声学 impedance ([latex]Z[/latex]) is a material’s intrinsic resistance to acoustic flow, defined as its density ([latex]\rho[/latex]) multiplied by its acoustic velocity ([latex]c[/latex]), so [latex]Z = \rho c[/latex]. The percentage of ultrasonic energy reflected at the boundary between two materials is governed by the difference, or mismatch, in their respective acoustic impedances. This principle is what makes flaw detection possible.
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.
相反，如果两种材料的声阻抗非常接近，则大部分声能将穿过界面，反射极小。超声耦合剂（用于换能器和测试件之间的凝胶或液体）和换能器匹配层的设计正是利用了这一原理。这些耦合剂和匹配层的阻抗介于换能器元件和测试材料之间，以最大限度地提高能量传输并改善信号质量。
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