the basis of the signal variation of the received ultrasonic echo signal by the amplitude of the ultrasonic echo signal between at least one ultrasonic half wave before this amplitude and an ultrasonic half wave after this amplitude being of an amount greater than the amplitude of these half waves this is explained in more detail in fig5 on the basis of a variation given by way of example of an ultrasonic echo signal s detected within the chosen time window with this method amplitudes that are detected as relevant are highlighted by an arrow on the basis of the transit time t depth each relevant amplitude is assigned a layer it can be seen in the figure that relatively great time ranges t 0 in which a relevant amplitude is not measured may also occur so that in the case where this time window is greater than the layer thickness there is no relevant amplitude for this layer in the diagram according to fig6 measurements on two porous cfr components are reproduced in curves g h the amplitude a of the ultrasonic echo signal is plotted against the depth d transit time it can then be seen from the different slope of the individual curves g and h that in the case of the component reproduced by the curve g the porosity in the proximity of the surface is greater on account of the greater steepness than the porosity in the middle of the component or in the proximity of the back wall in curve h it can then be seen that a rise in the amplitude of the backscattered echo signal takes place in the middle of the component with subsequently a slope of a greater amount steeper drop than in the region near the surface.