SONAR AND RADAR 



biological research, for now the timing of the vertical 

 migrations could be studied with great precision. Of 

 course the echoes from a deep scattering layer do not 

 identify the actual animals, so we still do not know for 

 certain whether the principal sources of these echoes are 

 shrimp-like animals, fish, or possibly squid. 



Sonar systems effective at echolocating submarines 

 were used with great success in World War II. One of 

 these sonar systems has a transmitting hydrophone, or 

 underwater loudspeaker which broadcasts sound whose 

 power level is 600 watts. For comparison, the minimum 

 sound power level audible in a quiet room at the fre- 

 quencies to which the human ear is most sensitive is 

 10-16 ^att, while a very loud shout at close range has 

 a power of 10~^ watt. Thus this sonar system puts into 

 the ocean a sound power roughly equivalent to that of 

 6,000,000 loud shouts. These intense probing sounds 

 are emitted as short pulses lasting one or two tenths of 

 a second. The frequency can be set anywhere between 

 10,000 and 26,000 c.p.s. Since the velocity of sound in 

 sea water is about 1500 meters/second, the actual wave 

 lengths of these sounds are from 5 to 13 centimeters, 

 and the length of the pulse is from 150 to 300 meters. 



Because this system emits some frequencies above 

 the range of human hearing, there has to be a system to 

 make these frequencies audible. You may be famihar 

 with the "beat note," or "beat frequency" which is con- 

 spicuous when two nearly identical notes are sounded 

 simultaneously. If one note is 500 c.p.s. and the other 

 is 600 c.p.s., you will hear a third note of 100 c.p.s. 

 Hence in the electrical circuit of the sonar apparatus, 

 by generating a local frequency and combining with it 

 the incoming echo, an audible beat note is generated. 

 For instance, an incoming echo of 22,000 c.p.s. and a 

 local frequency of 23,000 c.p.s. produce an audible note 



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