ECHOES OF BATS AND MEN 



of 1000 c.p.s. Since the emitted sounds were of short 

 duration, the beat note was also short and sounded Hke 

 "ping." So familiar was this noise to antisubmarine 

 sailors that probing with sound came to be called 

 "pinging." 



In selecting the frequencies of the underwater sound 

 which will produce the most useful echoes, the same gen- 

 eral considerations apply as apply to echolocation by 

 bats or bUnd men. Short pulses are desirable because 

 they allow the emitted sound to end before the echo re- 

 turns. This means that the frequency of the waves within 

 the pulse cannot be too low; otherwise the pulse duration 

 allows time for only one or two sound waves. Even sub- 

 marines are small enough targets that long wave lengths 

 could become inefficient owing to the smaller echo re- 

 turned by an object smaller than one wave length. 

 Furthermore, the background noise always present in 

 the sea is greater at lower frequencies. On the other 

 hand, in water as in air, there is an increasing loss of 

 sound energy as the frequency increases because of the 

 absorption of sound as it travels through the water. Bats 

 have evolved a most satisfactory machinery for echolo- 

 cation, but men designing sonar systems had to balance 

 all these factors against one another in reaching the com- 

 promise choice of 10,000-26,000 c.p.s. as a useful range 

 for practical echo ranging. 



In view of the fact that many of the most successful 

 bats use signals with a rapid frequency change during 

 each brief pulse of sound, it is interesting to find that 

 sonar engineers developed a somewhat similar procedure 

 which sometimes improves the performance of the sys- 

 tem. In one type of operation the frequency of the 

 emitted sonar signal was varied continuously from 800 

 c.p.s. above to 800 cycles below the regular frequency. 

 This change was made to occur, as it does in the pulses 



112 



