Sound beam intensity is irregular in the area called:

• A. The near field
• B. The far field
• C. The main lobe
• D. The rear field

Answer: (D)

Beam intensity is irregular in the near field, which is the region closest to the transducer (the Fresnel zone). Here, waves emitted from different parts of the crystal interfere with each other, creating a pattern of hot and cold spots and fluctuations in intensity. As the distance increases into the far field (the Fraunhofer zone), the waves spread out and interference averages out, producing a more uniform beam with a distinct main lobe in the forward direction. The rear field refers to energy radiated backward from the transducer and is not the region where the irregularity of the forward beam is typically described. So, the irregular intensity characterizes the near field.