PART III 

 HEARING 



PAGES 



SECTION 1. Properties of sound .......... 695 



,, 2. Structure of Auditory Apparatus . ... . . . . 600 



,, 3. Auditory Sensations . . . 611 



SECTION I 

 PROPERTIES OF SOUND 



SOUND is propagated by waves consisting of alternate compressions 

 and rarefactions which travel through the medium. Any medium 

 which has the properties of elasticity and mass can conduct sound ; thus 

 solids, liquids and gases are all efficient conductors. Since sound is a form 

 of wave motion it exhibits many of the properties which are found in the 

 case of light, namely Reflection, Refraction, Diffraction, etc. But 

 owing to the long wavelengths of sound waves compared with those of 

 light, the effects of diffraction are relatively of greater importance. Therefore 

 sound does not form sharp shadows, such as light does, and is found to bend 

 round obstacles and to be conducted down speaking tubes, etc., in a way that 

 would be impossible if sound were of shorter wavelength. ' 



SOURCES OF SOUND are so well known to us that the fundamental 

 property of a source of sound tends to be forgotten, namely that, to 

 produce sound, motion has to be initiated in a .sound conductor 

 which has a velocity the same or greater than that of the trans- 

 mission of sound. Thus when a book is closed with a snap, the book 

 becomes a source of sound when the velocity at which the air is 

 squeezed from between its pages is equal to the velocity of sound in air. A 

 stick stirred in water becomes a source when the ripples (eddies) it produces 

 have the necessary velocity to cause a wave motion. Sounds have been 

 divided arbitrarily into two classes, namely tones and noises ; the former 

 produce pleasant and the latter unpleasant (harsh, grating, screeching, etc.) 

 sensations. Between the two extremes fall the sounds of everyday life. 

 Thus music as a rule consists of tones, but may be found to consist of chords 

 which examined singly could be grouped as noises. And so at the other 

 end of the scale, when we strike a single stick with a hammer the effect is 

 that of a noise. If however we take a series of sticks of different lengths 

 and strike them in succession, it will be noticed that the sound produced by 



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