278 NATURE. OF SOUXD. 



sounding body. The opposite holds true when the sounding body 

 and the ear recede from each other. This explains why the pitch 

 of the whistle of a railway locomotive is perceptibly higher when 

 the train is rapidly approaching the observer, than when it is rapidly 

 moving away from him. 



436. Relation between Pitch and Period. 



Rate of vibration and period are reciprocals. If the 

 rate of vibration be 256 per second, the period is ^ of a 

 second. The period may, therefore, be used to measure 

 the pitch ; the greater the period, the lower the pitch. 



437. Relation between Pitch and Wave 

 Length. Since, in a given medium, all sounds travel 

 with the same velocity, the rate of vibration determines 

 the wave length. If the sounding body vibrate 224 times 

 per second, 224 waves will be started each second. If the 

 velocity of the sound be 1120 feet, the total length of these 

 224 waves must be 1120 feet, or the length of each wave 

 must be five feet. If another body vibrate twice as fast, 

 it will crowd twice as many waves into the 1120 feet; each 

 wave will be only two and a half feet long. Thus wave 

 length may be used to measure the pitch the greater the 

 wave length, the lower the pitch. 



438. Refraction of Sound. We have a clear 

 idea of sound waves advancing as concentric, spherical 

 shells, but we are far more familiar with the idea of sound 

 advancing in definite straight lines. This idea is also cor- 

 rect, the lines being radii of the sphere. We may thus 

 speak of lines or "rays" of sound, meaning thereby the 

 direction in which the sonorous pulses are propagated. 

 The ray is necessarily perpendicular to the wave. When 

 the noise of the street is heard by a person in a closed room, 

 the sound must have passed from the air without to the 



