Which combination of material properties primarily determines the acoustic velocity?

• A. Density only
• B. Elasticity only
• C. Acoustic impedance
• D. Density and elasticity

Answer: (D)

The speed of a sound wave in a material is set by how stiff the material is and how much mass there is per unit volume. For longitudinal waves in solids, the velocity roughly follows the stiffness-to-density ratio: increasing stiffness (elastic modulus) raises velocity, while increasing density lowers it. This means both density and elasticity together determine how fast the wave travels through the material.

Acoustic impedance, while related, is the product of density and velocity (Z = ρv). It depends on both properties and reflects how a material will reflect or transmit sound at an interface, but it does not by itself set the wave speed.

So the combination of density and elasticity best explains the acoustic velocity.