other groups all 

 had some instruments producing frequencies extending to the upper 

 audible limit. An upper cut-off of 10,000 cycles did not affect the tone 

 of most of the instruments to a marked extent, but every instrument 

 except the bass drum and tympani was affected by the 5000 cycle cut- 

 off. A frequency range of 100 to 10,000 cycles was shown to be en- 

 tirely satisfactory for speech. 



Between the 80 per cent marks the bass viol required the greatest 

 range — 7 octaves — and the piccolo required the smallest range — 4 

 octaves. 



Noises in particular were characterized by high frequencies. Hand 

 clapping and key jingling were both found to be very definitely changed 

 by the 13,000 cycle filter, and informal listening tests on several other 

 noises indicated that high frequencies were very prominent. Probably 

 many noises also contain important frequencies below 100 cycles and 

 transmission of the entire audible range would seem much more impor- 

 tant for noise reproduction than for reproduction of musical sounds. 



Part U 



The measurements of the quality changes produced by the filters 

 were made using the same apparatus but a different testing technique. 

 The 18 piece orchestra furnishing the music was made up as follows: 



