Final answer:
The sound wave's diameter in the Fresnel zone narrows as it leaves the transducer and is not simply half the crystal's diameter nor equal to it. The diameter of the sound wave is influenced by diffraction and thus changes as the wave travels.
Step-by-step explanation:
The question is related to the diameter of a sound wave produced by a PZT (lead zirconate titanate) crystal transducer in the Fresnel zone. The sound wave's behavior in this zone is not explicitly defined by a fixed diameter; instead, the sound wave generally narrows as it leaves the transducer before eventually spreading out. This phenomenon is due to diffraction and the wave-like nature of sound. While the question posits options regarding the relationship between the crystal and the sound wave's diameter in the Fresnel zone, the most accurate statement concerning the behavior of the sound wave's diameter as it leaves the transducer is 'The sound wave narrows as it leaves the transducer.'. In the context of ultrasound and acoustics, the Fresnel zone is an intermediate field where the sound wave starts to diverge or spread out after leaving the transducer but has not yet reached the far field or Fraunhofer zone where the spread becomes more pronounced and the beam can be considered to diverge at a constant angle.