172 Our Surroundings 



travel to you. In this way you may calculate the speed of sound 

 in air. 



Sound does not travel at the same rate in different substances. 

 To prove this, stand on a railroad and watch someone in the 

 distance strike a blow on a rail. If the ear is placed against 

 the rail you will hear the sound through the rail and then hear 

 it a second time through the air. It must have traveled faster 

 in the rail than in the air. Air is very light and will not carry 

 sound as fast as iron, which is very heavy. Any heavy sub- 

 stance, such as iron, is said to have a high density, while any- 

 thing that is light is said to have a low density. The speed of 

 sound is affected by the density of the substance through which 

 it passes. The higher the density, the faster sound travels. 



Intensity of Sound. Intensity of sound, or loudness, depends 

 upon the force with which the body producing the sound is vibrat- 

 ing, and also upon the substance through which the sound is 

 carried. The experiment with the metronome under the glass 

 receiver showed that the intensity of the sound became less as 

 the air under the receiver became less dense. It is said that 

 at high elevations, as the air becomes rarer, explorers find that 

 the distance at which the voice can be heard becomes less. At 

 sea level, the firing of a gun produces a sound much louder than 

 on a mountain top. 



Sounds are always louder when the air surrounding the sound- 

 making body is dense than when it is rare. You have perhaps 

 noticed that the sound of a bell seems louder in cold than in 

 hot weather. Cold air is denser than warm air. 



Experiments to Show Production of Sound by Vibra- 

 tions. Ring a bell, blow a whistle, strike a drum, and 

 snap a taut wire. Do each of these things gently, and then 

 forcibly, and observe in each case what occurs. Listen to the 

 sound each makes. Stop the vibration of the wire. Does the 

 sound continue? Was it the vibration that caused the sound? 



When a bell is struck, the sound travels equally well in all 

 directions if the density of the medium through which it passes 

 is uniform and there are no objects to divert the waves. You 

 have noticed how waves form in the water when a stone is 



