The length of the zone adjacent to a transducer in which fluctuations in sound pressure occur 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 length of the zone where pressure fluctuations are significant right next to the transducer is determined mainly by the transducer’s aperture and the sound wavelength. For a circular aperture, the near-field extent is about D^2 / (4λ). Since the wavelength λ gets smaller as frequency increases (λ = c/f), higher frequency lengthens the near-field. At the same time, a larger diameter (D) also increases this length because it’s D squared in the formula. The transducer cable length doesn’t set the near-field extent; it may affect signal integrity but not how far the fluctuating pressure zone reaches. So the factors that mostly affect this zone’s length are the frequency and the diameter.