THE CAUSE AND CHARACTER OF THE SOUND WAVES 757 



of the earlier forms of stethoscopes (Laennec) which usually consisted 

 of a wooden cyUnder perforated through its axis and enlarged at its 

 ends. Furthermore, their initial energy may be protected against loss 

 by sending them through narrow tubes, because they are then no longer 

 propagated as concentric spheres, but are reflected from the walls of 

 the tube. We shall see later on that this is true of the sound waves 

 traversing the external auditory meatus. 



Any sound produced near at hand, seems to reach our ears instan- 

 taneously. In reality, however, there is a distinct interval between 

 the moment of its production and the moment when it produces its 

 stimulation in the internal ear. This latency is caused in part by a 

 certain sluggishness of the receptor, and in part by the fact that sound 

 waves require time for their propagation through the medium. A 

 distant locomotive or steam boat is seen to discharge a certain volume 

 of steam through its vibrator long before the sound produced thereby 

 actually reaches our ears, and the flash of lightning is seen long before 



I 



Fig. 375. — Laennec Stethoscope. 



the thunder is heard. While altitude, temperature and the general 

 character of the medium have much to do with the propagation of the 

 vibrations from molecule to molecule, it may be said that the velocity 

 of sound is 340 m. in a second. Its speed, however, is proportional to 

 its intensity, i.e., loud sounds travel more rapidly than those possessing 

 a low quality. Through water sound is propagated at the rate of 

 about 1450 m. in a second, and through wood at the rate of about 

 13,000 m. in a second. 



Sound waves may be reflected and refracted. In the ear we deal 

 chiefly with reflections from curved surfaces in which the reflection 

 takes place on the opposite side of the perpendicular, drawn to the 

 point of impact of the incident wave. The angle of reflection in- 

 variably equals the angle of incidence, and both occupy the same plane. 

 In the ear we have curved surfaces which are constructed in such a way 

 that the inclinations of the planes of which any curved surface is com- 

 posed, gives rise to a convergence of the sound waves. Thus, the 

 external ear of man possesses a curvature arranged to reflect these 

 undulations into the auditory meatus. The same is true of the ear 

 trumpet and of the flexible stethoscope. Both appliances collect the 

 sound waves by means of their cup-shaped free ends and reflect them 

 into the meatus. 



Noises and Sounds. — It is not always easy to distinguish between a 

 noise and a sound. In general, however, it may be said that the former 

 consists either of a brief vibration, as may be produced by the discharge 

 of a cannon, or of a mixture of vibrations as may be caused by the 

 wheels of a carriage. It lacks, therefore, a definite wave length and 



