Acoustic waves emitted by ultrasonics transducer crystals exhibit a property known as self-focusing (or natural focusing).
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 volume near the transducer which exhibits this phenomenon is called the near field, as opposed to the far field where wave intensity decays approximately exponentially.
However, there exist techniques, such as apodization that may be used to reduce the near field size.