Which two characteristics of a medium affect the speed of sound?

• A. Stiffness
• B. Both stiffness and density
• C. Density
• D. None of the above

Answer: (B)

The speed of sound in a medium is set by how easily the medium can be compressed (stiffness) and how much mass there is to move (density). A stiffer material resists compression, so pressure disturbances are passed along more quickly, increasing the speed. Higher density adds inertia, meaning more mass has to be accelerated as the wave passes, which slows the propagation. In many materials the speed roughly scales with the square root of stiffness divided by density, so both factors matter. That’s why the best answer is that the speed of sound depends on both stiffness and density. For context, air has low stiffness and low density, water has much higher stiffness (and a higher density), and steel has very high stiffness, leading to fast sound speeds despite its greater density.