Which statement is true about acoustic impedance?

Which statement is true about acoustic impedance?

• A. It increases with increasing density or increasing propagation speed, and is affected by stiffness, and is not affected by frequency.
• B. It decreases with increasing density and stiffness.
• C. It only depends on wave frequency.
• D. It is constant across all tissues.

Answer: (A)

Acoustic impedance is defined as Z = ρc, where ρ is density and c is the speed of sound in the medium. It rises when density increases or when the speed of sound increases. Stiffness tends to raise the speed of sound, so impedance increases with stiffness as well. In a non-dispersive, lossless medium, impedance is independent of frequency, so frequency doesn’t affect Z in that ideal case. This is why the statement is the best: it correctly links higher density or faster propagation speed to higher impedance, notes the role of stiffness (through its effect on speed), and states that frequency doesn’t alter impedance in the simple model. The other options contradict the basic relation Z = ρc or imply frequency dependence or constancy across tissues that isn’t true.

Acoustic impedance is defined as Z = ρc, where ρ is density and c is the speed of sound in the medium. It rises when density increases or when the speed of sound increases. Stiffness tends to raise the speed of sound, so impedance increases with stiffness as well. In a non-dispersive, lossless medium, impedance is independent of frequency, so frequency doesn’t affect Z in that ideal case. This is why the statement is the best: it correctly links higher density or faster propagation speed to higher impedance, notes the role of stiffness (through its effect on speed), and states that frequency doesn’t alter impedance in the simple model. The other options contradict the basic relation Z = ρc or imply frequency dependence or constancy across tissues that isn’t true.