324 EARLE C. GREGG, JR. 



to cover a given frequency range. It should be kept in mind, how- 

 ever, that a crystal vibrating with a reasonable amplitude at reso- 

 nance in a liquid will certainly shatter if driven at the same voltage in 

 air. 



For producing very high sound intensities at a point, Greutz- 

 macher (7) developed a special cut of quartz crystal with a concave 

 face. In this manner, the energy was concentrated at a focal point 

 resulting in an increase of as much as 150 times the energy of a small 

 surface element close to the crystal. This t3^pe crystal cut obviously 

 allows the production of high sound intensities at relatively low 

 voltages ^^^th little danger of shattering the crystal. Using a crystal 

 of this nature, at frequencies from 638 to 1000 kilocycles, Tumanski 

 (8) produced jets of oil 70 cm. high, projected above the free surface 

 of the liquid. Figure IE is a photograph by Willard (23) of the sound 

 field produced by such a concave ultrasonic crj^stal. Other investiga- 

 tors have used combinations of crystals all aimed toward a focal point, 

 but under these circumstances care must be taken to assure the cor- 

 rect phasing of the sound waves at the focal point. 



As was mentioned i^reviously, it is possible to construct acoustic 

 lenses that will focus sound just like glass lenses in optics. While 

 this has not yet been applied to biological research, it has proved re- 

 markably useful in other phases of ultrasonics (27). 



As long as the wavelength of the sound is small compared to the 

 dimensions of the lens, the lens laws of optics apply to ultrasonics 

 (for example, the "thick lens" relationship). However, it is impor- 

 tant to remember that if the sound velocity in the lens material is 

 higher than that of the surrounding medium (as is usually the case) 

 the relative refractive index is less than unity and an acoustic lens 

 shaped like an optical diverging lens will actually converge the sound. 

 In ultrasonics, lens materials such as polystyrene or carbon tetra- 

 chloride in smooth aluminum containers (thickness and imperfections 

 small compared to X) have been used. In general, such devices 

 should prove much more economical than special cuts of quartz 

 crystals if their losses are small. Figure ID shows the sound field 

 produced by such a lens. 



Figure 10 shows two methods of exciting quartz crystals. Both 

 are simple electronic oscillators of the Hartley type and differ only 

 in the manner of coupling to the crystal. In a the crystal is connected 

 directly across the tank capacity, which is a source of high alternating 

 voltages. The disadvantages of this scheme are that the voltage at- 



