254 BELL SYSTEM TECHNICAL JOURNAL 



Speech Sounds 



Let me outline briefly some of the results of these studies on speech. 

 The source of any voiced sound is in the larynx. On both sides of 

 this larynx there are two muscular ledges called the vocal cords. 

 When we breathe, these two ledges are widely separated, but when a 

 voiced sound is produced, they come close together, forming a long 

 narrow slit. As they come close together, the air passing through the 

 resulting slit is set into vibration producing a sound. It has been 

 generally supposed that the pitch of the tone thus produced was 

 determined by the natural frequency of vibration of the two vocal 

 cords, and that by changing the tension of these cords, the pitch of 

 the tone can be raised or lowered at will. As most of you know, 

 their natural frequency of vibration is the rate that they would vibrate 

 to and fro if they were plucked and set into vibration like a banjo 

 string or an elastic band. Our studies revealed that the natural pitch 

 of these cords while a tone is being produced is considerably below 

 that of the pitch of the tone. It is true that the pitch of the tone 

 produced is affected, somewhat, by the elasticity of the vocal cords, 

 but it is principally controlled by the size of the air opening between 

 them. The little plug of air between the two vocal cords vibrates 

 through a very much larger amplitude than the amplitude of the cords 

 themselves and is the real source of the sound. The mass of this 

 small plug is controlled by the size of the opening and by the elastic 

 forces pushing it to and fro — namely, the air pressures on either side 

 of it. It is evident that these oscillating pressures will be influenced 

 by the size and shape of the trachea leading into the lungs on one side 

 and by the size and shape of the tongue, mouth, and nasal cavities 

 on the other. The mechanical action involved is analogous to the 

 electrical action in a vacuum-tube oscillator. The sound which is 

 generated at the vocal cords is modified as it passes through the 

 throat, mouth, and nasal passages. The real character of the sound 

 which enables us to identify words is wholly dependent upon the man- 

 ner in which this cord tone is modified by the changing sizes, shapes, 

 and characters of these passages and the outlet to the outside air. 



After the various speech sounds leave the mouth, they are trans- 

 mitted to the ear of the listener by means of air vibration. As an 

 example of the type of disturbance created in the air, consider the 

 sentence, "Joe took Father's shoe bench out." This silly sounding 

 sentence is chosen because it is used in our laboratories for making 

 tests on the efficiency of telephone systems. The sentence, together 

 with its mate, "She was waiting at my lawn," contains all the funda- 

 mental sounds in the English language that contribute appreciably 



