224 Destructive Effects of High Intensity Ultrasound / 1 2. : 3 



magnetostriction transducers are used. Frequencies from 0.1-10 mc 

 have several advantages. It is comparatively easy to generate and 

 control the necessary electrical power. Quartz transducers make it 

 possible to operate at sharply denned frequencies, whereas uarium 

 titanate elements permit focusing the sound field so that the region of 

 maximum acoustic pressure appears away from the transducer face. 



One problem which never has been completely solved is monitoring 

 the ultrasonic intensity. Hydrophones usually are sensitive to pressure 

 variations and fail just where the interesting region of cavitation starts. 

 Various ingenious devices have been used, although the most common 

 method is simply to measure the power input to the transducer (or current 

 through it) . This is easiest, but fails to take into account the dependence 

 of transducer efficiency on load or cavitation. (The latter may vary 

 with the dirt or biological cells suspended in the liquid.) 



3. Biological Cells and Cavitation 



The destruction of biological cells in suspension occurs when cavitation 

 is taking place. For many years, it was suspected that some other 

 parameter of the acoustic field or perhaps heating associated with 

 cavitation could be responsible for the cellular destruction. A large 

 number of experiments with many types of cells including blood cells, 

 protozoans, bacteria, and algae have all confirmed that the cells are 

 broken by mechanical rupture as a result of cavitation. 



At one time, it was believed that the large acoustic pressures, or 

 perhaps the local particle velocities and accelerations, were in some way 

 responsible for the observed cellular rupture. All of these properties 

 are, if anything, enhanced when cavitation is suppressed, either by 

 increasing the applied pressure or by vacuum degassing. In contrast, 

 cellular destruction disappears or is greatly diminished when cavitation 

 is suppressed. 



In some ultrasonic phenomena, such as the detonation of explosives 

 under water in the presence of cavitation, it has been shown that the 

 local heating was the important parameter. To prove that this heating 

 is not the active agent when cavitation occurs near biological cells 

 demanded additional experiments. If local heating were important, the 

 rate of cellular disruption should be strongly temperature dependent. 

 Experiments with bacteria, red blood cells, and protozoa have shown 

 that quite the reverse is true, that the rate of cell breakage is almost 

 independent of the temperature from 0° to 30°C. Another indication is 

 to compare red blood cells heated to boiling with those agitated by a 



