If two media share the same propagation speed but have different densities, how does impedance change?

If two media share the same propagation speed but have different densities, how does impedance change?

• A. Impedance increases with density
• B. Impedance decreases with density
• C. Impedance remains the same
• D. Impedance becomes zero

Answer: (A)

Acoustic impedance is the product of tissue density and the speed of sound in that tissue (Z = ρc). If two media share the same propagation speed but have different densities, the impedance must vary directly with density. The denser medium has a higher impedance, and the less dense medium has a lower impedance. This happens because the speed term is the same, so the only factor driving changes in Z is density. This difference in impedance at a boundary is what causes partial reflection of the sound wave—the greater the impedance difference, the more sound is reflected and less is transmitted.

Acoustic impedance is the product of tissue density and the speed of sound in that tissue (Z = ρc). If two media share the same propagation speed but have different densities, the impedance must vary directly with density. The denser medium has a higher impedance, and the less dense medium has a lower impedance. This happens because the speed term is the same, so the only factor driving changes in Z is density. This difference in impedance at a boundary is what causes partial reflection of the sound wave—the greater the impedance difference, the more sound is reflected and less is transmitted.