CARRIER NATURE OF SPEECH 501 



wave although this is not true of the damped wave carriers of multi- 

 frequency type once commonly used in spark wave radio telegraphy. 

 The carrier wave in speech is not a simple sine wave. Such a sound 

 would be like a whistle and so too limited for the rich flexibility of 

 speech. Instead the voice carrier is a compound tone having a multi- 

 plicity of components of dififerent frequencies which together cover the 

 audible range fairly completely. While these components may be 

 considered as a multiplicity of separate carriers it is simpler to think 

 of the ensemble as a single complex carrier; so this terminology has 

 been used in the earlier carrier illustration and elsewhere in this paper. 



Aside from this compound nature of the voice carrier, the voice has 

 two distinct types of carrier, one for voiced and one for unvoiced sounds. 

 Some sounds such as "z" have both types present at the same time 

 but this case may be treated as the superposition of one carrier on the 

 other. For voiced sounds the carrier is the vocal cord tone, an acoustic 

 wave produced by the vibration of the vocal cords consisting of a 

 fundamental frequency component and the upper harmonics thereof. 

 These decrease in amplitude with increasing frequency. For unvoiced 

 sounds the carrier is the breath tone, a complex tone resulting from a 

 constriction formed somewhere in the vocal tract through which the 

 breath is forced turbulently to produce a continuous spectrum of fre- 

 quency components in the audible range. 



These carrier waves must be dissociated from any effects of resonant 

 vocal chambers, for such characterize the speech message rather than 

 the carrier. Furthermore, these carrier waves must be mentally pic- 

 tured as sustained indefinitely with the starting and stopping of them 

 also characterizing the message wave. Pauses for breath, due to in- 

 cidental human limitations, do not invalidate the fundamental theory. 



The Speech Message 



Since a sustained voice carrier has no dynamic flow of information 

 there is need for a source of message waves and a modulating mecha- 

 nism for imprinting the message on the carrier. Conversely, any varia- 

 tion from the sustained carrier infers the presence of a message wave 

 molding the carrier. The message consists of those articulating, 

 phonating and inflecting motions of the vocal parts which imprint 

 the information on the carrier sound stream. The importance of the 

 message waves cannot be stressed too much. Any impairment of 

 them is an impairment of the message. 



The message waves include the motions producing speech changes 

 at infra-syllabic rates, such as the effect of anger when a talker may be 

 high-pitched for many minutes. When the carrier is thus altered over 



