Acoustic Impedance

How much ultrasound is reflected at a boundary depends on a property of each medium called its acoustic impedance Z. It is defined as the product of the density ρ of the medium and the speed of sound c in it:Z = ρcThe unit of Z is kg m⁻³ × m s⁻¹ = kg m⁻² s⁻¹.Materials that are dense and in which sound travels fast, such as bone, have a high acoustic impedance. Air has a very low acoustic impedance because its density is tiny. Soft tissues all have similar values, close to that of water.Worked example: soft tissue has density 1060 kg m⁻³ and speed of sound 1540 m s⁻¹.Z = 1060 × 1540 = 1.63 × 10⁶ kg m⁻² s⁻¹For comparison, typical values are:• air: 1.3 × 330 ≈ 430 kg m⁻² s⁻¹
• water: 1.5 × 10⁶ kg m⁻² s⁻¹
• muscle: about 1.7 × 10⁶ kg m⁻² s⁻¹
• bone: about 7 to 8 × 10⁶ kg m⁻² s⁻¹The bigger the difference in Z between two media, the more ultrasound is reflected at the boundary between them. This is why boundaries between soft tissue and bone, or between tissue and air, give strong echoes, while boundaries between two soft tissues give weak ones.