The near zone length increases with transducer diameter. Which statement best describes this relationship?

• A. It increases with diameter
• B. It decreases with diameter
• C. It is independent of diameter
• D. It is inversely proportional to diameter

Answer: (A)

The near field length is determined by the transducer’s aperture and the wavelength. For a circular aperture, the near field length L_n is proportional to D^2/λ. If you keep the frequency (and thus the wavelength) fixed, increasing the diameter D makes L_n grow, and it grows in a quadratic fashion. In plain terms, a larger transducer delivers a beam that stays fairly narrow for a longer distance before it starts to diverge, so the near field extends farther. Therefore, the statement that the near zone length increases with diameter is correct. (If you change frequency, λ changes too, but the dependence on diameter remains quadratic when frequency is fixed.)