Self-focusing transducers

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Acoustic waves emitted by ultrasonics transducer crystals exhibit a property known as self-focusing (or natural focusing). Note that this is distinct from the electronically controlled focusing employed in diagnostic ultrasound devices which employ arrays of transducers. The self-focusing effect exists even for a single crystal.

Self-focusing refers to the narrowing of the ultrasonic beam in the near-field. The near-field extends from the crystal face out to

$D = \frac{d^2}{4 \lambda}$

where $d =$ crystal diameter and $λ =$ wavelength of emitted wave. At the edge of the near-field, where the focusing reaches its maximum, the beam width reaches $d / 2$ .

The self-focusing effect occurs because the ultrasound wave is coherent and because the vibration mode of the crystal at the emitting surface generates wavelets that constructively interfere in a narrowing region from the surface out to a distance $D$ .

[edit] References

"Investigations of compact self focusing transducers using stacked piezoelectric elements for strong sound pulses in therapy", Dreyer, T.; Krauss, W.; Bauer, E.; Riedlinger, R.E. Ultrasonics Symposium, 2000 IEEE Volume 2, Issue , Oct 2000 Page(s):1239 - 1242 vol.2, cited at IEEE Xplore - Login, retrieved May 6, 2008