338 EARLE r. GREGG, JR. 



held in the field with the fingers closed. When the fingers were held 

 open, the sensation was relieved. A temperature increase at the hand 

 of 45°C. was measured with a thermocouple while glass and rubber 

 showed only a 1° rise. Their concept is that the heating is due to the 

 damping (absorption) of vibrations, not necessarily of the same fre- 

 quency as the sound, that are set up in the fingers by the intense sound 

 field in the region between them. 



A study of the effects of these amounts of power on airborne bac- 

 terial and larger organisms should prove both interesting and fruitful. 



6. Natural Sources of Ultrasonic Sound 



The production and use of ultrasonic sound is by no means con- 

 fined to the laboratory. Fish, shrimp, grasshoppers, crickets, birds, 

 and bats all produce acoustic energy in the ultrasonic frequencies. 

 In the case of the grasshopper, for example, energy has been de- 

 tected as high as 40 kilocycles. As far as fish and shrimp are con- 

 cerned, while some energy has been detected in the higher frequencies, 

 most of the energy is concentrated in the audible range at about 2.5 

 kilocycles. Some birds, the canary is a notable example, actually sing 

 at these higher frequencies (on the order of 20 kilocycles) , while other 

 animals such as cats and dogs can only detect and not produce them. 

 While there is some question as to what benefit most animals derive 

 from ultrasonic frequencies, the bat actually depends upon them for 

 navigation in flight in a manner similar to ultrasonic (sonar) and 

 radar echo ranging devices. 



Galambos and Griffin (26) have shown that the bat actually pro- 

 duces three types of sound. One, a shrill cry of anger, is usually at a 

 frequency of about 7 kilocycles. The second, a series of clicks, is 

 associated with the production of the third type of sound — a series 

 of pulses (sound wave trains) at a frequency of about 70 kilocycles. 

 Each pulse lasts about 0.01 second. If the bat is at rest, only about 

 five to ten clicks per second are observed. When in flight, this in- 

 creases to twenty to thirty per second and, in the \ncinity of objects, 

 to fifty per second. At these frequencies (70 kilocycles), the sound is 

 not only beamed from the source (seemingly the larynx), but also is 

 reflected quite well from nearby objects. The time between sending 

 and receiving a pulse obviously gives the bat a measure of the distance 

 to the reflector, while the number of pulses per second increases his 

 accuracy and speed of detection at close range. 



