The focus is located at ________.

The focus is located at ________.

• A. The transducer surface
• B. The center of the active element
• C. The far zone
• D. The end of the near zone

Answer: (D)

In ultrasound, the beam converges as it travels through the near field and reaches its smallest diameter at the focal point, after which it diverges in the far field. The focus is located at the end of the near field—the distance from the transducer to that narrow waist is the focal length. So the correct location is the end of the near zone. The surface of the transducer and the center of the active element aren’t where the beam concentrates, and the far zone lies beyond the focus. (Near-field length depends on aperture and wavelength, with higher frequency shortening the near field.)

In ultrasound, the beam converges as it travels through the near field and reaches its smallest diameter at the focal point, after which it diverges in the far field. The focus is located at the end of the near field—the distance from the transducer to that narrow waist is the focal length. So the correct location is the end of the near zone. The surface of the transducer and the center of the active element aren’t where the beam concentrates, and the far zone lies beyond the focus. (Near-field length depends on aperture and wavelength, with higher frequency shortening the near field.)