Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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 effect occurs because if the ultrasound wave generated crystal is coherent, edges of the emitting surface generate wavelets that constructively interfere with plane wave generated.