Self-focusing transducers

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Self-focusing transducers are ultrasonic transducers that can focus their own sound without any external focusing electronics. This natural focusing happens in the near-field, the region close to the crystal, and can occur with a single crystal. When the wave is coherent, waves from the edges of the crystal interfere with the main wave and narrow the beam. The near-field length is roughly D = d^2 / (4λ), where d is the crystal diameter and λ is the wavelength; at the end of this near field the beam width is about d/2. Techniques like apodization (gradually reducing edge amplitude) can reduce the near-field size. This self-focusing is different from the electronic focusing used in ultrasound devices that rely on transducer arrays.