In which medium listed below would the velocity of sound be lowest?

• A. Plastic
• B. Air
• C. Water
• D. Aluminum

Answer: (B)

The speed of sound in a medium depends on how easily a pressure disturbance can be transmitted, which is controlled by how stiff the material is and how dense it is. Sound travels faster when the material is stiff and the particles are closely held together, and slower when the material is more compressible and less rigid. Air is a gas with very low stiffness (high compressibility) and, despite its low density, the ratio of stiffness to density is small, so pressure changes propagate slowly. In contrast, liquids and especially solids transmit vibrations much more quickly due to greater stiffness and the way particles interact. That’s why air has the slowest sound speed among the listed media, with typical values around 343 m/s, while water and metals like aluminum are much faster.