ECHOES AS MESSENGERS 



velocity of motion. Once started, they move through a 

 given medium at a constant rate under any particular set 

 of conditions. They usually become weaker and weaker 

 as they progress, and eventually die out. But as long as 

 they are detectable at all their velocity remains the same. 

 Nor does the velocity vary with the frequency of sound. 

 This means that when a sound contains more than one 

 frequency (that is, the waves have more complicated 

 shapes than simple sine curves), the different parts of 

 the complex sound wave move together without one 

 component lagging or gaining on the others. The actual 

 speed of sound depends primarily upon the medium 

 where the sound waves travel, but temperature and other 

 factors affect it slightly. For example, in air at 20° C 

 sound travels 344 meters per second (about 1130 

 feet/ sec), and in sea water at 0° C its velocity is 

 about 1550 meters/sec (or 4700 feet/sec). While these 

 distances are fairly large, they are, of course, far less 

 than the 300 X 10^ meters (or 186,000 miles) cov- 

 ered in one second by light and radio waves. Hun- 

 dreds or thousands of meters are less convenient to 

 think about than shorter distances more comparable to 

 our own dimensions. Consequently it will often be con- 

 venient to specify the velocity of sound in terms of dis- 

 tance traveled in 1 millisecond, or thousandth of a 

 second; 344 meters/sec is 34.4 centimeters, or about 

 one foot, per millisecond, a helpful figure to keep in 

 mind when dealing with sounds of very short duration. 

 Another important property of a sound is its wave 

 length or wave lengths. Wave length is the distance be- 

 tween successive zones of maximum or minimum pres- 

 sures as the wave travels along. Since the velocity of 

 sound is constant, the waves, which cover 344 meters 

 in 1 second, may either be numerous and short or few 

 in number and longer in wave length. If the waves are 



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