54 Special Uses of Hearing and Vision /3 : 2 



flight. Many experiments have shown that bats navigate, sense their 

 surroundings, and hunt by a process known as echo-location. 



Echo-location has been used for many years to determine the depth of 

 the ocean. During World War II, two practical applications of echo- 

 location were developed. Systems using electromagnetic echoes are 

 called radar, whereas those employing acoustic echoes are named sonar. 

 In either case, a pulse of energy is sent out, reflected from an object, and 

 the returning echo is detected. By measuring the time for the echo to 

 return, one can compute the object distance. For radar, if the object is 

 at a distance d of 60 km, the pulse will return in the time t necessary to 

 travel 2d or. 120 km. That is 



, 2d 120 km _ . .... 



t = — = r-^-j : — = 0.4 millisecond 



c i x 10 s km/sec 



The echo will be weaker than the original pulse emitted. To detect the 

 echo, the original pulse must have stopped before the echo returns. 

 Thus, very short pulses are necessary. To aid in distinguishing the echo 

 from noise, among other reasons, the original pulse is emitted at a carrier 

 frequency which is high compared to the reciprocal of the pulse length. 

 For radar, frequencies of 10 9 to 3 x 10 10 cps are used. 



An echo-location system like that described above will determine 

 distance but not shape. To find the latter, it is necessary to emit many 

 pulses, each one in a slightly different direction. These echoes must all 

 occur before the object has moved very far. Thus, a high pulse- 

 repetition rate is needed to find detail. By contrast, a low rate is needed 

 to find distant objects. 



Finally, to be useful for determining distance and shape, there must 

 be some way of rapidly displaying the echoes as a function of direction 

 and time of return because there is not time to do a paper-and-pencil 

 calculation for most uses of echo-location. A similar rapid sensing of 

 shape and motion occurs when watching the wheel of a moving car. 

 One does not see that each point on the wheel describes a curve of 

 complex form and then figure out that the wheel is turning; rather, one 

 perceives this directly. Therefore, any successful echo-location system 

 must reveal directly the shape, size, and distance of the objects. Human 

 brains do not do this with echoes. Therefore, radar and sonar equip- 

 ment display their results after electronic computation. The bat brain 

 apparently makes a similar calculation directly. 



Radar works well in air but is useless under water since the electro- 

 magnetic waves are rapidly absorbed. Sonar, although less effective 

 than radar in air, can be used to locate objects under water. The speed 

 of sound in water is only 1.5 km/sec, so much longer pulses can be used 



