61.Ultrasonics, Ferroelectrics, and Frequency Control by John G. Webster (Editor)
By John G. Webster (Editor)
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Additional info for 61.Ultrasonics, Ferroelectrics, and Frequency Control
When the bubble is smaller than resonant size, its volumetric pulsations will be out of phase with the driving pressure. ) Under these circumstances the bubble scatters sound (because of its geometric properties and impedance mismatch with the surrounding fluid) and emits sound (via the compression and rarefaction of the liquid surrounding the bubble) at the frequency of the applied signal. Generally, the 372 BIOLOGICAL EFFECTS OF ULTRASOUND emitted sound has a larger amplitude than the scattered sound.
Even more successful (by a factor of 1000 or so) has been work (102,103) at lower frequencies (around 20 kHz) using pulsed ultrasound (with pulse lengths on the order of 100 ms applied every second for as long as one hour) with smaller average intensities, typically a few hundred milliwatts per square centimeter. ] We take the time to discuss sonophoresis in some detail because its study sheds light on a number of bioacoustic phenomena and illustrates the complexity of identifying acoustic mechanisms behind bioeffects.
Surgeons in Brazil in the 1980s discovered that removing a chunk of the heart muscle somehow resets the heart’s contraction pattern. Motivated by this work, Kluiwstra et al. (85,86) demonstrated that ablative ultrasound applied to heart muscle achieves the same result, promising that this treatment for cardiac arrhythmia can be applied from outside of the body, thus avoiding open-heart surgery. Their technique merits some attention here because of the interesting biological and engineering problems they had to solve to create their desired bioeffect.