4 HAYES— DETECTION OF SUBMARINES. 



A submarine sound having an amplitude of /4o^° inches is near 

 the limit of audibility. This represents a movement of the particles 

 of the medium through a distance less than %o the diameter of the 

 smallest atom. 



The fact that water transmits sound energy with slight loss and 

 that the relation between intensity and distance is more favorable 

 than the inverse square law makes it appear reasonable that sounds 

 can be heard at great distances in water if the energy of sound waves 

 of such minute amplitude can be efficiently collected and brought to 

 the ear. 



General Nature of Sound. 



Sound is a longitudinal wave motion having some vibrating body 

 as a source. It travels through any material medium with a definite 

 velocity depending upon the physical properties of the medium. The 

 ratio of the velocity of sound in air to the velocity in water at a tem- 

 perature of 60 degrees Fahr. is about ^Moo- 



A sustained sound or tone has three physical characteristics : 

 loudness, pitch and quality. Loudness or intensity depends upon 

 the amplitude, (the distance the particles of the medium vibrate 

 back and forth) ; pitch, the highness or lowness of the tone, depends 

 upon the frequency or number of waves which pass a fixed point per 

 second ; quality depends upon the number and intensity of overtones 

 or harmonics present in the sound. It is the quality of a sound that 

 enables a listener to name the instrument upon which it is produced. 



A sound which varies from moment to moment, as it does when 

 produced by an engine or rotating propeller on a boat, has other 

 characteristics, the most important of which is rhythm. Rhythm is 

 more or less a characteristic of each type of boat. A trained listener 

 can detect the faint rhythm of a distant boat through a mass of 

 louder confusing noises and can tell the type of boat and judge its 

 speed by the character and the period of the rhythm. 



The general laws of reflection, refraction, and interference of 

 light hold for sound, but there are certain practical differences be- 

 cause the wave-length of sound is much greater than the wave- 

 length of light. As a result of this greater wave-length, sound has a 

 greater tendency than light to bend around the edges of obstacles 

 and not travel in straight lines. It results also from this that mir- 



