CHAPTER 4 



PHYSICS OF SOUND 



Sonar is an electronic device that utilizes 

 sound energy as a means of locating submerged 

 objects, such as submarines. The equipment 

 may be either active or passive. Sonar of an 

 active nature transmits the sound energy into 

 the water and must depend on the returning 

 echoes bouncing off the target to provide bearing 

 and range information. Passive sonar depends 

 for bearing information on sounds originated by 

 the target (such as screw cavitation, machinery 

 noises, and so forth). 



This chapter, then, deals with sound and its 

 behavior in sea water. 



WHAT SOUND IS 



Sound is the physical cause of your sensation 

 of hearing. Anything that you hear is a sound. 



Sound travels in the form of waves, which 

 vary in length according to their frequency. 

 A sound having a long wavelength is heard at 

 a low pitch; one with a short wavelength is 

 heard at a high pitch. A complete wavelength 

 is called a cycle, and the number of cycles 

 per second is the sound's frequency. Frequencies 

 are now measured in the Hertz system, 1 hertz 

 (Hz) being equal to 1 cycle per second. Fre- 

 quencies of 1000 cps or more are measured in 

 kilohertz (kHz). Normally, sounds below 20 Hz 

 or above 15 kHz are beyond the human hearing 

 range. Between these two frequencies is the 

 average human audible range. More information 

 on the characteristics of sound is given in the 

 next section of this chapter. 



REQUIREMENTS FOR SOUND 



71.18 

 Figure 4-1. — The three elements of sound. 



You may recall the experiment in which a 

 bell was placed inside a jar containing a vacuum. 

 You could see the bell ringing, but you could 

 hear nothing because there was no medium to 

 transmit sound from the bell to you. What about 

 the third element, the detector? You may see 

 a source (such as an explosion) apparently 

 producing a sound, and you know the medium 

 (air) is present, but you are too far away to 

 hear the noise. So far as you are concerned, 

 there was no detector and, therefore, no sound. 

 For purposes of this text, we must assume 

 that sound can exist only when an auditory 

 vibration is caused by a source, is transmitted 

 through a medium, and is heard by a detector. 

 Figure 4-1 illustrates this assumption. The 

 bell vibrates on being struck, thus acting as a 

 sound source. The vibrating bell moves the 

 particles of air — the medium — in contact with 

 it. And the sound waves travel to the ear, which 

 acts as the detector. 



Before sound can be produced, three basic 

 elements must be present. (See fig. 4-1.) These 

 elements are a source of sound, a medium to 

 transmit the sound, and a detector to hear it. 

 If there is no source to generate a noise, then 

 there can be no sound. The same theory holds 

 true for the other required elements. 



Source 



Any object that moves rapidly to and fro, 

 or vibrates and thus disturbs the medium around 

 it, may become a sound source. Bells, radio 

 loudspeaker diaphragms, and stringed instru- 

 ments are familiar sound sources. 



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