When a mechanical stress pulse, which is propagating in an elastic medium, encounters a material- or phase interface, which generally represents a change of the acoustic impedance, it is split up into a part which propagates further into the new material and another part which is reflected. The amplitude ratio of the reflected and the transmitted part is governed by the normalized difference of the acoustic impedances only, provided that the impedance change is a pure step function in space. If the acoustic impedance change is broadened spatially, the ratio of the transmitted and reflected part becomes frequency dependent and the effect can therefore be used for filter-, damping-, acoustic isolation-, and/or spectrum analysis purposes or for a quantitative analysis of the interface. The effect is of growing importance for micro- and nanostructures since the relative size of interface layers is generally larger than in macroscopic structures. Oxidation or diffusion processes might lead to 'smooth' acoustic interface layers which are characterized by gradually varying mechanical properties like density, Young's- and shear moduli, which need to be quantified by nondestructive in-depth profiling methods.