Which lens curvature is used to focus sound energy to enter cylindrical surfaces normally or along a line focus?

• A. Cylindrical curvatures
• B. Spherical lens curvatures
• C. Convex shapes
• D. Concave shapes

Answer: (A)

Focusing sound energy with a lens depends on how the lens bends the waves in the two perpendicular planes. A cylindrical lens has curvature in only one plane, so it concentrates energy into a line rather than a point. That line focus is ideal for cylindrical surfaces because it allows the energy to enter along a line that runs along the cylinder, effectively providing normal incidence along that line around the curved surface. A spherical lens, with curvature in both planes, would focus to a point, which doesn’t align well with the geometry of a cylinder and can cause uneven incidence around the surface. Convex or concave shapes describe the overall form but don’t specify this line-focused behavior needed for cylindrical geometries.