3 : 2/ Special Uses of Hearing and Vision 55 



for sonar; to limit sound absorption, much lower frequencies are em- 

 ployed, usually around 3 x 10 4 cps. 



Griffin and Galambos showed in 1941 that bats use a sonar type of 

 echo-location. Since then, Griffin and his associates have studied echo- 

 location in bats in great detail. The bats emit and detect airborne 

 sound pulses; these travel at the speed of sound, 0.34 km/sec. A bat 

 can use comparatively long pulse lengths and still recognize close objects. 

 The pulse lengths used by an individual bat may vary from around 

 1 millisecond to 5 milliseconds. The lower limit allows a bat to dis- 

 tinguish echoes from objects as close as 15 cm (6 inches). Other species 

 of bats with poorer acoustic orientation use pulses of 10 to 100 milli- 

 seconds. 



To be effective, the wavelength of the sounds in the bat's pulse must 

 be of the order of the linear size of the smallest objects the bat chases. 

 The wavelength A is equal to the velocity of sound divided by the fre- 

 quency. The latter is easier to measure electronically. The frequency 

 of the sound which the bat emits varies during the pulse by a factor of 

 close to two. The highest frequency in some species is around 100 kc, 

 where the wavelength A of sound in air is about 0.3 cm. Appreciable 

 echoes should occur until the diameter of the object is about A/2, in this 

 case, about 0.05 cm (20 mils). Behavioral experiments with wire grids 

 show that such bats are extremely successful in avoiding wires 0.3 cm 

 in diameter but cannot detect wires 0.025 cm in diameter. The shape 

 of the pulse is shown in Figure 1 . 



The role of the frequency changes during the sound pulse is not 

 known. One possibility is that it is used to indicate size, the lower 

 frequencies being reflected less by small objects than the higher ones. 

 The directivity pattern of the sound emitted by the bat also changes 

 during the pulse in such a manner as to support this hypothesis. The 

 higher frequencies are concentrated into a narrower beam, favoring 

 their reflection from smaller objects directly ahead of the bat. 



Another possibility is that the bat uses its own particular frequency 

 variations to distinguish its echoes from those of other bats close by 

 or from surrounding noises. Bats must be very skillfull at distinguishing 

 their own pulses from others because they navigate well in the presence 

 of thousands of other bats in dark caves with hard reflecting walls. 

 They are also able to detect their own pulses from loud noises. Attempts 

 at "jamming" bat sonar, with broad-band noise, have so far failed. The 

 sound pressure level (see Chapter 1) near the bat's mouth is about 

 120 db, that is, about 175 dynes/cm 2 . Even broad-band noise signals 

 at these sound pressure levels in the bat's frequency range have failed to 

 jam the bat's sonar, although the echoes are very weak compared to the 

 over-all noise levels. 



