24.1.3 Specific Acoustic Impedance

Definition: Specific Acoustic Impedance of a Medium

• Product of density of the medium and speed of ultrasound in the medium.

Math

Specific Acoustic Impedance can be calculated using the equation: $Z=\rho c$

• Where:
  • $Z$ = specific acoustic impedance ($\text{kg}{\text{ m}}^{-2}{\text{ s}}^{-1}$)
  • $\rho$ = density of the medium ($\text{kg}{\text{ m}}^{-3}$)
  • $c$ = speed of sound in the medium ($\text{m}{\text{ s}}^{-1}$)

Concept

• The specific acoustic impedance of a medium quantifies how much resistance an ultrasound wave encounters as it passes through the medium.
• A larger specific acoustic impedance means that ultrasound experiences more resistance in the medium.

Concept

• At the boundary between media of different specific acoustic impedances, some of the wave is reflected and some of the wave is transmitted.
• The greater the difference in acoustic impedance between the two media, the greater the reflection and the smaller the transmission.