Sound waves require a medium to travel and travel fastest in which type of material?

• A. Gas
• B. Liquid
• C. Solid
• D. Vacuum

Answer: (C)

Sound needs something to push against, and how fast it travels depends on how easily the disturbance can be transmitted through the material. The key factor is stiffness versus density: the stiffer and less dense a medium is, the quicker a compression or rarefaction can pass along.

In a solid, the particles are tightly bound in a lattice, so the restoring forces are strong and vibrations propagate rapidly. That combination makes longitudinal waves move through solids much faster than through liquids or gases. In a vacuum there is no medium at all, so sound cannot travel. In liquids particles are less tightly bound than in solids, so the transmission is slower, and in gases the particles are far apart with weak interactions, leading to the slowest propagation.

Typical speeds illustrate the idea: air is about a few hundred meters per second, water about a thousand or so, and solids like steel can reach several thousand meters per second. Therefore, the fastest travel is in a solid.