A novel linearization apparatus for reducing power amplifier distortion does not require delay lines. The shut-off body is movable between a closed position, in which at least one of the shut-off sections of the shut-off body blocks the flow path, and an open position, in which both shut-off sections open the flow path. An optic and imaging system exterior of the light diffuser are used to inspect the plurality of surfaces of the substrate including specular surfaces. A linear piecewise approximation technique is used to approximate the voltage at the output of the amplifier for gaps that were not measured. Further, at least a first outer nonwoven layer of the front panel and at least a second outer nonwoven layer of the back panel may be folded to form an upper edge in the front and back regions such that a distal end of each of the first and second outer nonwoven layer may be disposed between the upper edge and the front and back end edges of the absorbent core, respectively.