Acoustic energy propagates in different modes. Which of the following represent a mode?

• A. All of the above
• B. Surface wave
• C. Longitudinal wave
• D. Shear wave

Answer: (B)

A mode is a distinct way that acoustic energy can travel, defined by a specific pattern of motion and how the wave is confined or guided in the medium. A surface wave is a clear example of this: it travels along the boundary of two media and its energy is concentrated near the surface, with motion and velocity that differ from waves moving through the bulk.

Longitudinal and shear waves are fundamental types of waves that propagate through the material, but they are bulk waves, not surface-confined modes. The question highlights a propagation mode as seen along a boundary, which is what makes a surface wave the best representative choice.