Using smaller diameter PZT crystals in annular arrays tends to produce beams with which characteristic?

Using smaller diameter PZT crystals in annular arrays tends to produce beams with which characteristic?

• A. Shallower focus
• B. Higher frequency
• C. Deeper focus
• D. Larger beam width

Answer: (A)

Smaller diameter crystals reduce the overall aperture of the annular array, so the focused beam comes to a point closer to the transducer. Focusing depth is largely set by how large the aperture is and how the waves from the rings interfere; a smaller aperture moves the focus to a shorter distance, giving a shallower focus. The center frequency is determined mainly by the crystal thickness, not the diameter, so you wouldn’t get a higher frequency from smaller rings. Deeper focus would require a larger aperture or a different focusing arrangement, and while a smaller aperture can increase beam divergence (leading to a wider beam), the characteristic most directly affected here is the shallower focal depth.

Smaller diameter crystals reduce the overall aperture of the annular array, so the focused beam comes to a point closer to the transducer. Focusing depth is largely set by how large the aperture is and how the waves from the rings interfere; a smaller aperture moves the focus to a shorter distance, giving a shallower focus. The center frequency is determined mainly by the crystal thickness, not the diameter, so you wouldn’t get a higher frequency from smaller rings. Deeper focus would require a larger aperture or a different focusing arrangement, and while a smaller aperture can increase beam divergence (leading to a wider beam), the characteristic most directly affected here is the shallower focal depth.