144 BELL SYSTEM TECHNICAL JOURNAL 



three sounds when frequencies below 1000 are eliminated. The 

 elimination of frequencies above 2000 produces almost no effect. 



The fricative consonants s, z and th are seen to be affected very 

 differently from those in the other two classes. These sounds are 

 very definitely affected when frequencies above 5000 are eliminated. 

 The sounds s and z are not affected by the elimination frequencies 

 below 1500. It is principally due to these three sounds that the 

 syllable articulation is reduced from 98 per cent to 82 per cent when 

 frequencies above 2500 cycles are eliminated. 



A more detailed analysis of the articulation results on all the speech 

 sounds showing the kind as well as the number of errors will be given 

 in a future paper. 



Conclusion 



In conclusion then we see that the intensity of undistorted speech 

 which is received by the ear can be varied from 100 times greater 

 to one-millionth less than the initial speech intensity without notice- 

 ably affecting its interpretation. The intensity must be reduced 

 to one-ten-billionth of that initial speech intensity to reach the thres- 

 hold of audibility for the average ear. Also it is seen that any ap- 

 paratus designed to reproduce speech and preserve all of its char- 

 acteristic qualities must transmit frequencies from 100 to above 

 5000 cycles with approximately the same efficiency. Although most 

 of the energy in speech is carried by frequencies below 1000, the 

 essential characteristics which determine its interpretation are carried 

 mostly by frequencies above 1000 cycles. In ordinary conversation 

 the sounds th, f and v are the most difficult to hear and are responsi- 

 ble for 50 per cent of the mistakes of interpretation. The character- 

 istics of these sounds are carried principally by the very high 

 frequencies. 



It is evident that progress in the knowledge of speech and hearing 

 has a great human interest. It will greatly aid the linguists, the 

 actors, and the medical specialists. It may lead to improved devices 

 which will alleviate the handicaps of deaf and dumb persons. Fur- 

 thermore this knowledge will be of great importance to the telephone 

 engineer, and since the telephone is so universally used, any improve- 

 ment in its quality will be for the public good. 



These humanitarian and utilitarian motives as well as the pure 

 scientific interest have already attracted a number of scientists to 

 this field. Now that new and powerful tools are available, it is 

 expected that in the near future more will be led to pursue research 

 along those lines. 



