CHAPTER XX 



HEARING, VOICE AND SPEECH 



FIBST SECTION 



AUDITORY SENSATIONS 



1. STIMULI APPROPRIATE FOR THE ORGAN OF HEARING 



THE organ of hearing is stimulated by the vibrations of elastic bodies 

 which we perceive as sound. Helmholtz, whose presentation of the subject * 

 we shall follow here in the main, divides auditory sensations into two groups : 

 namely, noises and musical tones. 



A musical tone is produced by regular periodic movements of the sounding 

 body, which are communicated by it to the air or some other elastic medium. 

 By a regular periodic movement we mean a movement which is repeated at 

 exactly the same interval of time, and always exactly in the same manner. 

 The length of the interval from the beginning of one movement to the next 

 repetition of it is called the wave length or the period. 



As a rule, the vibrations are conveyed to the ear through the air. The 

 particles of air must therefore execute regular periodic vibrations, moving to 

 and fro within narrow limits so that the air is alternately condensed and 

 rarefied (wave crest and wave trough). Sound is propagated in the form of 

 concentric spherical waves, new particles of air in all directions from the 

 sounding body being successively set in motion. 



Three qualities of sound are to be distinguished: loudness, pitch and 

 timbre. 



A. LOUDNESS 



The loudness of a sound depends upon the amplitude of the vibrations. 

 The greater the excursions which, for example, a vibrating piano string 

 describes, the louder is the sound at a given distance from its source. The 

 greater the distance from the source, the weaker is the sound, the loudness 

 being inversely as the square of the distance. 



B. PITCH 



Pitch is determined by the vibration frequency, or in other words by the 

 number of vibrations per second, and is independent of the form of the vibra- 



1 " Lehre von den Tonempfindungen als physiologische Grundlage fur die Theorie der 

 Musik." 



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