220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



A pulse technique has been developed for the location of flaws in 

 metals and other solid materials. A crystal is used to send a short 

 pulse of ultrasonic waves into the object to be tested. The same crystal 

 is used (through the direct piezoelectric effect) to receive reflections 

 of the primary pulse. The amplified electrical output of the crystal, 

 portrayed on the screen of a cathode-ray oscilloscope, depicts the pri- 

 mary pulse and all reflected pulses. Keflections caused by flaws permit 

 the presence and location of imperfections to be detected. This tech- 

 nique possesses advantages over X-ray testing in that the equipment 

 is portable, and far greater depths of material can be penetrated. 

 Masses of raw materials can be tested to avoid the machining of de- 

 fective material, and periodic fatigue checks can easily be made on 

 parts which are under strain as they work without the dismantling of 

 the machinery. One major rubber company tests its entire output of 

 tires by such ultrasonic methods. 



The violent agitation produced by high-intensity sound waves has 

 a marked dispersive effect on solids and liquids in liquids, producing 

 true colloidal solutions and fine emulsions. By means of ultrasonic 

 irradiation while in the molten state, alloys can be produced of metals 

 such as iron and lead which are ordinarily not miscible in the liquid 

 state. New bearing materials have been made in this way. By such 

 means it has also been possible to produce photographic emulsions of 

 improved homogeneity, stability, and sensitivity. The homogeniza- 

 tion of milk through ultrasonic irradiation is today an industrial 

 process. The coarse crystals of sulf athiazole have been broken down 

 by ultrasonics to form a creamy emulsion which can be injected through 

 fine hypodermic needles, a technique which was previously impossible. 



In spite of the fact that ultrasonic waves have this strong dispersive 

 effect on hydrosols, their effect on aerosols is exactly the opposite — 

 namely, coagulation. Irradiation by intense high-frequency sound 

 causes almost immediate agglomeration and precipitation of the solid 

 and liquid particles in mist and smoke. At an installation in Kings- 

 mill, Tex., this technique is used to recover carbon black from a smoke- 

 stack. At the Naval Landing Aids Experiment Station, Areata, 

 Calif., intense sound has been used in this way to turn heavy fog to 

 rain. 



Chemical reactions can also be influenced by ultrasonic irradiation. 

 Certain reactions are accelerated, and even depolymerization can be 

 brought about. The chain molecule of starch has been broken down 

 into several fragments to produce dextrine, and gum arable and 

 gelatine have been decomposed. The aging of whiskey by ultrasonics 

 has been proposed, inasmuch as in the aging process there is a gradual 

 change in the structure of complex molecules, a change which perhaps 

 could be accomplished much more rapidly by sound irradiation. 



