340 E A R L E C. GREGG, JR. 



sound, it is possible to send out a short train of sound waves and to 

 receive a reflection back from nearby objects that are in the same di- 

 rection as the projected sound. The time interval between repeated 

 pulses determines the maximum range of the device while the time 

 interval between pulse and echo determines the distance to the ob- 

 ject. A crystal transducer in this case produces adequate intensity 

 and sensitivity. The major problem has been to translate the re- 

 ceived pulses into some recognizable signal. This has been done in 

 one system by frequency modulating the sound source in such a 

 manner that the addition of the received signal to that being trans- 

 mitted at that instant produces an audible tone proportional to the 

 distance. Objects and discontinuities such as curbs have been de- 

 tected successfully up to about fifty feet. 



References 



1 . Wood, A. B., A Textbook of Sound. Macmillan, New Yoik, 1930. 

 la. Beranek, L. L., Acoustic Measurements. Wiley, New York, 1949. 



2. Bergman, L., and H. S. Hatfield, Ultrasonics and Their Scientific and 



Technical Applications. Wiley, New York, 1938. 

 Sa. Hiedemann, E., Ultraschallforschung. DeGruyter, Berlin, 1939. 

 _ 3b. Carlin, B., Ultrasonics. McGraw-Hill, New York, 1949. 

 Sc. Fry, W. J., J. M. Taylor, and B. W. Henvis, Design of Crystal Vibrating 



Systems for Projectors and Other Applications. Dover, New York, 



1948. 

 Sd. Camp, L., J. Acoust. Soc. Atn., 20, 289, 611, 616 (1948) (magnetostric- 



tive transducer designs). 

 Se. Dranetz, A. I., G. N. Howatt, and J. W. Crownover, Tele-Tech, 1949, 



May, p. 28, and June, p. 36 (barium titanate as a piezoelectric crystal 



element). 



4. Cady, W. G., Piezoelectricity. McGraw-Hill, New York, 1946. 



5. Pierce, G. W., Proc. Am. Acad. Arts Sci., 63, 1 (1928); Proc. Inst. 



Radio Engrs. and Waves and Electrons, 17, 42 (1929). 



6. Salisbury, W. W., and C. W. Porter, Rev. Sci. Instruments, 10, 142 



(1939); 10,269(1939). 



7. Greutzmacher, J., Z. Physik, 96, 342 (1935). 



8. Tumanski, S. S., J. Tech. Phys. U. S. S. R., 7, 2019 (1937). 



9. Smith, A. W., and D. K. Weimer, Rev. Sci. Instruments. 18, 188 (1917). 



10. Harvey, E. N., and A. L. Loomis, /. Gen. Physiol., 15, 147 (1931). 



11 . Wood, R. W., and A. L. Loomis, Phys. Rev.. 29, 373 (1927). 



12. Schmitt, F. 0., and B. Uhlemeyer, Proc. Soc. Exptl. Biol. Med., 27, 626 



(1930). 



