12 : 2/- Destructive Effects of High Intensity Ultrasound 



223 



by the cavities. If the acoustic pressure is measured as a function of the 

 energy applied to the transducer used to generate the ultrasonic field, a 

 curve such as that sketched for/? in Figure 1 is obtained. As the point of 

 cavitation is reached, the curve bends over because the liquid tears 

 rather than sustaining higher pressures. As the liquid fills with cavities, 

 its density decreases; this permits the particle velocity to increase 

 linearly with the square root of the applied energy to values well above 

 the threshold for cavitation. 



Cellular fracture and harmonic distortion of the pressure wave are 

 observed at acoustic pressures lower than those shown by the break in 



P*V 



(4) 



Cavitation 



(2) (3) v 





/, Acoustic Particle Velocity 



p, Acoustic Pressure Amplitude 



[Energy]*' 2 



Figure I. Variation of acoustic pressure/) and particle velocity 

 V with energy supplied to transducer. The arrows indicate 

 points at which cavitation occurs based on (1) cellular 

 disruption; (2) break in the pressure curve; (3) generation of 

 noise; and (4) break in the particle velocity curve. 



the curve in Figure 1, labeled "cavitation." However, both cellular 

 fracture and harmonic distortion can be suppressed by increasing the 

 atmospheric pressure, a change which would interfere only with effects 

 due to cavitation. These indicate that cavitation is occurring, at least 

 to a limited degree, before the break in the curve. Thus, different tests 

 for cavitation lead to different thresholds for cavitation. 



Of course, care must be taken to distinguish cavitation from heating. 

 When an intense ultrasonic field is generated in a small body of liquid, 

 acoustic energy must be dissipated as heat. If no method is provided to 

 remove this excess heat energy, the temperature will rise. Then all 

 biological effects of ultrasound are masked by heating. Any practical 

 exposure technique must involve either very short periods of time or 

 some form of cooling. 



All types of transducers can be built to include a cooling system. At 

 the lowest frequencies practical, 0-10 kc, vibrating plates or diaphragms 

 have been used to generate cavitating sound fields. From 6-60 kc, 



