THE LARYNX AND THE EAR 35! 



They are transmitted chiefly by the air, which, because of 

 its lightness, elasticity, and abundance, readily takes up the 

 vibrations and spreads them in all directions (Fig. 148). 



While these vibratory movements of the atmosphere are correctly 

 classified as waves, they bear little resemblance to the waves on water. 

 Instead of being made of crests and troughs, as are the water waves, 



FIG. 148. Diagram illustrating the spreading of sound 

 waves through air. 



the sound waves consist of alternating successions of slightly condensed 

 and rarefied layers of air. Then, while the general movement of the 

 water waves is that of ever widening circles over a surface, the sound 

 waves spread as enlarging spherical shells through the air. In sound 

 waves, as in all other waves, however, it is only the form of the wave 

 that moves forward. The individual particles of air that make up the 

 wave simply vibrate back and forth. 



How Sound Waves act as Stimuli. Any sound wave 

 represents a small but definite amount of energy, this 

 being a part of the original force that acted on the vibrating 

 body to set it in motion. The hammer, for instance, in 

 striking a bell imparts to it a measurable quantity of 

 energy, which the bell in turn imparts to the air. This 

 energy is in the sound waves and is communicated to the 



must vibrate rapidly, making not less than sixteen vibrations per second. The 

 upper limit of hearing being about 40,000 vibrations per second, certain bodies 

 may even vibrate too rapidly to be heard. 



