A larger width transducer produces a longer near zone length, while a smaller width transducer produces a shorter near zone length. Which term describes this relationship?

• A. Near Zone Length
• B. Far Field Length
• C. Focal Length
• D. Wavelength

Answer: (A)

Near field length describes how the size of the transducer aperture determines how far the beam remains in its converging, developing state. For a circular aperture, the near field length is proportional to the square of the diameter and inversely proportional to the wavelength (often written as L_n ≈ D^2/(4λ)). This means a larger aperture (greater width) creates a longer near field, while a smaller aperture shortens it. The key idea is that the beam is still evolving and not yet settled into the stable pattern of the far field within this region. Wavelength and focal considerations also play roles, but the described relationship is specifically about near field length increasing with aperture width.