Ultrasonics' 



By Arthur R. Laufer 

 Assistant Professor of Physics, University of Missouri 



[With 3 plates] 



Some 3,000 years ago, according to the Old Testament, Joshua, the 

 son of Nun, led the Israelites over the river Jordan into the promised 

 land. And then, once each day for 6 days, seven priests carrying seven 

 trumpets made of rams' horas circled the walled city of Jericho. And 

 on the seventh day the priests circled the walls seven times, and on the 

 seventh time the priests blew a loud blast on the trumpets, and the Isra- 

 elites shouted a loud shout, and the walls of Jericho fell down flat. 

 Thus must history reach back into antiquity to find the first allusion to 

 the use of sound energy for a purpose other than hearing. 



Thirty centuries later, in the field of ultrasonics, spectacular use is 

 again being made of "sound" energy. Although inaudible to the hu- 

 man ear, ultrasonic "sound" waves have all the physical properties of 

 audible sound waves, differing only in frequency. But it is this dif- 

 ference in frequency, and the consequent concentration of energy, 

 which lead to the very different effects obtainable with ultrasonic 

 waves. 



Audible sounds, or sonic waves, range in frequency from about 20 

 cycles per second to about 20 kilocycles per second. Ultrasonic 

 waves are defined as vibrational or "sound" waves Avhich have a fre- 

 quency higher than 20 kilocycles. Whether inaudible vibrational 

 waves should be called sound waves is a debatable issue, depending for 

 its resolution on the definition of sound on a physical or on a psycho- 

 logical basis. Not many years ago the waves now known as ultrasonic 

 went under the name of supersonic. This latter name left a lasting 

 impression in the field of radio. Although radio waves are electro- 

 magnetic rather than sound waves, the intermediate frequency used in 

 the most popular type of radio receiver was in the "supersonic" fre- 

 quency range and led to the designation of this receiver as a supersonic 

 heterodyne, or, more briefly, a superheterodyne receiver. When the 



^ Reprinted by permission from Physics Today, vol. 3, No. 8, August 1950. 



213 



