. Biophysical science. Biophysics. 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 wel
. Biophysical science. Biophysics. 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 "; 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
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