# 19.3.2 Specific Acoustic Impedance

### 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 = ρc

• 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
• 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 skin, meaning that very little ultrasound is reflected

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 = ρc

#### Exam Tip

A common mistake is to confuse the c in the acoustic impedance equation for the speed of light – don’t do this! ### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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