Which pair of medium properties determines the propagation speed of ultrasound?

• A. Density and Elasticity
• B. Density and Impedance
• C. Viscosity and Density
• D. Elasticity and Impedance

Answer: (A)

Propagation speed of ultrasound is determined by how easily the medium can be compressed and how much mass there is per unit volume. This is captured by elasticity (stiffness, such as the bulk modulus) and density. A stiffer medium (higher elasticity) tends to accelerate the wave, while a denser medium (higher density) tends to slow it down. The relation c ≈ sqrt(K/ρ) (with K representing the appropriate elastic modulus) shows why both properties matter together. Viscosity mainly affects how quickly the waveform is damped as it travels, not the speed itself, and impedance (Z = ρc) is a derived product that influences reflections at boundaries but does not set the propagation speed. Therefore, density and elasticity together determine how fast ultrasound travels.