Specific acoustic impedance is the product of density and sound velocity, Z = ρ · c. It is the material property that decides how much sound crosses an interface and how much comes straight back. Units are kg·m⁻²·s⁻¹ (rayl); NDT normally uses MRayl (10⁶ rayl), so steel is about 46 MRayl, water 1.5 MRayl, air 0.0004 MRayl.

Impedance is what makes ultrasonics work at all. The mismatch between steel and air is so extreme that a lack of fusion, a lamination or a gas pore reflects essentially all of the incident energy, which is why a hairline air gap is a perfect reflector while a tight, metallurgically bonded interface may be almost invisible. The same mismatch is why couplant is mandatory: without it the probe-to-part interface is a steel/air boundary and nothing gets in.

Impedance is mode-dependent because velocity is. Shear impedance in steel (7850 × 3240 ≈ 25.4 MRayl) is well below compression impedance (≈ 46.3 MRayl); this is one reason shear waves do not couple through a liquid layer — liquids support no shear stress at all.

Use the value with the interface reflection/transmission calculation to work out the loss at a wedge-to-steel boundary, at a clad interface, or at a water path in immersion testing.

Compression or shear

Worked example

Wave mode	compression
Material velocity (override)	0 m/s
Density ρ	7850 kg/m³
Velocity used	5900 m/s
Acoustic impedance Z	46.315 MRayl
Acoustic impedance Z	46315000 kg/(m²·s)

Ferritic steel, compression: Z = 7850 × 5900 = 46 315 000 kg·m⁻²·s⁻¹ = 46.315 MRayl. The same steel in shear gives 7850 × 3240 = 25.434 MRayl.

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