ULTRASONICS — LAUFER 215 



trated in a point source, at a distance of 2 meters the intensity would 

 be about 1 ten-thousandth of a watt per square centimeter which is 

 the threshold of pain for the human ear. If all the residents of New 

 York City (population about 7 million) were to speak at the same 

 time, the total power they would generate would be just about enough 

 to light a 60-watt lamp. Obviously, the intensities attainable with 

 ultrasonic waves are enormous in comparsion with those in the audible 

 range. The phenomena discovered in the field of ultrasonics are the 

 direct result of the short wavelength and the concomitant high inten- 

 sity of ultrasonic waves. 



EARLY GENERATORS 



The first man-made generator of sustained ultrasonic waves was 

 designed as long ago as 1883, when a forced-air whistle reached the 

 frequency of 25 kilocycles. Nature, however, anticipated the work 

 of man by endowing the bat with an ultrasonic generator of its own. 

 Using the vocal cords in its larynx, the bat generates and emits sound 

 waves in pulses of 2-milliseconds duration at a rate of about 30 per 

 second. The frequency in each pulse ranges from about 30 to 100 

 kilocycles. Eeflections received by the bat's ears indicate the location 

 of obstacles, and, in this radarlike manner, ultrasonic radiation is used 

 by the bat to guide itself in flight. 



Following the forced-air whistle, a tuning fork with tines only 

 several millimeters in length was developed near the end of the last 

 century, with a frequency ranging as high as 90 kilocycles. Both the 

 whistle and the fork, however, yielded frequencies which could not be 

 controlled accurately and output powers which were relatively small. 

 There was but little further progress in the development of ultrasonic 

 generators until the first World War when Prof. Paul Langevin, 

 director of the School of Physics and Chemistry in Paris, was re- 

 quested by the French Government to devise some method of detecting 

 submarines to combat the U-boat menace. 



A few years earlier, following the Titanic disaster, an Englishman 

 named L. F. Richardson suggested that a hydraulic whistle be used 

 to locate underwater navigational hazards such as icebergs through 

 the echo of a narrow beam of ultrasonic waves, but experiment proved 

 his apparatus to be ineffective. Then, in 1915, a Russian engineer 

 named Chilowski proposed that ultrasonic vibrations be excited in a 

 mica condenser by a Poulsen arc and that the radiation from the 

 vibrating condenser be used for underwater detection. Prof. Lange- 

 vin tested and then developed Chilowski's idea to such an extent that 

 a transmission range of 2,000 yards in the Seine River was attained 

 early in 1916, despite the fact that the frequency stability and power 

 output of the generator still left much to be desired. Shortly there- 



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