Which of the following best describes acoustic impedance?

• A. The wavelength of the sound
• B. The product of the medium's density and the speed of sound in that medium
• C. The amount of reflection at an interface
• D. The frequency of the sound

Answer: (B)

Acoustic impedance is the property that links how much pressure a sound wave generates to how easily the medium allows particle motion. For a plane wave in a uniform medium, impedance is the product of the medium’s density and the speed of sound in that medium: Z = ρc. This is why the correct description is the density times the speed of sound. The impedance value tells you how much of a wave is reflected versus transmitted at an interface: a big mismatch in Z between two media causes a strong reflection, while similar impedances yield more transmission. Wavelength, given by c/f, and frequency itself do not define impedance, and while the amount of reflection at an interface depends on impedance mismatch, that reflection is not the definition of impedance itself. An example helps: soft tissue has Z ≈ 1.54 × 10^6 Rayls (ρ ≈ 1000 kg/m^3, c ≈ 1540 m/s), while air has a much lower Z ≈ 400 Rayls, so the air–tissue boundary reflects most of the energy.