What is the equation for acoustic impedance?

• A. Density x Propagation Speed
• B. Density / Propagation Speed
• C. Propagation Speed / Density
• D. Density + Propagation Speed

Answer: (A)

Acoustic impedance describes how much pressure a sound wave creates for a given particle velocity in a medium. For a plane wave traveling through a uniform medium, impedance is the ratio of pressure to particle velocity, Z = p / u. This ratio simplifies to Z = ρ c, where ρ is the density and c is the speed of sound (propagation speed) in that medium. So the impedance is the product of density and propagation speed.

If you double either density or speed of sound, the impedance doubles, reflecting a medium that resists motion more strongly. The other forms would not match the actual relationship between pressure and velocity or would have the wrong units (they wouldn’t represent how pressure couples to velocity in a wave). For reference, in air the impedance is roughly ρ c ≈ 1.21 kg/m^3 × 343 m/s ≈ 416 Pa·s/m, illustrating the concept with concrete numbers.