Acoustic impedance equals the product of which two properties?

• A. Density and speed
• B. Density and wavelength
• C. Velocity and frequency
• D. Density and attenuation

Answer: (A)

Acoustic impedance is the ratio of acoustic pressure to particle velocity in a medium, and for a plane wave in a lossless medium this impedance equals the density times the speed of sound: Z = ρ c. So the two properties that multiply to give impedance are density and the speed of sound. The speed of sound comes from how quickly disturbances propagate through the medium, while density describes how much mass there is per unit volume—together they set how much pressure is produced by a given particle motion. Wavelength and frequency relate to speed but do not directly form impedance (λ = c/f), and attenuation is about energy loss, not the intrinsic resistance described by impedance. Higher density or a faster medium increases impedance, influencing how waves reflect or transmit at boundaries.