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.

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