Acoustic Impedance in Ultrasonic Reflection

The concept of acoustic impedance is analogous to electrical impedance in circuits and is fundamental to understanding how ultrasonic waves interact with materials. When a wave traveling through a material (Material 1) encounters an interface with a second material (Material 2), part of the wave is reflected and part is transmitted. The amount of reflection is quantified by the reflection coefficient ([latex]R[/latex]), which depends on the acoustic impedances of the two materials, [latex]Z_1[/latex] and [latex]Z_2[/latex].

For a wave at normal incidence, the pressure reflection coefficient is given by [latex]R = (Z_2 – Z_1) / (Z_2 + Z_1)[/latex]. The intensity of the reflected wave, which is what is typically measured, is proportional to the square of this value. A large mismatch in impedance, such as between steel ([latex]Z approx 45 times 10^6[/latex] Pa·s/m) and air ([latex]Z approx 415[/latex] Pa·s/m), results in a very high reflection coefficient (nearly 100%). This is why internal cracks and voids, which are filled with air or gas, are so easily detectable with ultrasound; they act as near-perfect reflectors.

Conversely, if two materials have very similar acoustic impedances, most of the sound energy will pass through the interface with minimal reflection. This principle is exploited in the design of ultrasonic couplants (gels or liquids used between the transducer and the test piece) and transducer matching layers, which are designed to have an impedance intermediate between the transducer element and the test material to maximize energy transmission and improve signal quality.