EFFECTS OF PHASE DISTORTION 551 



Nature of Speech and Hearing 



Since the manifestation of phase distortion depends upon the type of 

 signal and the method of observation, it is of interest to first consider 

 the nature of the waves of speech sounds and certain facts of audition. 



Speech waves may be regarded as non-periodic in that they start at 

 some time, take on some finite values, and then approximate zero again 

 as may be seen from the wave form - of the word "seems" in Fig. 1. 

 In this particular word the wave form of each sound and the transition 

 periods are readily distinguishable. Although in other cases of con- 

 nected speech this may not be done so easily it is usually possible to 



s 



EE 



M 



-^^-\r\/\fV^[\/^\j^^\f^^\\ 



500 CYCLE 



Fig. 1 — Wave form of the word "seems." 



approximately distinguish between sounds and to ascribe to each an 

 initial period of growth, an intermediate period which in some cases 

 approximates a steady state and then a final period of decay. The 

 duration intervals of different sounds vary from about .03 to as much 

 as .3 or .35 seconds. 



Hearing appears to be concerned more with the spectra of sound 

 waves, i.e., something corresponding to the amplitudes of the Fourier 

 components, than with the actual wave form of the disturbance. For 

 the type of steady state complex waves that the speech sound waves 

 approximate for a considerable portion of their duration intervals, it 

 has been observed that phase changes in the component waves cause 



^Speech and Hearing, H. Fletcher, D. Van Nostrand Co., 1929. 

 36 



