310 



EARLE C. GREGG, JR. 



reach the sample being treated. Figure 2 shows the relationship be- 

 tween sound transmission and the ratio of thickness to sound wave- 

 length for a glass plate immersed in water. Since Vz for glass is on 

 the order of 5500 m. per second, the wavelength of sound in glass at a 

 frequency of 1000 kilocycles is 0.55 cm. While Figure 2 shows that 

 for this condition a thin-bottomed flask will suffice for a relatively 

 large transmission of sound energy, the effect cannot be overlooked 

 at higher frequencies. 



Figure IF is a photograph by Willard (23) showing the selective 

 thickness transmission of 10 megacycle ultrasonics by an aluminum 

 wedge. 



>- 

 o 

 on 



UJ 



z 



UJ 



1.0 



0.8 



LlJ 



§0.6 



>-0. 

 o 



CE 

 UJ 



UJ 

 U. 



o 



o 



t- 

 < 

 a: 



0.2 







Fig. 2. 

 plate of 



0.2 0.4 0.6 0.8 1.0 1.2 



RATIO OF THICKNESS TO WAVELENGTH 



Behavior of plane sound waves incident normally on a glass 

 thickness d\ ( — ) reflected energy; ( ) transmitted energy. 



When sound waves in general meet a boundary, they exert upon 

 it, in addition to the alternating pressure mentioned, a steady uni- 

 directional pressure. This steady pressure is called the radiation 

 pressure and is responsible in the case of a liquid-gas boundary for 

 the actual distortion of the liquid surface. For very intense sound 

 waves in a small liquid volume, as is usually encountered in chemical 

 and biological investigations, particles of the liquid may actually be 

 ejected as far as 10 to 30 cm. above the surface. 



