The length of the near-field zone next to a transducer is mostly affected by?

• A. Both frequency and diameter
• B. The frequency of the transducer
• C. The diameter of the transducer
• D. The transducer cable length

Answer: (A)

The near-field length is governed by how the ultrasonic beam is formed by the transducer’s aperture and the wavelength of the sound. For a circular transducer, the region where the beam is still converging—the near field—extends roughly as D^2/λ, where D is the transducer diameter and λ is the wavelength. Since wavelength is inversely related to frequency (λ = c/f), increasing either the transducer diameter or the frequency makes the near-field longer. The transducer’s cable length doesn’t change the beam’s size or geometry, so it doesn’t affect the near-field length. In short, the near-field length grows with both frequency and diameter.