Which pair best describes the factors that determine the speed of sound in a medium?

• A. Elasticity of the medium and frequency of the sound beam
• B. Density of the medium and stiffness of the medium
• C. Density of the medium and power of the sound beam
• D. Stiffness of the medium and elasticity of the medium

Answer: (B)

Speed of sound in a medium is set by how easily the medium can be compressed (stiffness or elastic properties) and how much inertia the particles have (density). A stiffer medium transmits pressure changes more rapidly, increasing the speed, while a denser medium has more mass to move and slows the propagation. The relationship can be described roughly by c ≈ sqrt(stiffness / density), so both density and stiffness determine the speed. The other options mix factors that don’t set speed—frequency and beam power don’t change how fast sound travels through a medium, and focusing only on elastic properties without density misses a key influence.