Specific Acoustic Impedance (Cambridge (CIE) A Level Physics) : Revision Note

Specific acoustic impedance

• The acoustic impedance, Z, of a medium is defined as:

  The product of the speed of the ultrasound in the medium and the density of the medium

• This quantity describes how much resistance an ultrasound beam encounters as it passes through a tissue

• Acoustic impedance can be calculated using the equation:

• Where:

  • Z = acoustic impedance (kg m^-2 s^-1)

  • ρ = the density of the material (kg m^-3)

  • c = the speed of sound in the material (m s^-1)

• This equation tells us:

  • The higher the density of a tissue, the greater the acoustic impedance

  • The faster the ultrasound travels through the material, the greater the acoustic impedance also

• This is because sound travels faster in denser materials

  • Sound is fastest in solids and slowest in gases

  • This is because the closer the particles in the material, the faster the vibrations can move through the material

• At the boundary between media of different acoustic impedances, some of the wave energy is reflected and some is transmitted

• The greater the difference in acoustic impedance between the two media, the greater the reflection and the smaller the transmission

  • Two materials with the same acoustic impedance would give no reflection

  • Two materials with a large difference in values would give much larger reflections

• Air has an acoustic impedance of Z_air = 400 kg m^-2 s^-1

• Skin has an acoustic impedance of Z_skin = 1.7 × 10⁶ kg m^-2 s^-1

  • The large difference means ultrasound would be significantly reflected, hence a coupling gel is necessary

  • The coupling gel used has a similar Z value to the skin, meaning that very little ultrasound is reflected

Light travelling through two materials with different acoustic impedance

Refraction and reflection of ultrasound waves at a boundary between two materials with different acoustic impedances (in this case, Z₁ < Z₂ )