The only significant sound wave mode that will travel through a liquid is:

• A. Rayleigh
• B. Surface
• C. Longitudinal
• D. Shear

Answer: (C)

In a liquid, the only waves that can propagate meaningfully are longitudinal (compressional) waves. This is because liquids have no shear rigidity; the molecules can contract and expand along the direction of travel, producing density changes as the wave passes. Transverse or shear waves, which rely on the material’s ability to resist shear stress, cannot be sustained in a liquid and are quickly damped. Rayleigh waves are a type of surface wave that require a solid’s shear resistance to travel along a boundary, so they don’t propagate through liquids. Hence, the significant mode through a liquid is longitudinal waves, whose speed depends on the liquid’s bulk modulus and density.