The near zone length is equal to what fraction of the beam diameter?

• A. 1/4
• B. 1/2
• C. 3/4
• D. 1

Answer: (B)

The near field, or Fresnel zone, is the region directly in front of the transducer where the beam is still forming and the pattern isn’t yet fully stable. A common practical rule for a circular transducer is that this near field extends about halfway along the beam’s width, so its length is roughly half the beam diameter. That’s why this item’s best answer is one-half. Of course, the exact distance can be calculated from the transducer diameter and the wavelength (the precise expression uses D^2 and λ), but for a simple fraction used in testing, half the beam diameter is the standard takeaway.