318 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



the bearing of the object can be accurately determined. In radar the 

 energy used is in the form of electromagnetic waves; in sonar (the 

 method by which the depth of water is measured) ordinary sound 

 waves are used. Li their direction-finding bats use not audible sound 

 waves, but, as Hartridge suggested and as Griffin and Galambos have 

 proved, supersonic waves. The range of frequencies that a normal 

 human being can hear is from about IG vibrations a second to about 30,- 

 000 a second. Middle C is 256 vibrations a second. The range of fre- 

 quency of the supersonic waves used by bats is from about 25,000 to 

 70,000 a second, and is thus mostly above the limit of human hearing. 



Griffin and Galambos began their work by confirming that blind- 

 folded bats are able to fly surely. They then confirmed Jurine's dis- 

 covery that if the hearing of a bat is impaired it is unable to avoid 

 obstacles when flying. Indeed, they found that a bat with both ears 

 covered is most reluctant to take wing at all, but that a bat with one 

 ear covered will fly with moderate success, though it will be unable 

 to avoid all obstacles. These simple experiments indicated that bats 

 are made aware of the position of obstacles which they cannot see by 

 means of sound waves reflected from them. They then covered the 

 noses and mouths of their bats, but left the ears uncovered, and found 

 that again the animals were unable to fly with certainty. They thus 

 proved that the sound waves reflected by objects must be produced by 

 the vocal apparatus of the bats themselves. 



Their further experiments were conducted with the aid of an elec- 

 tronic apparatus known as a supersonic analyzer. This consists essen- 

 tially of a microphone sensitive to supersonic vibrations, a magnifier 

 which amplifies them and converts them to vibrations of a lower 

 frequency, and a recorder which traces a graph on paper when super- 

 sonic sounds are received. The analyzer measures the frequency of 

 any supersonic vibrations it may pick up. By means of this instru- 

 ment it was discovered that bats make supersonic sounds at frequent 

 intervals almost all the time. The frequency of the vibrations varies, 

 of course, but it is most usually about 50,000 a second, and at this pitch 

 each squeak lasts for a little less than one two-hundredth of a second. 



Now, it is evident that the more frequently squeaks are emitted the 

 fuller the information received. It has been proved that a bat at rest 

 emits a supersonic squeak about 10 times a second, but that as soon as 

 it takes wing the rate goes up to about 30 a second. That was to be 

 expected, since a bat on the wing obviously needs more information 

 than a bat at rest. Griffin and Galambos further found that the closer 

 a bat approached an obstacle the faster became the rate of squeaks, 

 rising to 50 and sometimes even to 60 a second, and dropping to normal 

 as soon as the obstacle was passed. The rate of squeak is, of course, 

 governed by the distance from the object, for there must be time for 

 the echo to come back before the ne;st squeak is sent out. 



