Assuming a constant frequency, what happens if the diameter of an unfocused circular transducer is increased?

Assuming a constant frequency, what happens if the diameter of an unfocused circular transducer is increased?

• A. The distance to the far field is reduced
• B. The beam width in the near field is reduced
• C. The beam width in the near field is increased
• D. The ultrasound wavelength is increased

Answer: (C)

In an unfocused circular transducer, the transducer diameter acts as the emitting aperture. In the near field, the beam footprint is essentially the size of that aperture at shallow depths. So when you increase the diameter, you illuminate a larger area right at the surface, making the beam width in the near field larger. The frequency is held constant, so the wavelength stays the same, and the near-field length actually grows with D^2, but the immediate near-field cross-section becomes wider as D increases. (In contrast, in the far field a larger diameter would produce a narrower beam due to diffraction, since the beam’s angular spread decreases with increasing D.)

In an unfocused circular transducer, the transducer diameter acts as the emitting aperture. In the near field, the beam footprint is essentially the size of that aperture at shallow depths. So when you increase the diameter, you illuminate a larger area right at the surface, making the beam width in the near field larger. The frequency is held constant, so the wavelength stays the same, and the near-field length actually grows with D^2, but the immediate near-field cross-section becomes wider as D increases. (In contrast, in the far field a larger diameter would produce a narrower beam due to diffraction, since the beam’s angular spread decreases with increasing D.)