Which statement about acoustic impedance is true?

Which statement about acoustic impedance is true?

• A. It is independent of density
• B. It is inversely proportional to speed
• C. It equals density times speed
• D. It is unrelated to frequency

Answer: (C)

Acoustic impedance tells you how much pressure a medium must generate to move air (or itself) at a certain particle velocity. For a plane sound wave in a non-dispersive medium, pressure and velocity are related by p = Z u, where Z is the acoustic impedance. The medium’s characteristic impedance is Z = ρ c, with ρ being the density and c the speed of sound in that medium. So the statement that impedance equals density times speed is the correct description. Intuitively, higher density means more inertia resisting motion, and a faster wave speed c means the medium transmits pressure changes more quickly, both contributing to greater impedance. If either density or speed increases, impedance increases accordingly, which is why Z is proportional to both factors. Regarding the other ideas: impedance is not independent of density, nor is it inversely proportional to speed. While in an ideal, lossless medium the basic definition makes Z independent of frequency, the essential relationship you should memorize is Z = ρ c, which ties impedance directly to density and wave speed.

Acoustic impedance tells you how much pressure a medium must generate to move air (or itself) at a certain particle velocity. For a plane sound wave in a non-dispersive medium, pressure and velocity are related by p = Z u, where Z is the acoustic impedance. The medium’s characteristic impedance is Z = ρ c, with ρ being the density and c the speed of sound in that medium. So the statement that impedance equals density times speed is the correct description.

Intuitively, higher density means more inertia resisting motion, and a faster wave speed c means the medium transmits pressure changes more quickly, both contributing to greater impedance. If either density or speed increases, impedance increases accordingly, which is why Z is proportional to both factors.

Regarding the other ideas: impedance is not independent of density, nor is it inversely proportional to speed. While in an ideal, lossless medium the basic definition makes Z independent of frequency, the essential relationship you should memorize is Z = ρ c, which ties impedance directly to density and wave speed.