Syllabus Edition

First teaching 2023

First exams 2025

|

Specific Acoustic Impedance (CIE A Level Physics)

Revision Note

Test yourself
Ashika

Author

Ashika

Last updated

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:

Z space equals space rho c

  • 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 Zair = 400 kg m-2 s-1
  • Skin has an acoustic impedance of Zskin = 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

Specific Acoustic Impedance, downloadable AS & A Level Physics revision notes

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

Worked example

The table shows the speed of sound acoustic impedance in four different materials.

medium speed of ultrasound / m s–1 acoustic impedance / kg m–2 s–1
air 330 4.3 × 102
gel 1500 1.5 × 106
soft tisuse 1600 1.6 × 106
bone 4100 7.0 × 106

Use this information to calculate the value for the density of bone.

Answer:

Step 1: Write down known quantities

  • Acoustic impedance of bone, Z = 7.0 × 106 kg m-2 s-1
  • Speed of ultrasound in bone, c = 4100 m s-1

Step 2: Write out the equation for acoustic impedance

Z space equals space rho c

Step 3: Rearrange for density and calculate

rho space equals space Z over c space equals space fraction numerator 7.0 space cross times space 10 space over denominator 4100 end fraction space equals space 1700 space kg space straight m to the power of negative 3 end exponent

Examiner Tip

A common mistake is to confuse the c in the acoustic impedance equation for the speed of light - don’t do this!

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.