318 SPEECH, MUSIC AND HEARING 



When a sound wave impinges upon the ear, it enters the ear canal and 

 causes the ear drum to vibrate. The vibration of the ear drum is communi- 

 cated by the lever action of the middle ear to the inner ear or cochlea. 

 In this mechanical transformer the amplitude reduction is one-sixtieth 

 and the force variation is increased by sixty times. From various experi- 

 ments the highest frequencies are associated with the portion of the basilar 

 membrane near the oval window. The lowest frequency is associated 

 with the extreme end removed from the oval window. For example, in 

 the case of 1000 cycles the nerves which are stimulated are those near the 

 midpoint of the basilar membrane. 



13.3. Speech Mechanism ^^. — The energy required for speech is 

 provided by the lungs in the form of an air stream. In the larynx a pair 

 of muscular strips, termed the vocal chords, form a slit through which the 

 air passes. When speaking they are tensed by muscular contraction and 

 the passage of air causes them to vibrate. The air then issues in a series 

 of puffs of a frequency controlled by the natural frequency of the vocal 

 chords. The sound thus produced is not a pure tone but very complex. 

 The overtone structure is somewhat under control of the muscular system 

 and the air pressure. 



The sound from the larynx passes through the cavity called the pharynx, 

 then through an aperture at the back of the mouth, then through the 

 mouth cavity and out of the aperture formed by the lips. The two cavities 

 are acoustic capacitances and the apertures are two inertances. The 

 nasal cavity and nose aperture form a shunt to the mouth cavity. The 

 complex tone produced by the vocal chords is modified by the resonances 

 of the capacitances and inertances. The size of the cavities and apertures 

 is controllable and hence the tone structure of the sound which issues 

 from the mouth varies as these constants are changed. 



The vowel sounds are produced by the above mechanism. The con- 

 sonants are produced by the air rushing through the various outlets, 

 usually of small dimensions. The consonants are of course influenced by 

 the resonances of the cavities and apertures. 



A. Artificial Larynx ^^. — In cases where the larynx has been removed 

 by an operation, speech is possible by the use of an artificial larynx. The 

 artificial larynx consists of a reed actuated by the air from an opening in 

 the front of the throat through which breathing takes place. The com- 

 plex tone generated by the reed is conducted by a tube into the mouth 

 cavity. The quality of the sounds is modified by the resonances in the 



^^ Fletcher, " Speech and Hearing," D. Van Nostrand Co., New York. 

 ^^Riesz, R. R., Jour. Acous. Soc. Amer., Vol. I, No. 2, p. 273, 1930. 



